ASM is an all purpose Java bytecode manipulation and analysis framework. It can be used to modify existing classes or to dynamically generate classes, directly in binary form. ASM provides some common bytecode transformations and analysis algorithms from which custom complex transformations and code analysis tools can be built. ASM offers similar functionality as other Java bytecode frameworks, but is focused on performance. Because it was designed and implemented to be as small and as fast as possible, it is well suited for use in dynamic systems (but can of course be used in a static way too, e.g. in compilers).

ASM is used in many projects, including:

ASM API Basics

The ASM API provides two styles of interacting with Java classes for transformation and generation: event-based and tree-based.


Package Description
org.objectweb.asm Provides a small and fast bytecode manipulation framework.
org.objectweb.asm.commons Provides some useful class and method adapters.
org.objectweb.asm.signature Provides support for type signatures.
org.objectweb.asm.tree Provides an ASM visitor that constructs a tree representation of the classes it visits.
org.objectweb.asm.tree.analysis Provides a framework for static code analysis based on the asm.tree package.
org.objectweb.asm.util Provides ASM visitors that can be useful for programming and debugging purposes.

Event-based API

This API is heavily based on the Visitor pattern and is similar in feel to the SAX parsing model of processing XML documents. It is comprised, at its core, of the following components:

  • ClassReader – helps to read class files and is the beginning of transforming a class
  • ClassVisitor – provides the methods used to transform the class after reading the raw class files
  • ClassWriter – is used to output the final product of the class transformation

The ClassVisitor methods in the event-based API are called in the following order:

( visitAnnotation | visitAttribute )*
( visitInnerClass | visitField | visitMethod )*

Tree-based API

This API is a more object-oriented API and is analogous to the JAXB model of processing XML documents.

It’s still based on the event-based API, but it introduces the ClassNode root class. This class serves as the entry point into the class structure.

Working With the Event-based ASM API

the ClassVisitor class contains all the necessary visitor methods to create or modify all the parts of a class.

Working With Fields

public FieldVisitor visitField(int access, String name, String desc,
        String signature, Object value) {
    if (name.equals(fieldName)) {
        isFieldPresent = true;
    return tracer.visitField(access, name, desc, signature, value);

public void visitEnd() {
    if (!isFieldPresent) {
        FieldVisitor fv = tracer.visitField(access, fieldName,
                Type.BOOLEAN_TYPE.toString(), null, null);
        if (fv != null) {

The visitEnd method is the last method called in order of the visitor methods. This is the recommended position to carry out the field insertion logic.

It’s important to be sure that all the ASM components used come from the org.objectweb.asm package

public byte[] addField() {
    addFieldAdapter = new AddFieldAdapter("aNewBooleanField",
            org.objectweb.asm.Opcodes.ACC_PUBLIC, writer);
    reader.accept(addFieldAdapter, 0);
    return writer.toByteArray();

Working With Methods

For most practical uses, however, we can either modify an existing method to make it more accessible (perhaps make it public so that it can be overridden or overloaded) or modify a class to make it extensible.

public MethodVisitor visitMethod(int access, String name, String desc,
        String signature, String[] exceptions) {
    if (name.equals("toUnsignedString0")) {
        return cv.visitMethod(ACC_PUBLIC + ACC_STATIC, name, desc,
                signature, exceptions);
    return cv.visitMethod(access, name, desc, signature, exceptions);
public byte[] publicizeMethod() {
    pubMethAdapter = new PublicizeMethodAdapter(writer);
    reader.accept(pubMethAdapter, 0);
    return writer.toByteArray();

Working With Classes

Along the same lines as modifying methods, we modify classes by intercepting the appropriate visitor method. In this case, we intercept visit, which is the very first method in the visitor hierarchy:

public void visit(int version, int access, String name, String signature,
        String superName, String[] interfaces) {
    String[] holding = new String[interfaces.length + 1];
    holding[holding.length - 1] = CLONEABLE_INTERFACE;
    System.arraycopy(interfaces, 0, holding, 0, interfaces.length);
    tracer.visit(V1_5, access, name, signature, superName, holding);

Using the Modified Class

In addition to simply writing the output of writer.toByteArray to disk as a class file, there are some other ways to interact with our customized Integer class.

Using the TraceClassVisitor

The ASM library provides the TraceClassVisitor utility class that we’ll use to introspect the modified class. Thus we can confirm that our changes have happened.

Because the TraceClassVisitor is a ClassVisitor, we can use it as a drop-in replacement for a standard ClassVisitor

public MethodVisitor visitMethod(int access, String name, String desc,
        String signature, String[] exceptions) {
    if (name.equals("toUnsignedString0")) {
        return tracer.visitMethod(ACC_PUBLIC + ACC_STATIC, name, desc,
                signature, exceptions);
    return tracer.visitMethod(access, name, desc, signature, exceptions);

public void visitEnd() {

All the visiting will now be done with our tracer, which then can print out the content of the transformed class, showing any modifications we’ve made to it.

Using Java Instrumentation

This is a more elegant solution that allows us to work with the JVM at a closer level via Instrumentation.

To instrument the java.lang.Integer class, we write an agent that will be configured as a command line parameter with the JVM. The agent requires two components:

  • A class that implements a method named premain
  • An implementation of ClassFileTransformer in which we’ll conditionally supply the modified version of our class

Building and packaging our code so far produces the jar that we can load as an agent. To use our customized Integer class in a hypothetical YourClass.class:

java -javaagent:jdk8.jar -cp . t5750.asm.instrument.PremainTest