Questions tagged [multi-release-jar]

A JAR file has a content root, which contains classes and resources, as well as a META-INF directory which contains metadata about the JAR. By adding some versioning metadata to specific groups of files the JAR format can encode, in a compatible way, multiple versions of a library for different target Java platform releases. A multi-release JAR ("MRJAR") will contain the main attribute:

Multi-Release: true

declared in the main section of the JAR MANIFEST.MF. The attribute name is also declared as a constant java.util.jar.Attributes.MULTI_RELEASE. Like other main attributes, the name declared in the MANIFEST.MF is case insensitive. The value is also case-insensitive, but there must be no preceding or trailing white space (such a restriction helps ensure the performance goal is met).

A multi-release JAR ("MRJAR") will contain additional directories for classes and resources specific to particular Java platform releases. A JAR for a typical library might look like this:

jar root
  - A.class
  - B.class
  - C.class
  - D.class

Suppose there are alternate versions of A and B that can take advantage of Java 9 features. We can bundle them into a single JAR as follows:

jar root
  - A.class
  - B.class
  - C.class
  - D.class
  - META-INF
     - versions
        - 9
           - A.class
           - B.class