javabytecodeinstrumentationjavassistjavaagents

Inserting return before method causes VerifyError without message

I've just tried to change a behavior of a method: replace the return value:

ctMethod.insertBefore("return null;");

Transformation works perfectly, I even write clazz.toBytecode() to a file and IntelliJ IDEA decompiles that .class file successfully and I can see my change there.

However, there is a error that blocks retrasformation of the class, it seems it's thrown after my transformer is executed, it's in native code, and it doesn't have a message:

java.lang.VerifyError
    at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses0(Native Method)
    at java.instrument/sun.instrument.InstrumentationImpl.retransformClasses(InstrumentationImpl.java:167)
    at [place where I call retransformClasses]...

What does it mean? What am I doing wrong?

If I do ctMethod.insertBefore("System.out.println(1);"); instead, it starts to work.

Version of javassist is 3.26.0-GA. The same happens on 3.27.0-GA and on 3.27.0-GA-bugfix-328.

Solution

  • setBody (ctMethod.setBody("return null;");) solves the problem: now I have nulls returned.