plaufer1980 / java-migration-playground

Experiments with multiple Java versions in the same Maven reactor.

java-migration-playground

What's the setup?

The project consists of two libraries. They have been created like this (Maven is used as a build tool):
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DgroupId=com.github.plaufer1980.java.migration.playground -DartifactId=j6-lib
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DgroupId=com.github.plaufer1980.java.migration.playground -DartifactId=j7-lib

And two apps that depend on these libraries:
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DgroupId=com.github.plaufer1980.java.migration.playground -DartifactId=j6-app
mvn archetype:generate -DarchetypeGroupId=org.apache.maven.archetypes -DgroupId=com.github.plaufer1980.java.migration.playground -DartifactId=j7-app

Everything is tied together by a multi-module POM so it can be built conveniently:
mvn clean install

As the names suggest there is one library with JDK 1.6 compatible source (j6-lib) and one that uses JDK 1.7 features (j7-lib). Same is true for the apps. Every module uses the maven-compiler-plugin to specify the required Java source and target versions.

Single apps can be built using Maven's project list support:
mvn clean install -am -pl j7-app
mvn clean install -am -pl j6-app

Additionally, the maven-shade-plugin is used to obtain some executable jars for the j6-app and j7-app modules which can be executed like this (after a successful build):
java -jar j6-app/target/j6-app-1.0-SNAPSHOT.jar
java -jar j7-app/target/j7-app-1.0-SNAPSHOT.jar

Observations

• Using JDK 1.7 features directly in a module configured to compile to JDK 1.6 target results in a failing build due to compilation errors as expected. (Uncomment Java6Foo.baz() in j6-lib to see this in action.)
• Building the j6-app (which is configured to be built with JDK 1.6 target) doesn't raise any warnings or errors regarding the JDK 1.7 sources from j7-lib, as Maven doesn't check the JDK compliance of the dependency module (even though its in the same reactor)!
• Running the j6-app with JDK 1.7 doesn't produce any issues.
• Running the j6-app with JDK 1.6 produces the following error:
  Exception in thread "main" java.lang.UnsupportedClassVersionError: com/github/plaufer1980/java/migration/playground/Java7Foo : Unsupported major.minor version 51.0

Conclusion

• The maven-compiler-plugin doesn't protect you from UnsupportedClassVersionError at runtime.
• Do not build multiple projects for multiple Java target versions in one Maven reactor.