JEP 282: jlink: The Java Linker

Summary

Create a tool that can assemble and optimize a set of modules and their dependencies into a custom run-time image as defined in JEP 220.

Non Goals

This JEP does not commit the resulting tool to generate anything other than a modular run-time image.

It is not a goal of this JEP to define a standard or supported API for plugins. Instead, the plugin API will be strictly experimental. A future JEP may re-visit this topic once experience has been obtained using the plugin API defined by this JEP.

Motivation

JEP 261 defines link time as an optional phase between the phases of compile time (the javac command) and run-time (the java run-time launcher). Link time requires a linking tool that will assemble and optimize a set of modules and their transitive dependencies to create a run-time image or executable.

Link time is an opportunity to do whole-world optimizations that are otherwise difficult at compile time or costly at run-time. An example would be to optimize a computation when all its inputs become constant (i.e., not unknown). A follow-up optimization would be to remove code that is no longer reachable.

Description

A basic invocation of the linker tool, jlink, is:

$ jlink --module-path <modulepath> --add-modules <modules> --limit-modules <modules> --output <path>

where:

• --module-path is the path where observable modules will be discovered by the linker; these can be modular JAR files, JMOD files, or exploded modules

• --add-modules names the modules to add to the run-time image; these modules can, via transitive dependencies, cause additional modules to be added

• --limit-modules limits the universe of observable modules

• --output is the directory that will contain the resulting run-time image

The --module-path, --add-modules, and --limit-modules options are described in further detail in JEP 261.

Other options that jlink will support include:

• --help to print a usage/help message
• --version to print version information

Alternatives

The alternative to a linking tool is to use platform-specific JDK and JRE image-build scripts. This approach would make it difficult to create custom run-time images.

Testing

Beyond the expected set of unit tests to exercise the tool I, the JDK build will regularly exercise jlink by creating the JDK and JRE run-time images.

Risks and Assumptions

The set of requirements for the tool is not complete at this time. The tool's extensibility is expected to evolve.

Dependences

• This JEP depends on the module system that will be specified by JSR 376 (Java Platform Module System) and implemented by JEP 261: Module System.

• The initial implementation of JEP 220 in JDK 9 uses a custom build-time tool to construct JRE and JDK images. That tool will be replaced by jlink.