(Originally posted here)

I took my very first step with Javassist by writing a SSCCE that tries to modify a field access.  It compiles and runs without error, but the output is not what I expected.

package test;

import javassist.CannotCompileException;

import javassist.ClassPool;

import javassist.CtClass;

import javassist.CtMethod;

import javassist.NotFoundException;

import javassist.expr.ExprEditor;

import javassist.expr.FieldAccess;

public class Test1 {

    public static void main(String[] args) {

        try {

            // instrument the class

            final CtClass ctClass = ClassPool.getDefault().get(Foo.class.getName());

            final CtMethod ctMethod = ctClass.getDeclaredMethod("setValue");

            ctMethod.instrument(new ExprEditor() {

                @Override

                public void edit(FieldAccess f) throws CannotCompileException {

                    //System.out.printf("instrumenting field access in %s, line %d\n", f.getFileName(), f.getLineNumber());

                    f.replace("System.out.println(\"fnord\");");

                }

            });

            // test the instrumentation

            Foo foo = new Foo();

            foo.setValue(42);

            System.out.println(foo.getValue());

        }

        catch (NotFoundException | CannotCompileException e) {

            e.printStackTrace();

        }

    }

}

package test;

public class Foo {

    private int value;

    public int getValue() {

        return value;

    }

    public void setValue(int value) {

        this.value = value;

    }

}

I expected the output to be:

fnord

0

...because I expected that the field assignment would be replaced by the code that prints "fnord", and the value would remain uninitialized.  But instead, the output is:

42

The code that attempts to replace the field assignment definitely runs and operates on the correct line, as I can see if I uncomment the print statement there.  What am I doing wrong?