jar

JAR stands for Java ARchive, a package file format used to aggregate many Java class files and related metadata into a single archive for distribution and deployment. A JAR is based on the ZIP file format and can contain compiled Java classes, resources such as images and property files, and a META-INF directory with metadata. A key file is META-INF/MANIFEST.MF, which can declare metadata about the archive, including the Main-Class attribute that designates the entry point for executable JARs, as well as Class-Path, version, and sealing information.

Executable and creation methods are central to JAR use. An executable JAR can be run with the Java Runtime Environment using the command java -jar myapp.jar, provided the manifest specifies a Main-Class. JAR files are created and updated with the jar tool from the JDK, for example: jar cfm app.jar MANIFEST.MF -C build/classes .; many projects assemble JARs automatically using build tools such as Maven or Gradle.

Usage extends beyond standalone applications to libraries that are included on a Java classpath. JAR files can be signed with jarsigner to verify origin and integrity, enhancing security for distribution. They are typically platform independent, since Java bytecode runs on the Java Virtual Machine across different systems.

Related packaging formats include WAR (Web ARchive) for web applications and EAR (Enterprise Archive) for enterprise applications. JARs remain a fundamental mechanism for distributing Java code and resources in a compact, portable form.