Java 9 introduced a new feature called multi-release JARs, which allows developers to bundle different versions of classes and resources in a single JAR file. This feature is particularly useful when targeting different Java versions, as it allows the application to choose the appropriate version at runtime.
What are multi-release JARs?
A multi-release JAR is a JAR file that contains classes and resources for multiple Java releases. Each release-specific content is placed in a separate directory within the JAR, using the META-INF/versions directory. For example, a JAR that supports Java 8 and Java 11 would have directories META-INF/versions/8 and META-INF/versions/11, respectively.
How do multi-release JARs work?
When the JVM loads a JAR file, it examines the META-INF/versions directory to determine the correct version to load. It starts by looking for a directory that matches the current Java version. If a matching directory is found, the JVM loads the classes and resources from that directory. If no matching directory is found, it falls back to the base directory.
For example, if the application is running on Java 11 and there is a multi-release JAR with directories for both Java 8 and Java 11, the JVM will load classes and resources from the META-INF/versions/11 directory. If the application is running on Java 9 or earlier, it will load classes and resources from the base directory.
How to create a multi-release JAR?
To create a multi-release JAR, you need to organize the release-specific content into separate directories within the JAR. For example, if you want to support Java 8 and Java 11, you would have directories META-INF/versions/8 and META-INF/versions/11. Place the corresponding classes and resources in these directories.
You can use build tools like Maven or Gradle to automate the creation of multi-release JARs. These build tools provide plugins that handle the necessary configuration and packaging.
Benefits of using multi-release JARs
Using multi-release JARs provides several benefits:
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Backward compatibility: Multi-release JARs allow you to target different Java versions without having to maintain separate codebases or build processes.
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Reduced file size: By bundling version-specific classes in separate directories, you can avoid including unnecessary classes in the JAR for each Java version, resulting in a smaller file size.
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Improved performance: Loading only the version-specific classes at runtime can improve startup time and reduce memory consumption.
Limitations of multi-release JARs
While multi-release JARs offer many advantages, there are a few limitations to keep in mind:
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Package visibility: If a class in one version-specific directory references a class in another version-specific directory, it may result in a runtime exception due to package visibility restrictions.
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Dependency conflicts: When using multi-release JARs with dependency management tools, such as Maven or Gradle, you may encounter conflicts if different versions of the same dependency are required for different Java versions.
Conclusion
Multi-release JARs in Java 9 provide a convenient way to bundle multiple versions of classes and resources in a single JAR file. This feature offers benefits such as backward compatibility, reduced file size, and improved performance. However, it also has limitations related to package visibility and dependency conflicts. When used correctly, multi-release JARs can simplify the development and deployment of applications targeting different Java versions.
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