BCEL (Byte Code Engineering Library) gives a possibility to analyze, create and modify Java class files. It gives very nice API for manipulating inside bytecode. Here is an example of using BCEL. It adds instructions relevant to System.out.println(“About to call: METHOD_NAME METHOD_SIGNATURE”) before invocation of any method in the class.

import java.io.IOException;
import java.util.Iterator;

import org.apache.bcel.Repository;
import org.apache.bcel.classfile.ClassFormatException;
import org.apache.bcel.classfile.Method;
import org.apache.bcel.generic.*;

/**
 * Instruments byte-code by adding: 
 * System.out.println("About to call: METHOD_NAME METHOD_SIGNATURE");
 * 
 * before method invocation.
 *
 * @author Radoslaw Urbas
 * @since Oct 27, 2007
 *
 */
public class Transform
{

    private ClassGen classGen = null;

    /**
     *
     * @param className name of class to instrument
     * @throws ClassNotFoundException
     * @throws ClassFormatException
     * @throws IOException
     */
    public Transform(String className) throws ClassNotFoundException,
            ClassFormatException, IOException
    {
        this.classGen = new ClassGen(Repository.lookupClass(className));
    }

    /**
     * Instruments the class by displaying method information before invocation.
     *
     * @return instrumented ClassGen object
     */
    private ClassGen instrument()
    {
        ConstantPoolGen pgen = classGen.getConstantPool();
        String cname = classGen.getClassName();
        for (Method method : classGen.getMethods())
        {
            MethodGen methgen = instrumentMethod(pgen, cname, method);
            classGen.replaceMethod(method, methgen.getMethod());
        }
        return classGen;
    }

    /**
     * Instruments method by displaying information before method invocation.
     *
     * @param constantPoolGen constant pool for class containing method to
     *            instrument
     * @param className name of the class containing method to instrument
     * @param method method to instrument
     * @return instrumented MethodGen object
     */
    private MethodGen instrumentMethod(ConstantPoolGen constantPoolGen,
            String className, Method method)
    {
        InstructionFactory instructionFactory = new InstructionFactory(classGen);
        MethodGen methgen = new MethodGen(method, className, constantPoolGen);
        InstructionList originalInstructionList = methgen.getInstructionList();
        Iterator instructionIterator = originalInstructionList.iterator();
        while (instructionIterator.hasNext())
        {
            InstructionHandle ih = (InstructionHandle) instructionIterator
                    .next();
            if (ih.getInstruction() instanceof InvokeInstruction)
            {
                originalInstructionList.insert(ih, instructionFactory
                        .createPrintln(getMessage((InvokeInstruction) ih
                                .getInstruction(), constantPoolGen)));
            }
        }
        methgen.setMaxStack();
        return methgen;
    }

    /**
     *
     * @param invokeInstruction object representing invoke instruction
     * @param constantPoolGen
     * @return message based on method name and signature
     */
    private String getMessage(InvokeInstruction invokeInstruction,
            ConstantPoolGen constantPoolGen)
    {
        String text = "About to call: "
                + invokeInstruction.getMethodName(constantPoolGen)
                + invokeInstruction.getSignature(constantPoolGen);
        return text;
    }

    /**
     *
     * @param args requires one parameter: class name. Class should be available
     *            in directory ./classes
     * @throws ClassNotFoundException
     * @throws IOException
     */
    public static void main(String[] args) throws ClassNotFoundException,
            IOException
    {
        String className = args[0];
        Transform transform = new Transform(className);
        ClassGen classGen = transform.instrument();
        classGen.getJavaClass().dump(className + ".class");
    }
}

Homepage: http://jakarta.apache.org/bcel/