java - How safe is it to use -XX:-UseSplitVerifier?

Question

There are known compatibility issues with JDK7 compiled code using instrumentation. As for http://www.oracle.com/technetwork/java/javase/compatibility-417013.html

Classfiles with version number 51 are exclusively verified using the type-checking verifier, and thus the methods must have StackMapTable attributes when appropriate. For classfiles with version 50, the Hotspot JVM would (and continues to) failover to the type-inferencing verifier if the stackmaps in the file were missing or incorrect. This failover behavior does not occur for classfiles with version 51 (the default version for Java SE 7). Any tool that modifies bytecode in a version 51 classfile must be sure to update the stackmap information to be consistent with the bytecode in order to pass verification.

The solution is to use -XX:-UseSplitVerifier as summarised here: https://community.oracle.com/blogs/fabriziogiudici/2012/05/07/understanding-subtle-new-behaviours-jdk-7

How safe it is? I suppose Oracle has put this check in for a reason. If I don't use it, I may be risking some other issues.

What can be consequences of using -XX:-UseSplitVerifier?

Thanks,

Piotr.

Answer 1

In short, it's perfectly safe.

Since Java 6, Oracle's compiler has made class files with a StackMapTable. The basic idea is that the compiler can explicitly specify what the type of an object is, instead of making the runtime do it. That provides a tiny speedup in the runtime, in exchange for some extra time during compile and some complexity in the compiled class file (the aforementioned StackMapTable).

As an experimental feature, it was not enabled by default in the Java 6 compiler. The runtime defaults to verifying the object types itself if no StackMapTable exists.

Until Java 7. Oracle made it mandatory: the compiler generates them, and the runtime verifies them. It still uses the old verifier if the StackMapTable isn't there... but only on class files from Java 6 or earlier (version 50). Java 7 class files (version 51) are required to use the StackMapTable, and so the runtime won't cut them the same slack.

That's only a problem if your classfiles were generated without a StackMapTable. For instance, if you used a non-Oracle JVM. Or if you messed with bytecode afterwards -- like instrumenting it for use with a debugger, optimizer, or code coverage analyzer.

But you can get around it! Oracle's JVM provides the -XX:+UseSplitVerifier to force the runtime to fallback to the old type verifier. It doesn't care about StackMapTable.

In practice, the hoped-for optimization in runtime speed and efficiency hasn't materialized: if it exists, it hasn't been enough for anyone to notice. As the new type verifier doesn't provide any new features (just the optimization), it's perfectly safe to shut it off.

Oracle's explanation is at http://www.oracle.com/technetwork/java/javase/compatibility-417013.html if you search for JSR 202.