Java LanguageBytecode Modification

What is Bytecode?

Bytecode is the set of instructions used by the JVM. To illustrate this let's take this Hello World program.

public static void main(String[] args){
    System.out.println("Hello World");

This is what it turns into when compiled into bytecode.

public static main([Ljava/lang/String; args)V    
    getstatic java/lang/System out Ljava/io/PrintStream;
    ldc "Hello World"
    invokevirtual java/io/PrintStream print(Ljava/lang/String;)V

What's the logic behind this?

getstatic - Retreives the value of a static field of a class. In this case, the PrintStream "Out" of System.

ldc - Push a constant onto the stack. In this case, the String "Hello World"

invokevirtual - Invokes a method on a loaded reference on the stack and puts the result on the stack. Parameters of the method are also taken from the stack.

Well, there has to be more right?

There are 255 opcodes, but not all of them are implemented yet. A table with all of the current opcodes can be found here: Java bytecode instruction listings.

How can I write / edit bytecode?

There's multiple ways to write and edit bytecode. You can use a compiler, use a library, or use a program.

For writing:

For editing:

I'd like to learn more about bytecode!

There's probably a specific documentation page specificially for bytecode. This page focuses on the modification of bytecode using different libraries and tools.

How to edit jar files with ASM

Firstly the classes from the jar need to be loaded. We'll use three methods for this process:

  • loadClasses(File)
  • readJar(JarFile, JarEntry, Map)
  • getNode(byte[])
Map<String, ClassNode> loadClasses(File jarFile) throws IOException {
    Map<String, ClassNode> classes = new HashMap<String, ClassNode>();
    JarFile jar = new JarFile(jarFile);
    Stream<JarEntry> str =;
    str.forEach(z -> readJar(jar, z, classes));
    return classes;

Map<String, ClassNode> readJar(JarFile jar, JarEntry entry, Map<String, ClassNode> classes) {
    String name = entry.getName();
    try (InputStream jis = jar.getInputStream(entry)){
        if (name.endsWith(".class")) {
            byte[] bytes = IOUtils.toByteArray(jis);
            String cafebabe = String.format("%02X%02X%02X%02X", bytes[0], bytes[1], bytes[2], bytes[3]);
            if (!cafebabe.toLowerCase().equals("cafebabe")) {
                // This class doesn't have a valid magic
                return classes;
            try {
                ClassNode cn = getNode(bytes);
                classes.put(, cn);
            } catch (Exception e) {
    } catch (IOException e) {
    return classes;

ClassNode getNode(byte[] bytes) {
    ClassReader cr = new ClassReader(bytes);
    ClassNode cn = new ClassNode();
    try {
        cr.accept(cn, ClassReader.EXPAND_FRAMES);
    } catch (Exception e) {
    cr = null;
    return cn;

With these methods loading and changing a jar file becomes a simple matter of changing ClassNodes in a map. In this example we will replace all Strings in the jar with capitalized ones using the Tree API.

File jarFile = new File("sample.jar");
Map<String, ClassNode> nodes = loadClasses(jarFile);
// Iterate ClassNodes
for (ClassNode cn : nodes.values()){
    // Iterate methods in class
    for (MethodNode mn : cn.methods){
        // Iterate instructions in method
        for (AbstractInsnNode ain : mn.instructions.toArray()){
            // If the instruction is loading a constant value 
            if (ain.getOpcode() == Opcodes.LDC){
                // Cast current instruction to Ldc
                // If the constant is a string then capitalize it.
                LdcInsnNode ldc = (LdcInsnNode) ain;
                if (ldc.cst instanceof String){
                    ldc.cst = ldc.cst.toString().toUpperCase();

Now that all of the ClassNode's strings have been modified we need to save the changes. In order to save the changes and have a working output a few things have to be done:

  • Export ClassNodes to bytes
  • Load non-class jar entries (Ex: / other binary resources in jar) as bytes
  • Save all bytes to a new jar

From the last portion above, we'll create three methods.

  • processNodes(Map<String, ClassNode> nodes)
  • loadNonClasses(File jarFile)
  • saveAsJar(Map<String, byte[]> outBytes, String fileName)


Map<String, byte[]> out = process(nodes, new HashMap<String, MappedClass>());
saveAsJar(out, "sample-edit.jar");

The methods used:

static Map<String, byte[]> processNodes(Map<String, ClassNode> nodes, Map<String, MappedClass> mappings) {
    Map<String, byte[]> out = new HashMap<String, byte[]>();
    // Iterate nodes and add them to the map of <Class names , Class bytes>
    // Using Compute_Frames ensures that stack-frames will be re-calculated automatically
    for (ClassNode cn : nodes.values()) {
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        out.put(mappings.containsKey( ? mappings.get( :, cw.toByteArray());
    return out;

static Map<String, byte[]> loadNonClasses(File jarFile) throws IOException {
    Map<String, byte[]> entries = new HashMap<String, byte[]>();
    ZipInputStream jis = new ZipInputStream(new FileInputStream(jarFile));
    ZipEntry entry;
    // Iterate all entries
    while ((entry = jis.getNextEntry()) != null) {
        try {
            String name = entry.getName();
            if (!name.endsWith(".class") && !entry.isDirectory()) {
                // Apache Commons - byte[] toByteArray(InputStream input)
                // Add each entry to the map <Entry name , Entry bytes>
                byte[] bytes = IOUtils.toByteArray(jis);
                entries.put(name, bytes);
        } catch (Exception e) {
        } finally {
    return entries;

static void saveAsJar(Map<String, byte[]> outBytes, String fileName) {
    try {
        // Create jar output stream
        JarOutputStream out = new JarOutputStream(new FileOutputStream(fileName));
        // For each entry in the map, save the bytes
        for (String entry : outBytes.keySet()) {
            // Appent class names to class entries
            String ext = entry.contains(".") ? "" : ".class";
            out.putNextEntry(new ZipEntry(entry + ext));
    } catch (IOException e) {

That's it. All the changes will be saved to "sample-edit.jar".

How to load a ClassNode as a Class

 * Load a class by from a ClassNode
 * @param cn
 *            ClassNode to load
 * @return
public static Class<?> load(ClassNode cn) {
    ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
    return new ClassDefiner(ClassLoader.getSystemClassLoader()).get("/", "."), cw.toByteArray());

 * Classloader that loads a class from bytes.
static class ClassDefiner extends ClassLoader {
    public ClassDefiner(ClassLoader parent) {

    public Class<?> get(String name, byte[] bytes) {
        Class<?> c = defineClass(name, bytes, 0, bytes.length);
        return c;

How to rename classes in a jar file

public static void main(String[] args) throws Exception {
    File jarFile = new File("Input.jar");
    Map<String, ClassNode> nodes = JarUtils.loadClasses(jarFile);
    Map<String, byte[]> out = JarUtils.loadNonClassEntries(jarFile);
    Map<String, String> mappings = new HashMap<String, String>();
    mappings.put("me/example/ExampleClass", "me/example/ExampleRenamed");
    out.putAll(process(nodes, mappings));
    JarUtils.saveAsJar(out, "Input-new.jar");

static Map<String, byte[]> process(Map<String, ClassNode> nodes, Map<String, String> mappings) {
    Map<String, byte[]> out = new HashMap<String, byte[]>();
    Remapper mapper = new SimpleRemapper(mappings);
    for (ClassNode cn : nodes.values()) {
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES);
        ClassVisitor remapper = new ClassRemapper(cw, mapper);
        out.put(mappings.containsKey( ? mappings.get( :, cw.toByteArray());
    return out;

SimpleRemapper is an existing class in the ASM library. However it only allows for class names to be changed. If you wish to rename fields and methods you should create your own implemenation of the Remapper class.

Javassist Basic

Javassist is a bytecode instrumentation library that allows you to modify bytecode injecting Java code that will be converted to bytecode by Javassist and added to the instrumented class/method at runtime.

Lets write the first transformer that actually take an hypothetical class "" and add to the instructions of each method a log call.

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
public class DynamicTransformer implements ClassFileTransformer {
    public byte[] transform(ClassLoader loader, String className, Class classBeingRedefined,
        ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        byte[] byteCode = classfileBuffer;
        // into the transformer will arrive every class loaded so we filter 
        // to match only what we need
        if (className.equals("com/my/to/be/instrumented/MyClass")) {
            try {
                // retrive default Javassist class pool
                ClassPool cp = ClassPool.getDefault();
                // get from the class pool our class with this qualified name
                CtClass cc = cp.get("");
                // get all the methods of the retrieved class
                CtMethod[] methods = cc.getDeclaredMethods()
                for(CtMethod meth : methods) {
                    // The instrumentation code to be returned and injected
                    final StringBuffer buffer = new StringBuffer();
                    String name = meth.getName();
                    // just print into the buffer a log for example
                    buffer.append("System.out.println(\"Method " + name + " executed\" );");
                // create the byteclode of the class
                byteCode = cc.toBytecode();
                // remove the CtClass from the ClassPool
            } catch (Exception ex) {
        return byteCode;

Now in order to use this transformer (so that our JVM will call the method transform on each class at load time) we need to add this instrumentor this with an agent:

import java.lang.instrument.Instrumentation;
public class EasyAgent {
    public static void premain(String agentArgs, Instrumentation inst) {
        // registers the transformer
        inst.addTransformer(new DynamicTransformer());

Last step to start our first instrumentor experiment is to actually register this agent class to the JVM machine execution. The easiest way to actually do it is to register it with an option into the shell command:

java -javaagent:myAgent.jar MyJavaApplication

As we can see the agent/transformer project is added as a jar to the execution of any application named MyJavaApplication that is supposed to contain a class named "" to actually execute our injected code.