Example example_addReads_addExports

Part of the full Java 9 Jigsaw modules example suite.

Authors

Originally written by Martin Lehmann, Kristine Schaal and Rüdiger Grammes (cf. original repository).

Migrated for Java Modules support documentation of Apache MavenTM in the course of the Maven Support & Care program by Gerd Aschemann (and other team members) as forked repository. Please add discussions, requirements, bugfixes, etc. to the fork instead of the original.

What is this example about?

Modules in this example

  • modb, modc, modmain

  • modmain has a Main class which is started in run.sh

Module Dependency Graph, created with DepVis

Example’s Module Dependency Graph

Example shows …​

How to use --add-reads and --add-exports for javac compiler and java launcher.

All module-info.java files are empty. All configuration is done via command-line options --add-reads and --add-exports in compile.sh and run.sh for javac and java.

  1. modb needs a read relationship to modc

  2. modc needs to export its package pkgc to modb

  3. modmain needs a read relationship to modb

  4. modb needs to export its package pkgb to modmain

Don’t try this at home

This example demonstrates an anti-pattern. Using --add-reads and --add-exports in production/main code is strongly discouraged.

These flags should only be used for testing scenarios where you need test-only dependencies (like JUnit) without polluting production module descriptors.

For a realistic example of proper usage, see Issue #5: Realistic testing example.

Better alternatives: Declare proper requires and exports in your module-info.java files instead.

See also: Why you shouldn’t use --add-exports.

Output

This example uses golden master testing to ensure output consistency. The expected output is compared with actual output using verify.sh.

Expected Output

Main: pkgmain.Main, id=ID_Main_23, B: from B, C: pkgc.C, id=ID_B_21

Actual Output

Main: pkgmain.Main, id=ID_Main_23, B: from B, C: pkgc.C, id=ID_B_21

Maven 4 Output

Main: pkgmain.Main, id=ID_Main_23, B: from B, C: pkgc.C, id=ID_B_21

Maven 4 Migration

This example demonstrates module system boundary overrides using --add-reads and --add-exports flags. These flags must be configured for both compile-time and runtime.

Maven 4 Compiler Changes

The Challenge

This example shows how to override module boundaries at compile time and runtime. Without explicit configuration, the compiler will reject:

  • modb reading from modc (no requires declaration)

  • modmain reading from modb (no requires declaration)

  • modmain accessing non-exported packages from modb and modc

The Solution

Configure the compiler plugin with the necessary --add-reads and --add-exports arguments:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>4.0.0-beta-3</version>
  <configuration>
    <compilerArgs>
      <!-- Required for cross-module access without explicit requires/exports -->
      <arg>--add-reads</arg>
      <arg>modb=modc</arg>
      <arg>--add-reads</arg>
      <arg>modmain=modb</arg>
      <arg>--add-exports</arg>
      <arg>modc/pkgc=modb</arg>
      <arg>--add-exports</arg>
      <arg>modb/pkgb=modmain</arg>
    </compilerArgs>
  </configuration>
</plugin>

The runtime configuration in run.sh must include the same flags:

java --module-path mlib  \
     --add-modules modb,modc            \
     --add-reads   modmain=modb         \
     --add-reads   modb=modc            \
     --add-exports modb/pkgb=modmain    \
     --add-exports modc/pkgc=modb       \
     --module modmain/pkgmain.Main

Key Takeaways

  • Use <compilerArgs> to pass module system override flags to the compiler

  • Both compile-time and runtime configurations must specify the same overrides

  • --add-reads allows a module to read another without declaring requires

  • --add-exports makes non-exported packages accessible to specific modules

  • These flags are useful for migration scenarios or working with third-party modules