JAR (file format)
A JAR ("Java archive") file is a package typically used to aggregate many s and associated metadata and resources (text, images, etc.) into one file for distribution. JAR files are s that include a Java-specific . They are built on the and typically have a .jar . A JAR file allows Java runtimes to efficiently deploy an entire application, including its classes and their associated resources, in a single request. JAR file elements may be compressed, shortening download times. A JAR file may contain a manifest file, that is located at META-INF/MANIFEST.MF. The entries in the manifest file describe how to use the JAR file. For instance, a Classpath entry can be used to specify other JAR files to load with the JAR. The contents of a file may be extracted using any archive extraction software that supports the ZIP format, or the jar command line utility provided by the Java Development Kit. Developers can digitally sign JAR files. In that case, the signature information becomes part of the embedded manifest file. The JAR itself is not signed, but instead every file inside the archive is listed along with its checksum; it is these checksums that are signed. Multiple entities may sign the JAR file, changing the JAR file itself with each signing, although the signed files themselves remain valid. When the Java runtime loads signed JAR files, it can validate the signatures and refuse to load classes that do not match the signature. It can also support 'sealed' packages, in which the Classloader will only permit Java classes to be loaded into the same package if they are all signed by the same entities. This prevents malicious code from being inserted into an existing package, and so gaining access to package-scoped classes and data. The content of JAR files may be obfuscated to make reverse engineering more difficult. An executable Java program can be packaged in a JAR file, along with any libraries the program uses. Executable JAR files have the manifest specifying the entry point class with Main-Class: myPrograms.
