CODEWITHSUNDEEP
javainstrumentationjavassist
Rafael Aguilar
Rafael Aguilar

Reputation: 3269

How to obtain exception message to log it on catch blocks on already existing class with Javassist?

Case:

Try to insert a log method with a String parameter, in this case, with e.getMessage() on any catch Block of existing method.

The following snippets can inject a String.

...
CtMethod log =   CtNewMethod.make("public void log(String s){ " +
                    " System.out.println(\"Hello from injected log method \" + s); " +
                    "}",
                    ct);
ct.addMethod(log);

...
ControlFlow cf = new ControlFlow(m);
Block blocks[] = cf.basicBlocks();
for(Block block : blocks){
    Catcher catchers[] = block.catchers();
    ArrayList<Catcher> catchersList = new ArrayList<Catcher>(Arrays.asList(catchers));
    Collections.reverse(catchersList);
    for (Catcher catcher : catchersList){
        Block catchBlock = catcher.block();
        int pos = catchBlock.position();
    
        CodeIterator itr = m.getMethodInfo().getCodeAttribute().iterator();
    
        Bytecode code = new Bytecode(m.getMethodInfo().getConstPool(), 0, 0);
    
        code.addAload(0);
        code.addLdc("LogParameter");
        code.addInvokevirtual(ct,"log",log.getMethodInfo().getDescriptor());
        code.addGap(2);
        int n = itr.insertAt(pos,code.get());
    
        m.getMethodInfo().rebuildStackMapForME(cp);
    }
}

Problem

What i can't do is to get the e.getMessage() as input parameter to log(String).

Maybe someone could point me in the right direction.

Upvotes: 1

Views: 541

Answers (1)

Rafael Winterhalter
Rafael Winterhalter

Reputation: 43997

When you enter a catch block, you will find the exception lying on the stack. Thus, you can extract the message as follows:

  • duplicate the topmost value of the stack which is the exception such that you can call methods of this object without taking away the value for deeper instructions.

  • extract the message from the exception object by calling getMessage virtually on it.

This way, you now have the exception message lying on top of the stack.

Upvotes: 1

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