Java 9, like every release, has caused a load of code to stop building. This page attempts to capture some of the solutions.

-source/-target 1.5

The -source and -target arguments no longer accept values below 1.6.

Maven and ant have been patched to automatically move things to at least 1.6. The specific changes can be found at plexus compiler 2.8.3-1, ant 1.9.9-4 and and 1.9.9-5. Everything else needs to the build system fixing.

According to JEP-182 and then - after the 6-month release cycle was set - updated by JDK-8046172, JDK-8028563, and JDK-8173605, support for 1.6 will be dropped in "Java 11", so if you can move things to 1.7 immediately, then that will be fine.

However, the 1.6 source level introduces stricter encoding and generics checks: things that may previously have been warnings on javac -source 1.5, are errors on javac -source 1.6. Most of these things are easy, but annoying, to fix.

Note that many builds will succeed without the -source/-target arguments provided; if they are not present, then Java defaults to using the latest versions.

_ may not be used as a keyword

For things not specifying -source, the default source level has changed to 9, which prevents _ being used as an identifier. This breaks gettext users. The easiest solution is to change to -source 1.8.

javax.xml.bind / javax.activation

These packages have been removed from the JDK, but are still available as modules.

Some write-up here:

There are implementations on Maven, which used to be in Debian but have been removed. It's not known if these will actually work on Java 9.

Someone needs to decide what Debian is doing about this.

swing / *ComboBox

There have been some, ... mistakes in the addition of generics to some ComboBox and TreeNode related classes. We don't really have a solution to this at the moment, beyond hoping that one of the upstreams wants to make their codebase much worse to work around it.

Some complaining, with references:

Failed to find JDK / tools.jar

The path of everything inside $JAVA_HOME has changed, and the java -version / java.version values have changed in ways that break terrible build scripts.

There's a build system named "equivalence" kicking around, a perl, hand-maintained script named ./configure. There's so few of them that it's probably worth just patching all of them:

`cannot find symbol` during javadoc

Javadoc generation is now more strict about the classpath being wrong during documentation generation. This should be fixable by fixing the classpath.

Sometimes this happens with Maven, cause unknown:

When it happens with ant or shell as the build system, it's probably a problem with the build file?

There's a (hidden) option for javadoc to ignore these problems, named --ignore-source-errors.