What format and structure of code are packaged into a JAR file?

A JAR (Java ARchive) is an archive file that packages together:

• Compiled Java classes (.class files), organized in directories corresponding to packages.
• Resources (e.g., images, configuration files) needed by the application.
• Manifest file (META-INF/MANIFEST.MF) containing metadata about the JAR, such as the entry point (Main-Class).

The structure of a JAR file is similar to a ZIP archive with a specific organization:

/ (root of archive)
  com/example/MyClass.class
  resources/config.properties
  META-INF/
    MANIFEST.MF

A JAR file makes it convenient to distribute and run Java applications or libraries.