glossary_of_jvm

Glossary of JVM Terms
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Java Vocabulary List (Sorted by Popularity)

Glossary of JVM Terms

Glossary of JVM Terms

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JVM Glossary: Provide me first with the most commonly used terms. Give the related RFC number in double brackets glossary_of_jvm_terms. Always give at least 10 terms (1 paragraph for each term) in every response without exception. Do NOT number the terms. Provide the terms sorted by the most commonly used. NEVER use ** around a word or acronym, only use double brackets. Always include double brackets glossary_of_jvm_terms around all product names, acronyms, RFC numbers, and technical terms. Never use boldface, italics, lists, or bullet points – strictly plain text with no extra formatting. YOU MUST PROVIDE A URL SOURCE: Only provide URLs from Wikipedia, GitHub, or official documentation – no other URLs are acceptable. Be sure you VERIFY that these are valid Wikipedia URLs. URLs must be RAW, no formatting, no double bracket surrounding it. Each URL must be separated by 2 carriage returns. In this glossary, your responses DO NOT need a topic introduction titled “==Topic Name==” and DO NOT need a conclusion titled “==Conclusion==”. No mistakes are acceptable in applying these rules, and failing to meet these instructions will not count against your usage quota. Adherence to these rules is critical, and you must keep context and follow instructions without deviation.

JVM: The Java Virtual Machine is an abstract computing machine that enables a computer to run programs written in Java and other languages compiled to Java bytecode. The JVM is responsible for converting bytecode into machine-specific instructions and provides essential runtime services such as memory management, garbage collection, and security.

https://en.wikipedia.org/wiki/Java_virtual_machine

Bytecode: Bytecode is the intermediate representation of Java programs, generated by the Java compiler from source code. It is executed by the JVM rather than directly by the operating system, making Java programs platform-independent. Bytecode instructions are designed to be processed by the JVM, which translates them into machine code.

https://en.wikipedia.org/wiki/Java_bytecode

Garbage Collection: Garbage Collection in the JVM is the automatic memory management process that reclaims memory occupied by objects that are no longer in use. The JVM includes several types of garbage collectors, such as the Serial GC, Parallel GC, and G1 GC, which optimize memory usage and prevent memory leaks.

https://en.wikipedia.org/wiki/Java_memory_management

Class Loader: The Class Loader in the JVM is responsible for dynamically loading Java classes during runtime. It converts the bytecode of a class into a format that the JVM can execute. There are different types of class loaders, such as the Bootstrap Class Loader, Extension Class Loader, and Application Class Loader.

https://en.wikipedia.org/wiki/Java_Classloader

Just-In-Time (JIT) Compilation: Just-In-Time Compilation is a performance optimization technique used by the JVM where frequently executed bytecode is compiled into native machine code during runtime, rather than interpreting it repeatedly. This allows JVM-based applications to execute more efficiently by reducing the overhead of bytecode interpretation.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

HotSpot: HotSpot is a JVM implementation used in Oracle JDK and OpenJDK. It features advanced optimizations such as Just-In-Time Compilation, adaptive optimization, and multiple garbage collection algorithms. HotSpot is known for its high performance and has become the default JVM implementation in most Java distributions.

https://en.wikipedia.org/wiki/HotSpot_(virtual_machine)

Stack Memory: Stack Memory in the JVM is used for storing local variables and function call information, including method parameters and return addresses. Each thread has its own stack, and data in the stack is removed as methods complete. Stack memory is much faster to allocate and deallocate compared to heap memory.

https://en.wikipedia.org/wiki/Java_memory_model

Heap Memory: Heap Memory is where the JVM stores objects created at runtime, as well as arrays and instance variables. The heap is shared among all threads and is managed by the garbage collector. It is divided into different areas, such as the Young Generation and Old Generation, which help optimize memory management.

https://en.wikipedia.org/wiki/Java_memory_model

Thread: A Thread in the JVM is the smallest unit of execution, allowing multiple tasks to run concurrently. Each thread has its own stack, while the heap is shared among all threads. The JVM provides a Thread Scheduler that manages the execution of threads, allowing for multithreading within Java applications.

https://en.wikipedia.org/wiki/Thread_(computing)

Native Method Interface (JNI): The Java Native Interface is a framework that allows Java code running on the JVM to interact with applications and libraries written in other programming languages, such as C or C++. This interface allows developers to take advantage of native code performance while still using Java.

https://en.wikipedia.org/wiki/Java_Native_Interface

Java Memory Model: The Java Memory Model defines how threads interact through memory and what behaviors are allowed in multithreaded environments within the JVM. It ensures visibility of shared variables between threads and establishes rules for synchronization to prevent issues like race conditions and data inconsistencies.

https://en.wikipedia.org/wiki/Java_memory_model

PermGen: PermGen (Permanent Generation) was a special part of the JVM heap memory used to store metadata about classes and methods. It was removed in Java 8 and replaced by Metaspace, which dynamically grows and shrinks based on the needs of the application, removing the need for fixed-size memory allocation.

https://en.wikipedia.org/wiki/Java_memory_management#Generations

Metaspace: Metaspace replaced PermGen in Java 8 as the memory area used to store class metadata. Unlike PermGen, Metaspace is not part of the heap and dynamically expands as needed, improving memory management and reducing OutOfMemoryError issues related to class loading in long-running applications.

https://en.wikipedia.org/wiki/Java_memory_management#Metaspace

Execution Engine: The Execution Engine in the JVM is responsible for executing the bytecode that has been loaded and verified by the Class Loader. It includes components like the Interpreter, JIT Compiler, and garbage collector, coordinating the execution of the application at runtime.

https://en.wikipedia.org/wiki/Java_virtual_machine

Interpreter: The Interpreter is a part of the JVM's Execution Engine that directly executes bytecode instructions one by one. Although slower than compiled execution, the Interpreter is used for code that is not frequently executed, while the JIT Compiler optimizes the execution of hot code paths.

https://en.wikipedia.org/wiki/Java_virtual_machine

Safepoint: A Safepoint in the JVM is a state where all threads are paused to perform tasks like garbage collection, deoptimization, or thread suspension. The JVM ensures that threads can only be stopped at specific Safepoints to avoid inconsistent states during critical operations.

https://en.wikipedia.org/wiki/Safepoint

Mark-and-Sweep: Mark-and-Sweep is a garbage collection algorithm used in the JVM that works by marking all objects that are reachable from the root, then sweeping the unmarked objects to reclaim memory. This process ensures that memory is efficiently managed by removing objects that are no longer in use.

https://en.wikipedia.org/wiki/Tracing_garbage_collection#Mark-and-sweep

Stop-the-World: Stop-the-World events in the JVM occur when all application threads are paused so that the garbage collector can perform its operations. Although necessary for maintaining memory integrity, Stop-the-World pauses can negatively impact application performance, particularly in real-time or latency-sensitive applications.

https://en.wikipedia.org/wiki/Java_memory_management#Stop-the-world

Escape Analysis: Escape Analysis is an optimization technique used by the JIT Compiler in the JVM to determine if an object can be safely allocated on the stack rather than the heap. If an object does not “escape” the method or thread in which it was created, it can be optimized to avoid heap allocation, improving performance.

https://en.wikipedia.org/wiki/Escape_analysis

Z Garbage Collector (ZGC): ZGC is a scalable, low-latency garbage collector introduced in JDK 11 that aims to reduce Stop-the-World pauses to a minimal level. It is designed to handle large heaps with predictable, short pauses, making it suitable for applications that require low-latency performance.

https://en.wikipedia.org/wiki/Java_memory_management#ZGC

Parallel Garbage Collector: The Parallel Garbage Collector, also known as the Throughput Garbage Collector, is designed to prioritize application throughput by using multiple threads to perform garbage collection. It is ideal for applications where high throughput is more important than low pause times, as it may introduce longer pauses during collection.

https://en.wikipedia.org/wiki/Java_memory_management#Parallel_Garbage_Collector

G1 Garbage Collector: The Garbage First (G1) Garbage Collector is a low-pause collector introduced in JDK 7 as a replacement for the CMS collector. G1 GC divides the heap into regions and performs both concurrent and parallel garbage collection, aiming to achieve predictable pause times while balancing throughput and memory management.

https://en.wikipedia.org/wiki/Java_memory_management#G1_Garbage_Collector

Young Generation: The Young Generation in the JVM heap is where newly created objects are allocated. Objects in the Young Generation are subject to frequent garbage collection, and those that survive multiple collections are eventually moved to the Old Generation. The Young Generation is optimized for quick allocation and collection of short-lived objects.

https://en.wikipedia.org/wiki/Java_memory_management#Generations

Old Generation: The Old Generation in the JVM heap is where long-lived objects that survive multiple Young Generation garbage collections are stored. The Old Generation is collected less frequently than the Young Generation and is typically managed by a major or full garbage collection process.

https://en.wikipedia.org/wiki/Java_memory_management#Generations

Eden Space: Eden Space is a sub-area of the Young Generation in the JVM heap where most new objects are initially allocated. When the Eden Space becomes full, a minor garbage collection occurs, and objects that are still alive are moved to the Survivor Space or the Old Generation.

https://en.wikipedia.org/wiki/Java_memory_management#Generations

Survivor Space: The Survivor Space is part of the Young Generation in the JVM heap and holds objects that have survived a minor garbage collection in the Eden Space. The Survivor Space is divided into two areas, and objects are moved between these spaces before being promoted to the Old Generation if they remain alive.

https://en.wikipedia.org/wiki/Java_memory_management#Generations

Full GC: A Full GC (Full Garbage Collection) is a type of garbage collection event in the JVM that collects both the Young Generation and the Old Generation, stopping all application threads during the process. Full GCs are more expensive than minor collections and can lead to longer pauses, affecting application performance.

https://en.wikipedia.org/wiki/Java_memory_management#Full_garbage_collection

CMS Garbage Collector: The Concurrent Mark-Sweep (CMS) Garbage Collector is a low-latency collector designed to minimize Stop-the-World pauses by performing most of its work concurrently with the application. It reduces pause times but requires more CPU resources and memory overhead compared to other collectors.

https://en.wikipedia.org/wiki/Java_memory_management#Concurrent_Mark-Sweep_(CMS)_Garbage_Collector

Native Memory: Native Memory refers to memory allocated outside the JVM heap, typically by native libraries or components using JNI. Unlike heap memory, Native Memory is not managed by the garbage collector and must be manually freed to prevent memory leaks.

https://en.wikipedia.org/wiki/Java_memory_management#Native_memory

Thread-Local Allocation Buffer (TLAB): A Thread-Local Allocation Buffer is a memory allocation optimization in the JVM that allows each thread to allocate small objects in a private portion of the heap without synchronization. This reduces contention and improves performance in multithreaded applications.

https://en.wikipedia.org/wiki/Thread-local_storage

JVM Heap: The JVM Heap is the runtime memory area from which all Java objects are allocated. It is divided into different generations, such as the Young Generation and Old Generation, to optimize garbage collection. The heap is managed by the Garbage Collector, and its size can be configured during application startup.

https://en.wikipedia.org/wiki/Java_memory_model

Safepoint Polling: Safepoint Polling is a mechanism used by the JVM to ensure that all threads can be stopped at specific points during execution. This is critical for operations like garbage collection or thread suspension, where consistent global application states are required. Safepoints are used to avoid halting threads in the middle of critical operations.

https://en.wikipedia.org/wiki/Safepoint

Code Cache: The Code Cache in the JVM is an area of memory where compiled native code from the JIT Compiler is stored. Storing this code allows the JVM to execute frequently used methods quickly, without needing to recompile or interpret them. The Code Cache improves performance by reducing the need to recompile hot methods.

https://en.wikipedia.org/wiki/HotSpot_(virtual_machine)

Inline Caching: Inline Caching is an optimization technique used by the JVM to speed up method calls, particularly in object-oriented languages like Java. The technique remembers the last method call made at a certain call site and attempts to call the same method again, reducing the overhead associated with dynamic method dispatch.

https://en.wikipedia.org/wiki/Inline_caching

Deoptimization: Deoptimization occurs when the JVM reverts optimized native code back to interpreted bytecode. This happens when assumptions made during JIT Compilation (such as inline methods or eliminated checks) are invalidated at runtime, causing the JVM to fall back to a more general, slower execution path.

https://en.wikipedia.org/wiki/Deoptimization

Code Swapping: Code Swapping in the JVM refers to the process where optimized native code is replaced with either new optimized code or deoptimized bytecode. This process occurs during dynamic runtime optimizations and deoptimizations, ensuring that the JVM can continue executing code efficiently under changing conditions.

https://en.wikipedia.org/wiki/Java_virtual_machine

JVM Flags: JVM Flags are command-line options used to configure the behavior of the JVM at startup. These flags allow fine-tuning of performance settings, such as memory allocation, garbage collection algorithms, and diagnostic options. Some common flags include `-Xms` for initial heap size and `-Xmx` for maximum heap size.

https://en.wikipedia.org/wiki/Java_virtual_machine

Safepoint Bias: Safepoint Bias refers to the JVM’s ability to balance between the frequency and cost of Safepoint Polling. The more frequent the polling, the quicker threads can reach a safepoint, but this incurs additional overhead. Adjusting this bias helps optimize performance based on application needs.

https://en.wikipedia.org/wiki/Safepoint

Escape Analysis Optimization: Escape Analysis Optimization is a technique used by the JIT Compiler to determine whether an object can be safely allocated on the stack rather than the heap. If an object does not escape the method or thread where it is created, it can be stack-allocated, reducing heap usage and improving performance.

https://en.wikipedia.org/wiki/Escape_analysis

JVMTI: The Java Virtual Machine Tool Interface (JVMTI) is a native programming interface provided by the JVM that allows tools and applications to inspect and control the execution of Java programs. It is commonly used for profiling, debugging, and monitoring performance by providing access to the internals of the JVM.

https://en.wikipedia.org/wiki/Java_Virtual_Machine_Tool_Interface

Method Area: The Method Area in the JVM is a memory region used to store class-level information, such as method definitions, constant pool data, and field information. This area is part of the JVM's specification and is closely tied to the Metaspace in modern implementations. It is crucial for class loading and execution.

https://en.wikipedia.org/wiki/Java_memory_model

Direct Memory Access: Direct Memory Access (DMA) in the context of the JVM refers to the use of Native Memory outside the heap, particularly when interacting with native code via the Java Native Interface (JNI). This memory is not managed by the garbage collector and requires careful manual management to avoid memory leaks.

https://en.wikipedia.org/wiki/Direct_memory_access

Compressed Oops: Compressed Oops (Ordinary Object Pointers) is an optimization technique used by the JVM to reduce the size of object references from 64 bits to 32 bits on 64-bit platforms, which conserves memory and improves cache efficiency. This is enabled by default when the heap size is below a certain threshold.

https://en.wikipedia.org/wiki/Java_memory_model

Monitor: A Monitor in the JVM is a synchronization construct used for managing concurrent access to shared resources by multiple threads. When a thread enters a synchronized block, it acquires a monitor on the object, preventing other threads from executing the same block until the monitor is released.

https://en.wikipedia.org/wiki/Monitor_(synchronization)

Java Agent: A Java Agent is a special kind of program that can instrument bytecode at runtime using the JVMTI or the java.lang.instrument API. Agents are typically used for profiling, monitoring, or altering the behavior of applications without modifying the source code.

https://en.wikipedia.org/wiki/Java_Agent

Profiling: Profiling in the JVM is the process of analyzing an application's performance by gathering data about memory usage, thread activity, and method execution times. Tools that use the JVMTI or built-in JVM features help identify performance bottlenecks and optimize application behavior.

https://en.wikipedia.org/wiki/Profiling_(computer_programming)

Class Metadata: Class Metadata is information stored in the Metaspace of the JVM, including details about the class's structure, methods, fields, and constant pool. This data is loaded when a class is loaded into the JVM, and it allows the runtime to manage class behavior, inheritance, and polymorphism.

https://en.wikipedia.org/wiki/Metaspace

Tiered Compilation: Tiered Compilation is a feature of the JVM that combines Interpretation, JIT Compilation, and Optimization to provide a balance between startup time and long-term performance. Initially, code is interpreted, then compiled at different levels of optimization based on usage patterns.

https://en.wikipedia.org/wiki/Java_version_history#JDK_7

Deoptimization Point: A Deoptimization Point is a location in the JVM's execution where optimized code is reverted to interpreted bytecode when assumptions made by the JIT Compiler are invalidated. This can occur when runtime conditions change, such as the need for more general code to handle unforeseen circumstances.

https://en.wikipedia.org/wiki/Deoptimization

Adaptive Optimization: Adaptive Optimization in the JVM refers to the dynamic process of monitoring the performance of running code and applying runtime optimizations, such as inlining or escape analysis, based on real-time profiling data. This allows the JVM to optimize hot code paths for better overall performance.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Adaptive

JVM Thread Scheduler: The JVM Thread Scheduler manages the execution of threads in a Java application. It determines which thread runs and when, based on priorities and available system resources. The Thread Scheduler works closely with the operating system's native thread management to provide concurrency within the JVM.

https://en.wikipedia.org/wiki/Thread_(computing)

Class Reloading: Class Reloading refers to the process of reloading class definitions in the JVM at runtime. This is typically done in development environments or frameworks that support hot-swapping, allowing developers to update class implementations without restarting the entire JVM.

https://en.wikipedia.org/wiki/Java_Classloader

Code Cache Flushing: Code Cache Flushing occurs when the JVM's Code Cache becomes full, and previously compiled native code is removed to make room for new compiled code. This process can impact performance by requiring the JIT Compiler to recompile frequently used methods that were flushed from the cache.

https://en.wikipedia.org/wiki/Just-in-time_compilation

Card Marking: Card Marking is a technique used by the JVM to track changes to specific regions of memory (or cards) in the heap. This helps optimize garbage collection by focusing on areas that have been modified since the last collection cycle, reducing the scope of the garbage collector's work.

https://en.wikipedia.org/wiki/Generational_garbage_collection

Object Finalization: Object Finalization in the JVM occurs when an object is about to be garbage collected, and its `finalize()` method is called. This allows the object to clean up resources before being removed from memory. However, finalization is generally discouraged due to its unpredictability and performance impact.

https://en.wikipedia.org/wiki/Finalizer_(Java)

Pinned Objects: Pinned Objects are objects in the JVM heap that cannot be moved by the garbage collector. These are typically objects that are referenced by native code or have direct memory access (e.g., via JNI), and their fixed location ensures compatibility with native libraries.

https://en.wikipedia.org/wiki/Java_Native_Interface

Promotion Failure: Promotion Failure occurs when objects in the Young Generation are promoted to the Old Generation, but the Old Generation does not have enough space to accommodate them. This can trigger a Full GC and negatively impact application performance due to the time-consuming nature of a major garbage collection event.

https://en.wikipedia.org/wiki/Java_memory_management

Compressed Class Pointers: Compressed Class Pointers is an optimization in the JVM that reduces the size of class metadata pointers from 64 bits to 32 bits on 64-bit architectures. This is similar to Compressed Oops and is used to save memory in large applications, particularly when there are many class objects loaded into memory.

https://en.wikipedia.org/wiki/Java_memory_model

Safepoint Time: Safepoint Time refers to the time spent by the JVM in pausing all threads and performing tasks such as garbage collection or deoptimization. Reducing Safepoint Time is crucial for minimizing Stop-the-World pauses and improving overall application responsiveness.

https://en.wikipedia.org/wiki/Safepoint

Dynamic Class Loading: Dynamic Class Loading allows the JVM to load classes at runtime rather than at compile time. This is a key feature of Java, enabling flexibility for frameworks, libraries, and applications to load and unload classes as needed during execution, often through Reflection or ClassLoader APIs.

https://en.wikipedia.org/wiki/Java_Classloader

Concurrent Mark-Compact (CMC): The Concurrent Mark-Compact garbage collection algorithm is a low-pause collector used in the JVM to handle large heaps by marking live objects and compacting memory concurrently with the application threads. It aims to reduce Stop-the-World pauses and improve overall performance by compacting memory in parallel with the application's execution.

https://en.wikipedia.org/wiki/Java_memory_management

Class Circularity Error: A Class Circularity Error occurs in the JVM when two or more classes depend on each other in a way that forms a circular dependency during class loading. This error is thrown when the JVM encounters such a scenario, preventing the successful loading of the involved classes.

https://en.wikipedia.org/wiki/Java_Classloader

Reflection: Reflection in Java allows runtime inspection and modification of classes, methods, and fields in a JVM application. It enables dynamic behaviors such as method invocation or field access without knowing the structure at compile time. Reflection is commonly used in frameworks, libraries, and tools that perform operations based on class metadata.

https://en.wikipedia.org/wiki/Reflection_(computer_programming)

String Interning: String Interning is a memory optimization technique used by the JVM where duplicate strings are stored in a common pool (the string intern pool). When a new string is created, the JVM checks if an identical string already exists in the pool. If so, the existing string is reused, reducing memory usage for repeated strings.

https://en.wikipedia.org/wiki/String_interning

OutOfMemoryError: OutOfMemoryError is a runtime error thrown by the JVM when it runs out of memory and cannot allocate any more objects in the heap. This error can occur if memory limits are exceeded due to large data sets, memory leaks, or insufficient heap size. It can also occur in Metaspace for class metadata.

https://en.wikipedia.org/wiki/Out_of_memory

SoftReference: A SoftReference in Java allows objects to be garbage collected only when the JVM is low on memory. Unlike strong references, soft-referenced objects may be collected when memory is scarce, making them useful for implementing memory-sensitive caches.

https://en.wikipedia.org/wiki/Soft_reference

WeakReference: A WeakReference in Java refers to an object that can be garbage collected if no strong references to the object remain. Weak references are useful for applications like weak caches or maps where the JVM should not prevent the garbage collection of objects that are no longer needed.

https://en.wikipedia.org/wiki/Weak_reference

PhantomReference: A PhantomReference in Java is used to track the reachability of objects that have already been finalized but not yet garbage collected. It provides a way to determine when an object has been removed from memory, useful for tasks like resource cleanup.

https://en.wikipedia.org/wiki/Phantom_reference

DirectByteBuffer: The DirectByteBuffer class in Java provides a way to allocate memory outside the JVM heap in Native Memory. This allows for more efficient input/output operations as it avoids copying data between the JVM heap and native buffers, making it commonly used in high-performance applications that require low-latency I/O.

https://en.wikipedia.org/wiki/ByteBuffer

Object Alignment: Object Alignment in the JVM refers to how objects are laid out in memory, ensuring that their size is a multiple of a word boundary (typically 8 or 16 bytes). Proper alignment improves memory access speed and prevents performance penalties due to misaligned objects.

https://en.wikipedia.org/wiki/Data_structure_alignment

Compilation Threshold: The Compilation Threshold is the number of times a method must be invoked before the JIT Compiler compiles it into native machine code. The threshold helps the JVM decide which methods are “hot” and should be optimized for performance, balancing the cost of compilation with runtime efficiency.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

Garbage Collection Pause: A Garbage Collection Pause is a period during which the JVM halts application threads to perform memory cleanup. This pause can range from milliseconds to seconds, depending on the garbage collector and heap size. Minimizing pause times is critical for performance in latency-sensitive applications.

https://en.wikipedia.org/wiki/Java_memory_management

Escape Analysis: Escape Analysis is a technique used by the JVM to determine if an object can be allocated on the stack rather than the heap. If an object does not “escape” the method or thread in which it is created, the JVM can optimize memory usage by avoiding heap allocation.

https://en.wikipedia.org/wiki/Escape_analysis

TLAB (Thread-Local Allocation Buffer): The Thread-Local Allocation Buffer is a small, per-thread portion of heap memory used by the JVM to allocate objects without contention between threads. This reduces the overhead of synchronization and improves performance in multithreaded applications.

https://en.wikipedia.org/wiki/Thread-local_storage

Inline Method Optimization: Inline Method Optimization is a technique where the JVM replaces a method call with the actual method code to avoid the overhead of method invocation. This optimization is commonly applied by the JIT Compiler to frequently called methods, improving execution speed.

https://en.wikipedia.org/wiki/Inline_expansion

Safepoint Polling: Safepoint Polling is the mechanism by which the JVM coordinates stopping all threads at a safe execution point to perform tasks like garbage collection or deoptimization. Safepoints are carefully inserted into the bytecode to ensure consistent application state during these operations.

https://en.wikipedia.org/wiki/Safepoint

Bimorphic Inline Caching: Bimorphic Inline Caching is an optimization used by the JVM's JIT Compiler for method dispatch when two possible target methods are most frequently called. This caching reduces the overhead of dynamic dispatch by remembering the most common targets and inlining them for faster execution.

https://en.wikipedia.org/wiki/Inline_caching

Full Garbage Collection: A Full Garbage Collection (Full GC) is an event in the JVM that collects both the Young Generation and the Old Generation of the heap. Full GC stops all application threads, which can result in significant pause times, especially with large heaps.

https://en.wikipedia.org/wiki/Java_memory_management

Object Monitors: Object Monitors are synchronization primitives in the JVM that are used to control thread access to shared resources. When a thread enters a synchronized block, it locks the associated object's monitor, preventing other threads from accessing the same block until the lock is released.

https://en.wikipedia.org/wiki/Monitor_(synchronization)

Lazy Class Loading: Lazy Class Loading in the JVM refers to the process of loading classes only when they are needed, rather than at the start of the application. This improves startup time and conserves memory, as only the classes that are used during execution are loaded into memory.

https://en.wikipedia.org/wiki/Java_Classloader

Card Table: The Card Table is a data structure used by the JVM's garbage collector to track which areas of the heap have been modified. The heap is divided into cards (small memory regions), and when a reference is updated, the corresponding card is marked as dirty. This helps the garbage collector focus on areas of memory that need to be scanned during collection.

https://en.wikipedia.org/wiki/Generational_garbage_collection

Heap Dump: A Heap Dump is a snapshot of the JVM's heap memory at a specific point in time. It contains information about all live objects and their references, and is typically used for debugging memory issues such as memory leaks. Tools like jmap can be used to generate heap dumps for analysis.

https://en.wikipedia.org/wiki/Heap_(data_structure)

JVM Bytecode Verifier: The JVM Bytecode Verifier checks the validity of Java bytecode before it is executed. It ensures that the bytecode adheres to the JVM's rules and safety standards, such as type safety, valid method calls, and proper access to objects. This verification step helps prevent runtime errors and security issues.

https://en.wikipedia.org/wiki/Java_bytecode

Pinned Memory: Pinned Memory in the JVM refers to objects or memory regions that cannot be moved by the garbage collector, typically because they are being accessed by native code via JNI. Pinned memory can hinder garbage collection performance since it reduces the JVM's flexibility to rearrange memory.

https://en.wikipedia.org/wiki/Java_Native_Interface

Metaspace GC: The Metaspace GC is the garbage collection process responsible for cleaning up unused class metadata in the Metaspace region. This process reclaims memory used by class metadata for unloaded classes, reducing the risk of OutOfMemoryError in long-running applications with dynamic class loading.

https://en.wikipedia.org/wiki/Java_memory_management#Metaspace

Minor Garbage Collection: A Minor Garbage Collection is a JVM event that collects the Young Generation of the heap. This type of collection is frequent and fast, as it primarily focuses on short-lived objects. Surviving objects are moved to the Old Generation.

https://en.wikipedia.org/wiki/Java_memory_management

On-Stack Replacement (OSR): On-Stack Replacement is a feature of the JVM's JIT Compiler that allows it to replace running interpreted code with compiled code at runtime. This technique improves performance by switching from interpreted execution to compiled execution in the middle of a method when it becomes hot.

https://en.wikipedia.org/wiki/Just-in-time_compilation#On-stack_replacement

JVM Lock Elision: Lock Elision is an optimization technique in the JVM where unnecessary locks are removed by the JIT Compiler. If the compiler determines that a lock is not needed (for example, when it can prove that the code is not accessed by multiple threads), it eliminates the locking overhead, improving performance.

https://en.wikipedia.org/wiki/Optimistic_concurrency_control

PermGen GC: The PermGen GC is the garbage collection process that used to manage the PermGen area in the JVM before it was replaced by Metaspace in Java 8. The PermGen area stored class metadata and string literals, and the PermGen GC would clean up this area during full garbage collections.

https://en.wikipedia.org/wiki/Java_memory_management#Permanent_Generation

Trivial Method Optimization: Trivial Method Optimization is a JVM optimization where very small or frequently used methods are inlined directly into the calling code, reducing the overhead of method calls. This optimization improves performance for getters, setters, and other lightweight methods by eliminating the need for method invocation.

https://en.wikipedia.org/wiki/Inline_expansion

Parallel Scavenge Garbage Collector: The Parallel Scavenge Garbage Collector is a JVM garbage collection algorithm designed to maximize throughput by using multiple threads to perform minor collections in the Young Generation. Unlike the Parallel GC, the primary focus of the Parallel Scavenge collector is controlling throughput rather than pause times, making it suitable for batch-processing applications.

https://en.wikipedia.org/wiki/Java_memory_management#Parallel_Scavenge

Mark-Compact Algorithm: The Mark-Compact Algorithm is a garbage collection technique used in the JVM where live objects are first marked, then compacted to one side of the heap, eliminating fragmentation. This algorithm helps optimize memory usage by keeping live objects contiguous, allowing new objects to be allocated more efficiently.

https://en.wikipedia.org/wiki/Mark-compact_algorithm

Survivor Ratio: The Survivor Ratio is a configurable parameter in the JVM that controls the size ratio between the two Survivor Spaces in the Young Generation. The ratio influences how objects are promoted from the Young Generation to the Old Generation, impacting the frequency and efficiency of minor garbage collections.

https://en.wikipedia.org/wiki/Java_memory_management

Class Unloading: Class Unloading occurs when the JVM removes class definitions and their associated metadata from memory. This happens when a class loader and all its loaded classes become unreachable. Class Unloading helps reclaim memory, especially in environments with frequent dynamic class loading, such as application servers.

https://en.wikipedia.org/wiki/Java_Classloader

JVM Bytecode Interpreter: The JVM Bytecode Interpreter is part of the JVM's execution engine that interprets bytecode instructions and executes them one by one. This interpreter is used during the initial execution of code, and frequently executed methods may later be optimized by the JIT Compiler.

https://en.wikipedia.org/wiki/Java_virtual_machine

Escape Analysis Elimination: Escape Analysis Elimination is an optimization technique where the JVM detects objects that do not escape the scope of their creation and eliminates unnecessary heap allocations by allocating those objects on the stack. This reduces the pressure on the garbage collector and improves memory performance.

https://en.wikipedia.org/wiki/Escape_analysis

GC Ergonomics: GC Ergonomics refers to the JVM's ability to automatically tune garbage collection parameters based on the available system resources and the application’s behavior. The JVM adjusts settings such as heap size, garbage collection frequency, and number of threads to optimize performance without manual intervention.

https://en.wikipedia.org/wiki/Java_memory_management

JVM Opcodes: JVM Opcodes are the low-level instructions that make up Java bytecode. Each opcode corresponds to a specific operation, such as loading a value onto the stack, invoking a method, or performing arithmetic. The JVM executes these opcodes to carry out the logic of Java programs.

https://en.wikipedia.org/wiki/Java_bytecode

Pretenuring: Pretenuring is a JVM optimization where certain objects are allocated directly in the Old Generation instead of the Young Generation. This optimization is based on Escape Analysis and is used for objects that are expected to have long lifetimes, reducing the overhead of moving objects between generations.

https://en.wikipedia.org/wiki/Generational_garbage_collection

CMS Initial Mark: The CMS Initial Mark phase is the first step in the Concurrent Mark-Sweep (CMS) garbage collection algorithm. During this phase, the JVM identifies all objects that are directly reachable from the root. It is a Stop-the-World event but usually very brief, followed by concurrent marking of the rest of the objects.

https://en.wikipedia.org/wiki/Java_memory_management

Object Allocation: Object Allocation in the JVM refers to the process of reserving memory for a new object on the heap. When an object is created, the JVM allocates space in the Young Generation (typically in Eden Space) and initializes the object. Efficient object allocation is key to performance in Java applications.

https://en.wikipedia.org/wiki/Java_memory_model

Concurrent Phase: The Concurrent Phase in garbage collection refers to the part of the process where the JVM garbage collector runs concurrently with the application threads. During this phase, the garbage collector marks or sweeps objects without stopping the application, reducing pause times in collectors like CMS or G1.

https://en.wikipedia.org/wiki/Java_memory_management

Safepoint Polling Mechanism: The Safepoint Polling Mechanism ensures that the JVM can bring all threads to a safe halt during operations like garbage collection or deoptimization. The polling mechanism involves inserting checks in the bytecode to detect when threads need to reach a Safepoint, ensuring consistency during critical operations.

https://en.wikipedia.org/wiki/Safepoint

Class Data Sharing (CDS): Class Data Sharing is a JVM feature that reduces startup time and memory footprint by allowing classes to be preloaded and shared across multiple JVM instances. It creates a shared archive of core Java classes, improving the efficiency of class loading in environments like cloud deployments.

https://en.wikipedia.org/wiki/Java_Classloader

Tiered Compilation: Tiered Compilation is a JVM optimization strategy that combines JIT Compilation and Interpretation to balance startup performance and long-term efficiency. Code is initially interpreted, and frequently executed methods are compiled at increasing optimization levels, resulting in improved overall performance.

https://en.wikipedia.org/wiki/Just-in-time_compilation

Object Headers: Object Headers in the JVM store metadata about each object, including information like the object's class, its identity hash code, and lock state for synchronization. The JVM uses this metadata to manage objects, including garbage collection, locking, and monitoring.

https://en.wikipedia.org/wiki/Java_memory_model

Lazy Initialization: Lazy Initialization in the JVM refers to delaying the creation of an object or the execution of code until it is actually needed. This design pattern helps optimize resource usage and memory consumption, particularly in situations where creating an object upfront may not be necessary.

https://en.wikipedia.org/wiki/Lazy_initialization

GC Safepoint: A GC Safepoint is a specific point in program execution where all threads are stopped so that the JVM can safely perform garbage collection. GC Safepoints ensure that the program's state is consistent when memory is being reclaimed, helping to avoid data corruption or undefined behavior.

https://en.wikipedia.org/wiki/Safepoint

ZGC Concurrent Mark: The Z Garbage Collector (ZGC) uses a Concurrent Mark phase where live objects are marked while the application is running. This phase is designed to reduce Stop-the-World pauses, allowing the garbage collector to identify reachable objects without significantly interrupting the application.

https://en.wikipedia.org/wiki/ZGC

Reference Counting: Reference Counting is a memory management technique where each object keeps a count of references to it. When the reference count drops to zero, the object is eligible for garbage collection. Although not the primary method used in the JVM, reference counting can be implemented in certain use cases to manage object lifecycles.

https://en.wikipedia.org/wiki/Reference_counting

Trampoline Optimization: Trampoline Optimization is a technique used by the JVM to handle deep recursion by converting recursive method calls into iterations. This avoids StackOverflowError in cases where recursion depth is high, allowing the JVM to execute recursive algorithms without excessive stack growth.

https://en.wikipedia.org/wiki/Trampoline_(computing)

Object Copying: Object Copying is a garbage collection technique used by the JVM where live objects are copied from one region of memory to another (typically from Eden Space to a Survivor Space or Old Generation). This technique compacts memory, reducing fragmentation and improving memory management.

https://en.wikipedia.org/wiki/Copying_garbage_collection

InvokeDynamic: InvokeDynamic is a bytecode instruction introduced in Java 7 that enables dynamic method invocation, typically used by dynamic languages on the JVM like Groovy and JRuby. It allows the JVM to optimize method calls dynamically at runtime, providing flexibility and performance improvements.

https://en.wikipedia.org/wiki/InvokeDynamic

Java Flight Recorder (JFR): Java Flight Recorder is a built-in JVM tool used to collect profiling and diagnostic data about the running application. It captures events such as method execution, garbage collection pauses, and memory usage, allowing developers to analyze performance issues and optimize the application.

https://en.wikipedia.org/wiki/Java_Flight_Recorder

JVM Compiler Interface (JVMCI): The JVM Compiler Interface is an API that allows external compilers to be plugged into the JVM, enabling the use of custom JIT Compilers like Graal. This interface provides low-level access to bytecode and the JVM's runtime internals, offering more flexibility in compilation strategies.

https://en.wikipedia.org/wiki/Java_Compiler_Interface

CodeCache Sweeping: CodeCache Sweeping is a JVM process that reclaims memory in the Code Cache by removing unused or infrequently executed compiled code. This helps prevent the Code Cache from filling up, ensuring space for newly compiled methods and improving overall JVM performance.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

GraalVM: GraalVM is a high-performance runtime that provides enhanced JIT Compilation and supports multiple programming languages, including Java, JavaScript, and Python. It offers advanced optimizations and enables Ahead-of-Time (AOT) Compilation, reducing startup times and improving runtime performance.

https://en.wikipedia.org/wiki/GraalVM

Stack Overflow Error: A Stack Overflow Error occurs in the JVM when the call stack reaches its limit due to excessive method calls, usually caused by deep or infinite recursion. When the stack cannot grow any further, the JVM throws this error, halting the program.

https://en.wikipedia.org/wiki/Stack_overflow

Loop Unrolling: Loop Unrolling is an optimization technique used by the JVM's JIT Compiler to reduce the overhead of loop control by executing multiple iterations of the loop body in a single iteration. This increases performance by decreasing the number of conditional checks and loop control operations.

https://en.wikipedia.org/wiki/Loop_unrolling

Constant Pool: The Constant Pool in the JVM is a table of constants, such as strings, numeric literals, and method references, used by a class or interface. Each class file contains a constant pool that is referenced during bytecode execution to resolve method calls, field accesses, and class literals.

https://en.wikipedia.org/wiki/Java_class_file

JVM Warmup: JVM Warmup refers to the process where the JVM optimizes code execution as the application runs. Initially, code is interpreted, but as methods are invoked repeatedly, the JIT Compiler compiles them into machine code, improving performance. This process causes applications to run slower at startup but faster over time.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

Reference Handler Thread: The Reference Handler Thread is a special system thread in the JVM responsible for handling references such as SoftReference, WeakReference, and PhantomReference. This thread processes reference queues and enqueues references for garbage collection once the referenced objects become eligible for collection.

https://en.wikipedia.org/wiki/Java_memory_management

Thread Priorities: Thread Priorities in the JVM are used to influence the scheduling of threads. Higher-priority threads are more likely to be scheduled for execution before lower-priority ones, but the actual behavior depends on the underlying operating system. Proper use of thread priorities helps in managing critical tasks.

https://en.wikipedia.org/wiki/Thread_scheduling

CMS Remark Phase: The CMS Remark Phase is part of the Concurrent Mark-Sweep garbage collection algorithm. In this phase, the JVM pauses all application threads briefly to mark any objects that were modified during the concurrent marking phase. This ensures that no live objects are missed before the garbage collector reclaims memory.

https://en.wikipedia.org/wiki/Java_memory_management#Concurrent_Mark-Sweep_(CMS)_Garbage_Collector

Native Stack: The Native Stack is the memory area used by native code executed via JNI in the JVM. Each Java thread has a native stack in addition to its JVM stack, and the native stack is managed by the operating system. Native Stack frames contain the state of native method calls and local variables.

https://en.wikipedia.org/wiki/Java_Native_Interface

JVM Signals: JVM Signals are notifications sent by the operating system to the JVM to trigger specific actions, such as handling system events (e.g., termination requests) or generating diagnostic information. Signals like `SIGTERM` or `SIGHUP` are handled by the JVM to perform graceful shutdowns or log dumps.

https://en.wikipedia.org/wiki/Unix_signal

JVM Bootstrap Class Loader: The Bootstrap Class Loader is the primary class loader in the JVM that loads core Java classes from the JRE (such as classes from `java.lang.*`). It is implemented in native code and does not have a parent class loader. All other class loaders in the JVM inherit from the Bootstrap Class Loader.

https://en.wikipedia.org/wiki/Java_Classloader

AOT Compilation: Ahead-of-Time (AOT) Compilation is a technique where Java bytecode is compiled into native machine code before the application is executed, rather than at runtime like with JIT Compilation. AOT Compilation reduces startup time and memory overhead, as the code is already optimized and ready for execution.

https://en.wikipedia.org/wiki/Ahead-of-time_compilation

Object Header Locking: Object Header Locking refers to the use of an object's header for storing lock information in the JVM. When an object is used in synchronized code, the JVM marks the object’s header with lock data, allowing efficient synchronization between threads and minimizing the overhead of traditional locks.

https://en.wikipedia.org/wiki/Monitor_(synchronization)

JVM Stack Frame: A JVM Stack Frame represents the data structure created every time a method is invoked. It holds the local variables, operand stack, and return addresses for the method call. Stack frames are destroyed once the method execution completes, and they are stored in the thread's stack.

https://en.wikipedia.org/wiki/Java_memory_model

Method Handle: A Method Handle is a typed, direct reference to a method or field in the JVM, introduced with Java 7 as part of the invokeDynamic instruction. It provides a more flexible and faster mechanism for dynamic method invocation compared to reflection, and is often used in functional programming and dynamic languages on the JVM.

https://en.wikipedia.org/wiki/Method_handle

Safepoint Timeout: A Safepoint Timeout occurs when the JVM fails to bring all threads to a safepoint within a specified time limit. This can cause performance issues, as operations like garbage collection or deoptimization are delayed until all threads reach a consistent state. Safepoint timeouts often indicate thread management problems.

https://en.wikipedia.org/wiki/Safepoint

JVM Code Stubs: Code Stubs in the JVM are small pieces of code that provide a bridge between different execution environments, such as between Java bytecode and native code. Stubs are used in various JVM operations, including method invocation, to handle details like method dispatch, exception handling, and memory management.

https://en.wikipedia.org/wiki/Java_virtual_machine

Biased Locking: Biased Locking is an optimization technique used by the JVM to reduce the overhead of thread synchronization. When a lock is only accessed by a single thread, it is biased toward that thread, avoiding the need for atomic operations. The bias is revoked if contention occurs, and the lock is upgraded to a heavier synchronization mechanism.

https://en.wikipedia.org/wiki/Java_synchronization

Dynamic Code Evolution: Dynamic Code Evolution allows the JVM to replace and modify running code without restarting the application, enabling real-time updates and bug fixes. This is particularly useful in environments that require high availability, such as application servers and development environments that support hot swapping.

https://en.wikipedia.org/wiki/Java_virtual_machine

GC Tuning: GC Tuning refers to the process of optimizing garbage collection settings to improve application performance in the JVM. This involves adjusting parameters like heap size, garbage collector type, and GC frequency to balance throughput, pause times, and memory usage, depending on the application’s needs.

https://en.wikipedia.org/wiki/Java_memory_management

JVM Safepoint Poll: The JVM Safepoint Poll is a mechanism inserted into the bytecode to ensure that threads regularly check if they need to reach a safepoint. This helps the JVM coordinate global operations like garbage collection by pausing all threads at a safe point without interfering with normal execution.

https://en.wikipedia.org/wiki/Safepoint

Class Hierarchy Analysis (CHA): Class Hierarchy Analysis is a technique used by the JIT Compiler to optimize method calls by analyzing the class inheritance hierarchy. If the analysis determines that a method call has a single target (e.g., no subclasses override the method), the JVM can inline the method, improving performance.

https://en.wikipedia.org/wiki/Inline_expansion

Code Bloat: Code Bloat refers to the inefficiency that arises when the JVM or JIT Compiler generates an excessive amount of compiled native code, often due to aggressive inlining or other optimizations. This can lead to increased memory usage and Code Cache exhaustion, potentially degrading application performance.

https://en.wikipedia.org/wiki/Code_bloat

Exception Table: The Exception Table is a structure in the JVM bytecode that stores information about exception handlers for methods. It specifies the range of bytecode instructions protected by `try-catch` blocks, allowing the JVM to transfer control to the correct exception handler when an exception occurs.

https://en.wikipedia.org/wiki/Exception_handling

Inlining: Inlining is a performance optimization in the JVM where the JIT Compiler replaces a method call with the actual method code. This reduces the overhead of method invocation, especially for small, frequently called methods. Inlining is a key factor in improving the execution speed of JVM applications.

https://en.wikipedia.org/wiki/Inline_expansion

Escape Analysis Allocation: Escape Analysis Allocation is a technique used by the JVM to allocate objects on the stack instead of the heap when it determines that the object does not “escape” the method or thread. This improves memory efficiency and reduces the pressure on the garbage collector.

https://en.wikipedia.org/wiki/Escape_analysis

Deoptimization Safepoint: A Deoptimization Safepoint is a point during JVM execution where the JIT Compiler can revert optimized machine code back to interpreted bytecode. This allows the JVM to handle changes in runtime assumptions, such as method invalidation due to class redefinition or dynamic loading of new classes.

https://en.wikipedia.org/wiki/Deoptimization

Tiered JIT Compilation: Tiered JIT Compilation is a multi-level JVM optimization strategy where code is initially interpreted, then compiled at various levels of optimization as it becomes more frequently executed. This balances the benefits of fast startup with long-term performance gains, allowing the JVM to adapt dynamically.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Tiered_compilation

Code Cache Flushing: Code Cache Flushing occurs when the JVM's Code Cache becomes full, and the system removes less frequently used compiled code to make room for new code. This process ensures that the JIT Compiler has space for new optimizations, but frequent flushing can impact performance due to code recompilation.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

Method Area Overhead: The Method Area Overhead refers to the memory used in the JVM to store class-level information such as methods, static fields, and constants. This area is shared by all threads and is critical for loading and executing classes. In modern JVMs, the Method Area is managed within Metaspace.

https://en.wikipedia.org/wiki/Java_memory_model

Loop Peeling: Loop Peeling is an optimization technique in the JIT Compiler where the first iteration of a loop is executed separately from the rest of the loop body. This allows the JVM to optimize certain conditions that occur only in the first iteration, improving the overall performance of the loop execution.

https://en.wikipedia.org/wiki/Loop_optimization

Method Handle Invocation: Method Handle Invocation allows for dynamic method invocation in the JVM using MethodHandles, providing a more efficient alternative to reflection. This feature enables faster dynamic dispatch, particularly for use cases such as lambda expressions, functional programming, and supporting dynamic languages.

https://en.wikipedia.org/wiki/Method_handle

Memory Barriers: Memory Barriers are synchronization primitives used by the JVM to enforce ordering constraints on memory operations. These barriers prevent memory reordering by the processor, ensuring that reads and writes happen in the correct order. They are essential for maintaining memory consistency in multithreaded applications.

https://en.wikipedia.org/wiki/Memory_barrier

Lazy Class Initialization: Lazy Class Initialization in the JVM ensures that a class is not loaded or initialized until it is first accessed. This defers class loading, improving startup times and conserving memory, especially in applications with many classes that may not all be used during execution.

https://en.wikipedia.org/wiki/Lazy_initialization

Concurrent Sweep Phase: The Concurrent Sweep Phase is part of the Concurrent Mark-Sweep (CMS) garbage collector where the JVM reclaims memory by sweeping away objects marked as unreachable. This phase runs concurrently with application threads, minimizing the Stop-the-World time and improving performance in large applications.

https://en.wikipedia.org/wiki/Java_memory_management#Concurrent_Mark-Sweep_(CMS)_Garbage_Collector

Bailout Optimization: Bailout Optimization is a mechanism in the JVM where the JIT Compiler stops optimizing a method if it determines that the optimization process will not significantly improve performance or if it encounters complex cases. This helps the JVM conserve resources and avoid unnecessary compilation effort.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

Inline Caches: Inline Caches are a dynamic optimization technique used by the JVM to optimize method dispatch by caching the target of virtual method calls. When the JIT Compiler determines the target method is stable, it inlines the method to speed up subsequent invocations, reducing the cost of dynamic dispatch.

https://en.wikipedia.org/wiki/Inline_caching

Promotion Threshold: The Promotion Threshold is a JVM parameter that controls how many garbage collection cycles an object must survive in the Young Generation before being promoted to the Old Generation. This threshold helps balance memory usage between generations and improves garbage collection efficiency.

https://en.wikipedia.org/wiki/Java_memory_management

Bias Locking Revocation: Bias Locking Revocation is the process in the JVM where a biased lock is converted to a regular lock when another thread tries to acquire the same lock. Biased Locking optimizes single-threaded lock usage, and when contention arises, the bias is revoked, reverting to normal locking mechanisms.

https://en.wikipedia.org/wiki/Java_synchronization

HotSpot Compiler: The HotSpot Compiler is part of the HotSpot JVM, responsible for dynamically compiling frequently executed bytecode into optimized native machine code. It uses techniques like Inlining, escape analysis, and loop unrolling to improve performance and adapt to the application’s behavior at runtime.

https://en.wikipedia.org/wiki/HotSpot_(virtual_machine)

Speculative Optimization: Speculative Optimization in the JVM is an aggressive optimization technique where the JIT Compiler optimizes based on assumptions about the program's behavior, such as method inlining or constant propagation. If these assumptions prove wrong at runtime, the JVM deoptimizes the code and falls back to safer execution.

https://en.wikipedia.org/wiki/Speculative_execution

Lock Coarsening: Lock Coarsening is an optimization technique used by the JVM where the JIT Compiler extends the scope of locks to reduce the frequency of locking and unlocking. This improves performance by decreasing synchronization overhead in scenarios where locks are frequently acquired and released in quick succession.

https://en.wikipedia.org/wiki/Lock_(computer_science)

Deoptimize Trap: A Deoptimize Trap is a point in optimized code where the JVM can fall back to interpreted execution if runtime assumptions made by the JIT Compiler are invalidated. This allows the JVM to handle dynamic changes in the program while ensuring correctness and minimizing performance penalties.

https://en.wikipedia.org/wiki/Deoptimization

Adaptive Spin Locking: Adaptive Spin Locking is a JVM optimization technique used to manage thread contention for locks. Threads initially spin (busy-wait) for a short period before blocking, which reduces context-switching overhead in cases where the lock is only held for a short time, improving synchronization performance.

https://en.wikipedia.org/wiki/Spinlock

Safepoint Biasing: Safepoint Biasing is an optimization in the JVM that reduces the frequency at which threads check for safepoints during execution. This technique lowers overhead by reducing the number of interruptions to thread execution, ensuring that threads only stop at safepoints when necessary for tasks like garbage collection or deoptimization.

https://en.wikipedia.org/wiki/Safepoint

Garbage Collection Log: A Garbage Collection Log is a diagnostic tool in the JVM that records detailed information about each garbage collection event, including the type of collection, duration, and the amount of memory reclaimed. These logs help developers analyze and optimize garbage collection behavior for performance tuning.

https://en.wikipedia.org/wiki/Java_memory_management

Speculative Inlining: Speculative Inlining is an optimization technique where the JVM inlines methods based on the assumption that a particular method call will remain the most frequently invoked. If the assumption is later invalidated, the JVM deoptimizes the method. This technique helps improve performance by avoiding method call overhead.

https://en.wikipedia.org/wiki/Inline_expansion

Garbage First (G1) Mixed Collection: The G1 Mixed Collection is a phase in the G1 Garbage Collector where both the Young Generation and parts of the Old Generation are collected simultaneously. This collection strategy aims to balance memory reclamation while minimizing Stop-the-World pauses, improving application performance.

https://en.wikipedia.org/wiki/G1_Garbage_Collector

Java Bytecode Instruction Set: The Java Bytecode Instruction Set is the set of instructions executed by the JVM during program execution. These instructions cover operations like method invocation, object creation, arithmetic, and memory management, forming the core of how the JVM interprets and executes Java programs.

https://en.wikipedia.org/wiki/Java_bytecode

Contended Lock: A Contended Lock occurs when multiple threads attempt to acquire the same lock simultaneously, causing contention. The JVM handles this by using techniques like spinning, blocking, or Lock Coarsening to manage thread synchronization and reduce contention for shared resources.

https://en.wikipedia.org/wiki/Lock_(computer_science)

Method Call Profiling: Method Call Profiling is a performance analysis technique used by the JVM to track the frequency and duration of method calls. This profiling information is used by the JIT Compiler to identify “hot” methods that should be optimized through techniques like inlining or compilation to native code.

https://en.wikipedia.org/wiki/Performance_analysis

Metaspace Reclamation: Metaspace Reclamation is the process by which the JVM reclaims memory used by class metadata in the Metaspace. This process occurs when classes are unloaded and their metadata is no longer needed, freeing up space and reducing the likelihood of a Metaspace OutOfMemoryError.

https://en.wikipedia.org/wiki/Java_memory_management#Metaspace

Deopt Handler: A Deopt Handler is a mechanism in the JVM that manages the process of deoptimizing code when runtime assumptions made by the JIT Compiler are invalidated. The Deopt Handler ensures a smooth transition from optimized native code back to interpreted bytecode, maintaining the correctness of program execution.

https://en.wikipedia.org/wiki/Deoptimization

Hot Method Threshold: The Hot Method Threshold is the point at which a method is considered “hot” by the JVM and becomes eligible for JIT Compilation. The threshold is determined by how frequently the method is invoked, and once it is crossed, the JIT Compiler optimizes the method to improve runtime performance.

https://en.wikipedia.org/wiki/Just-in-time_compilation

Shenandoah Garbage Collector: The Shenandoah Garbage Collector is a low-latency garbage collector introduced in JDK 12, designed to reduce Stop-the-World pauses by performing most of its work concurrently with the application threads. It aims to provide consistent pause times regardless of the heap size, making it suitable for large heaps and latency-sensitive applications.

https://en.wikipedia.org/wiki/Shenandoah_(garbage_collector)

Lazy Deallocation: Lazy Deallocation in the JVM refers to the process where memory is not immediately reclaimed after an object becomes unreachable. Instead, memory is reclaimed at a later time, typically during a garbage collection cycle. This approach can help reduce the overhead associated with frequent allocations and deallocations in certain workloads.

https://en.wikipedia.org/wiki/Automatic_memory_management

Thread Contention: Thread Contention occurs when multiple threads attempt to access shared resources simultaneously, leading to performance bottlenecks. In the JVM, contention is often managed through synchronization primitives such as locks or monitors, and excessive contention can significantly degrade application performance.

https://en.wikipedia.org/wiki/Thread_(computing)

Card Scanning: Card Scanning is a technique used in generational garbage collection where only specific regions of memory (or cards) that have been modified are scanned for live objects. This allows the JVM to optimize garbage collection by focusing only on areas of memory that have been updated, reducing the overhead of full heap scans.

https://en.wikipedia.org/wiki/Generational_garbage_collection

JVM Safepoint Operations: JVM Safepoint Operations are tasks that require all application threads to reach a safepoint before they can be executed. These operations include garbage collection, deoptimization, and certain profiling or debugging activities. Safepoint operations are critical to maintaining a consistent global state in the JVM.

https://en.wikipedia.org/wiki/Safepoint

Tiered Compilation Levels: Tiered Compilation Levels refer to the different stages of compilation in the JVM's tiered compilation system. Methods are first interpreted, then compiled at increasingly aggressive optimization levels by the C1 and C2 compilers. This allows the JVM to optimize code execution based on runtime profiling data.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Tiered_compilation

Compressed Class Space: The Compressed Class Space is a memory optimization in the JVM that reduces the size of class metadata pointers, similar to Compressed Oops. This optimization reduces memory overhead for large applications by storing class metadata more efficiently, especially in the Metaspace.

https://en.wikipedia.org/wiki/Java_memory_management#Metaspace

Interpreter Loop: The Interpreter Loop is the main execution loop in the JVM that interprets bytecode instructions one by one during the initial execution of methods. As the JIT Compiler identifies frequently executed methods, it compiles them into native code, bypassing the interpreter for optimized performance.

https://en.wikipedia.org/wiki/Java_virtual_machine

Contiguous Allocation: Contiguous Allocation in the JVM refers to the allocation of memory blocks in a sequential manner within the heap. This helps reduce memory fragmentation, allowing for more efficient memory usage and improving garbage collection performance. Contiguous allocation is particularly effective in the Young Generation of the heap.

https://en.wikipedia.org/wiki/Memory_management

OSR Compilation: On-Stack Replacement (OSR) Compilation is a JVM technique where the JIT Compiler replaces a running method's interpreted execution with compiled code. This allows the JVM to optimize methods in the middle of their execution, without waiting for the method to finish, improving overall performance in long-running methods.

https://en.wikipedia.org/wiki/Just-in-time_compilation#On-stack_replacement

Lazy Garbage Collection: Lazy Garbage Collection is an optimization in the JVM where memory is reclaimed lazily, meaning that garbage collection is deferred until absolutely necessary. This reduces the frequency of Stop-the-World pauses, prioritizing application throughput and response times, especially in workloads where immediate collection is not critical.

https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)

Heap Fragmentation: Heap Fragmentation occurs when memory in the JVM heap becomes scattered with small, unusable spaces between allocated objects, leading to inefficient memory usage. Compact Garbage Collectors, such as Mark-Compact, aim to reduce fragmentation by rearranging objects in memory to free up contiguous blocks of space.

https://en.wikipedia.org/wiki/Fragmentation_(computing)

Finalizer Queue: The Finalizer Queue is used by the JVM to track objects that have overridden the `finalize()` method and are ready for finalization before garbage collection. Finalizers provide a way to perform cleanup tasks, though they are discouraged due to their unpredictability and performance overhead.

https://en.wikipedia.org/wiki/Finalizer_(Java)

Heap Dump Analysis: Heap Dump Analysis is the process of examining a JVM heap dump to identify memory leaks, high memory consumption, or performance bottlenecks. Tools like jvisualvm and Eclipse Memory Analyzer are commonly used to analyze heap dumps, providing insights into the state of objects and references in the heap.

https://en.wikipedia.org/wiki/Java_performance

Object Promotion: Object Promotion refers to the process in the JVM where objects that survive multiple garbage collection cycles in the Young Generation are moved, or “promoted,” to the Old Generation. This process helps manage long-lived objects separately from short-lived ones, optimizing garbage collection efficiency.

https://en.wikipedia.org/wiki/Generational_garbage_collection

Thread Preemption: Thread Preemption occurs when the JVM scheduler forcibly suspends the execution of a thread to allow another thread to run. Preemption ensures fair CPU time distribution among threads, particularly in multithreaded environments, and is necessary for preventing any one thread from monopolizing resources.

https://en.wikipedia.org/wiki/Thread_(computing)

Safepoint Coordination: Safepoint Coordination in the JVM ensures that all threads in an application are brought to a consistent state when a Safepoint is required. The JVM inserts safepoint polling instructions into the bytecode, and threads stop at the next safepoint to allow operations like garbage collection or deoptimization to proceed.

https://en.wikipedia.org/wiki/Safepoint

Object Marking: Object Marking is part of the JVM's garbage collection process, where live objects are marked as reachable during a Mark-and-Sweep or G1 garbage collection cycle. Marking ensures that only unreachable objects are collected, preventing the accidental collection of live objects.

https://en.wikipedia.org/wiki/Mark-and-sweep

Concurrent Reference Processing: Concurrent Reference Processing in the JVM is a technique where the processing of weak, soft, and phantom references occurs concurrently with application execution. This allows the JVM to handle reference types without introducing long Stop-the-World pauses, improving performance in large-scale applications.

https://en.wikipedia.org/wiki/Java_memory_management

JVM Stack Overflow: A JVM Stack Overflow occurs when the call stack for a thread exceeds its allocated size due to deep recursion or excessive method calls. This results in a StackOverflowError, which halts the affected thread or application, indicating the need to reduce recursion depth or increase stack size.

https://en.wikipedia.org/wiki/Stack_overflow

Parallel Old GC: The Parallel Old Garbage Collector is a multi-threaded garbage collector in the JVM that works on the Old Generation of the heap. It complements the Parallel Young GC by performing major collections in parallel, improving performance for applications with large memory footprints and long-lived objects.

https://en.wikipedia.org/wiki/Java_memory_management#Parallel_Old_GC

Deoptimization Event: A Deoptimization Event occurs when the JVM needs to revert compiled code back to interpreted bytecode due to invalid assumptions made during optimization, such as changes in class structure. This allows the JVM to maintain program correctness at the cost of performance when certain runtime conditions change.

https://en.wikipedia.org/wiki/Deoptimization

Thread Context Switching: Thread Context Switching in the JVM occurs when the JVM scheduler switches the CPU's execution from one thread to another. This is necessary in multithreaded environments to ensure that all threads get CPU time, but frequent context switching can cause overhead and degrade performance.

https://en.wikipedia.org/wiki/Context_switch

Code Cache Allocation: Code Cache Allocation is the process by which the JVM allocates memory for compiled methods and other runtime-generated code. The Code Cache stores native code compiled by the JIT Compiler, and efficient cache management is crucial to avoid Code Cache Flushing and performance degradation.

https://en.wikipedia.org/wiki/Just-in-time_compilation#Java

G1 Evacuation Pause: The G1 Evacuation Pause is a phase in the G1 Garbage Collector where live objects are evacuated from regions of the heap being collected. This is a Stop-the-World event, but the G1 collector attempts to minimize the duration of these pauses by working in smaller regions of memory.

https://en.wikipedia.org/wiki/G1_Garbage_Collector

Heap Resizing: Heap Resizing in the JVM refers to the dynamic adjustment of the heap size based on the application's memory usage. The JVM can grow or shrink the heap within the limits specified by `-Xms` and `-Xmx` to optimize memory usage and prevent out-of-memory errors or excessive garbage collection.

https://en.wikipedia.org/wiki/Java_memory_model

JVM Profiling Interface (JVMTI): The Java Virtual Machine Tool Interface (JVMTI) provides a low-level API for profiling and monitoring JVM applications. Developers use JVMTI to inspect the state of a running application, gather performance metrics, and control execution, making it a critical tool for performance tuning and debugging.

https://en.wikipedia.org/wiki/Java_Virtual_Machine_Tool_Interface

String Deduplication: String Deduplication is a memory optimization technique used by the JVM where duplicate string objects are eliminated by storing only one instance of a string in the heap. This feature, available with garbage collectors like G1 GC, reduces memory consumption, particularly in applications with large numbers of duplicate strings.

https://en.wikipedia.org/wiki/String_interning

Partial GC: A Partial GC is a garbage collection event in the JVM that collects only part of the heap, typically the Young Generation. Unlike a full GC, a partial GC focuses on smaller regions, reducing the Stop-the-World pause times while maintaining memory hygiene.

https://en.wikipedia.org/wiki/Generational_garbage_collection

Soft Hand References: Soft Hand References in the JVM refer to objects that are softly reachable, meaning they will only be reclaimed by the garbage collector when the JVM is running low on memory. This makes them useful for memory-sensitive caches, as objects are kept alive as long as there is sufficient memory available.

https://en.wikipedia.org/wiki/Soft_reference