A Java Archive file is called a JAR file. JAR files are archives compressed in the ZIP package. They are used by the Java Runtime Environment (JRE). JAR packages contain metadata files and resource files like texts, images or other data. These are packed into a single file for use in Java. A JAR file can also be a separate Java application if it has a main class specified.
To open a JAR file you need to have JRE installed. If JRE is set as the default program for JAR files, you can just double-click to run it. On Mac, Apple Jar Launcher passes the JAR file to JRE which runs it.
The main section of a JAR file has security and configuration details about the file itself. It also defines main attributes for every manifest entry. This section ends with an empty line. Other sections define attributes for packages or files in this JAR.
JAR files use specifications proprietary to Oracle. JRE uses the data in JAR files as libraries for Java programs. These run on platforms with Java Runtime Environment.
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