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A Java package organizes Java classes into namespaces, providing a unique namespace for each type it contains. Classes in the same package can access each other's package-private and protected members.
In general, a package can contain the following kinds of types: classes, interfaces, enumerations, records and annotation types. A package allows a developer to group classes (and interfaces) together. These classes will all be related in some way – they might all have to do with a specific application or perform a specific set of tasks. Programmers also typically use packages to organize classes belonging to the same category or providing similar functionality.
The Java Platform Module System (JPMS) specifies a distribution format for collections of Java code and associated resources. It also specifies a repository for storing these collections, or modules, and identifies how they can be discovered, loaded and checked for integrity. It includes features such as namespaces with the aim of fixing some of the shortcomings in the existing JAR format, especially the JAR Hell, which can lead to issues such as classpath and class loading problems.
The Java Module System was initially being developed under the Java Community Process as JSR 277 and was scheduled to be released with Java 7.
JSR 277 later was put on hold and Project Jigsaw was created to modularize the JDK. This JSR was superseded by JSR 376 (Java Platform Module System).
Project Jigsaw was originally intended for Java 7 (2011) but was deferred to Java 8 (2014) as part of Plan B, and again deferred to a Java 9 release in 2017. Java 9 including the Java Module System was released on September 21, 2017.
A package manager, or package management system, is software that supports installing, upgrading, configuring, and removing software for the host system in a consistent manner.
A package manager deals with packages, distributions of software and data in archive files. Packages contain metadata, such as the software's name, description of its purpose, version number, vendor, checksum (usually a cryptographic hash function), and a list of dependencies necessary for the software to run properly. Upon installation, metadata is stored in a local package database. Package managers typically maintain a database of software dependencies and version information to prevent software mismatches and missing prerequisites. They work closely with software repositories, binary repository managers, and app stores.
Package managers are designed to eliminate the need for manual installs and updates. This can be particularly useful for large enterprises whose operating systems (OSes) typically consist of hundreds or even tens of thousands of distinct packages.
Package format is a type of archive containing computer programs and additional metadata needed by package managers; an instance of this type of archive is called a package. While the archive file format itself may be unchanged, package formats carry additional metadata, such as a manifest file or certain directory layouts. Packages may contain either source code or executable files.
Packages may be converted from one type to another with software such as Alien.
Modular programming is a programming paradigm that emphasizes organizing the functions of a codebase into independent modules, each providing an aspect of a computer program in its entirety without providing other aspects.
A module interface expresses the elements that are provided and required by the module. The elements defined in the interface are detectable by other modules. The implementation contains the working code that corresponds to the elements declared in the interface.
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