Can Linkage Checker detect the bad Java8-incompatible byte code

[suztomo]

Can Linkage Checker (Linkage Monitor) detect the wrong byte code that does not work with Java 8 (case protocolbuffers/protobuf#7827)

Wrong byte code for Java8 that has reference to CharBuffer CharBuffer.flip() (java.nio.CharBuffer.flip()Ljava/nio/CharBuffer). In JRE 8, it's Buffer Buffer.flip() (the return type is different).

As of now, references to all system classes (including java.nio.CharBuffer) are skipped.

[suztomo]

For protocolbuffers/protobuf#7827, could Linkage Monitor detect this as part of the PR check, if Linkage Checker is capable to detect the invalid references?

It seems no. If the check builds protobuf-java JAR file with Java 11, then Linkage Monitor could detect it. If it was with Java 8, then the resulting JAR file is fine.

[elharo]

This is a really interesting one. Some take-aways:

1. The compiler doesn't catch all linkage errors, even to classes in the JDK.
2. We should report errors that the project (here Kristen) can't fix so they can understand them and report the errors to the team that can fix them.

[cpovirk]

Nice. It sounds like this will check for compatibility against the JDK that you're using to build? So, if you want to check for compatibility with JDK 8, then you'll need to build with JDK 8? That makes it helpful for detecting problems in your dependencies but not necessarily helpful for detecting problems in your own code (since you'd already detect/avoid these by building with JDK 8 in the first place). That's fine; I had just been initially hoping for an improvement on Animal Sniffer, too.

[suztomo]

compatibility against the JDK that you're using to build?

Yes, it detect the compatibility with the JDK Linkage Checker is running on.

If we enhance the check script to build protobuf-java in Java 11 and running Linkage Checker with Java 8, #1607 should detect the invalid method references.

Memo: script for current check https://github.com/protocolbuffers/protobuf/blob/master/tests.sh#L252 uses Java 8 for both.

[cpovirk]

Thanks.

If you're interested, it is probably possible to detect problems even when building with Java 11: Java 9+ comes bundled with API signatures of not only the current version of Java but also of older versions. See $JAVA_HOME/lib/ct.sym, which contains files like 876/java/nio/ByteBuffer.sig, which I believe is just a classfile that contains the signatures of ByteBuffer under Java 6, 7, and 8.

That still requires that users tell the Linkage Checker which version of Java to check against. I don't suppose it possible to guess a sensible default based on the <source>, <target>, or <release> configuration for plugins like maven-compiler-plugin??

[suztomo]

@cpovirk Thank you for information.