1. Introduction

In this tutorial, we’re going to talk about Java Instrumentation API. It provides the ability to add byte-code to existing compiled Java classes.

We’ll also talk about java agents and how we use them to instrument our code.

2. Setup

Throughout the article, we’ll build an app using instrumentation.

Our application will consist of two modules:

  1. An ATM app that allows us to withdraw money
  2. And a Java agent that will allow us to measure the performance of our ATM by measuring the time invested spending money

The Java agent will modify the ATM byte-code allowing us to measure withdrawal time without having to modify the ATM app.

Our project will have the following structure:


Before getting too much into the details of instrumentation, let’s see what a java agent is.

3. What Is a Java Agent

In general, a java agent is just a specially crafted jar file. It utilizes the Instrumentation API that the JVM provides to alter existing byte-code that is loaded in a JVM.

For an agent to work, we need to define two methods:

  • premain – will statically load the agent using -javaagent parameter at JVM startup
  • agentmain – will dynamically load the agent into the JVM using the Java Attach API

An interesting concept to keep in mind is that a JVM implementation, like Oracle, OpenJDK, and others, can provide a mechanism to start agents dynamically, but it is not a requirement.

First, let’s see how we’d use an existing Java agent.

After that, we’ll look at how we can create one from scratch to add the functionality we need in our byte-code.

4. Loading a Java Agent

To be able to use the Java agent, we must first load it.

We have two types of load:

  • static – makes use of the premain to load the agent using -javaagent option
  • dynamic – makes use of the agentmain to load the agent into the JVM using the Java Attach API

Next, we’ll take a look at each type of load and explain how it works.

4.1. Static Load

Loading a Java agent at application startup is called static load. Static load modifies the byte-code at startup time before any code is executed.

Keep in mind that the static load uses the premain method, which will run before any application code runs, to get it running we can execute:

java -javaagent:agent.jar -jar application.jar

It’s important to note that we should always put the –javaagent parameter before the –jar parameter.

Below are the logs for our command:

22:24:39.296 [main] INFO - [Agent] In premain method
22:24:39.300 [main] INFO - [Agent] Transforming class MyAtm
22:24:39.407 [main] INFO - [Application] Starting ATM application
22:24:41.409 [main] INFO - [Application] Successful Withdrawal of [7] units!
22:24:41.410 [main] INFO - [Application] Withdrawal operation completed in:2 seconds!
22:24:53.411 [main] INFO - [Application] Successful Withdrawal of [8] units!
22:24:53.411 [main] INFO - [Application] Withdrawal operation completed in:2 seconds!

We can see when the premain method ran and when MyAtm class was transformed. We also see the two ATM withdrawal transactions logs which contain the time it took each operation to complete.

Remember that in our original application we didn’t have this time of completion for a transaction, it was added by our Java agent.

4.2. Dynamic Load

The procedure of loading a Java agent into an already running JVM is called dynamic load. The agent is attached using the Java Attach API.

A more complex scenario is when we already have our ATM application running in production and we want to add the total time of transactions dynamically without downtime for our application.

Let’s write a small piece of code to do just that and we’ll call this class AgentLoader. For simplicity, we’ll put this class in the application jar file. So our application jar file can both start our application, and attach our agent to the ATM application:

VirtualMachine jvm = VirtualMachine.attach(jvmPid);

Now that we have our AgentLoader, we start our application making sure that in the ten-second pause between transactions, we’ll attach our Java agent dynamically using the AgentLoader.

Let’s also add the glue that will allow us to either start the application or load the agent.

We’ll call this class Launcher and it will be our main jar file class:

public class Launcher {
    public static void main(String[] args) throws Exception {
        if(args[0].equals("StartMyAtmApplication")) {
            new MyAtmApplication().run(args);
        } else if(args[0].equals("LoadAgent")) {
            new AgentLoader().run(args);

Starting the Application

java -jar application.jar StartMyAtmApplication
22:44:21.154 [main] INFO - [Application] Starting ATM application
22:44:23.157 [main] INFO - [Application] Successful Withdrawal of [7] units!

Attaching Java Agent

After the first operation, we attach the java agent to our JVM:

java -jar application.jar LoadAgent
22:44:27.022 [main] INFO - Attaching to target JVM with PID: 6575
22:44:27.306 [main] INFO - Attached to target JVM and loaded Java agent successfully

Check Application Logs

Now that we attached our agent to the JVM we’ll see that we have the total completion time for the second ATM withdrawal operation.

This means that we added our functionality on the fly, while our application was running:

22:44:27.229 [Attach Listener] INFO - [Agent] In agentmain method
22:44:27.230 [Attach Listener] INFO - [Agent] Transforming class MyAtm
22:44:33.157 [main] INFO - [Application] Successful Withdrawal of [8] units!
22:44:33.157 [main] INFO - [Application] Withdrawal operation completed in:2 seconds!

5. Creating a Java Agent

After learning how to use an agent, let’s see how we can create one. We’ll look at how to use Javassist to change byte-code and we’ll combine this with some instrumentation API methods.

Since a java agent makes use of the Java Instrumentation API, before getting too deep into creating our agent, let’s see some of the most used methods in this API and a short description of what they do:

  • addTransformer – adds a transformer to the instrumentation engine
  • getAllLoadedClasses – returns an array of all classes currently loaded by the JVM
  • retransformClasses – facilitates the instrumentation of already loaded classes by adding byte-code
  • removeTransformer – unregisters the supplied transformer
  • redefineClasses – redefine the supplied set of classes using the supplied class files, meaning that the class will be fully replaced, not modified as with retransformClasses

5.1. Create the Premain and Agentmain Methods

We know that every Java agent needs at least one of the premain or agentmain methods. The latter is used for dynamic load, while the former is used to statically load a java agent into a JVM.

Let’s define both of them in our agent so that we’re able to load this agent both statically and dynamically:

public static void premain(
  String agentArgs, Instrumentation inst) {
    LOGGER.info("[Agent] In premain method");
    String className = "com.baeldung.instrumentation.application.MyAtm";
public static void agentmain(
  String agentArgs, Instrumentation inst) {
    LOGGER.info("[Agent] In agentmain method");
    String className = "com.baeldung.instrumentation.application.MyAtm";

In each method, we declare the class that we want to change and then dig down to transform that class using the transformClass method.

Below is the code for the transformClass method that we defined to help us transform MyAtm class.

In this method, we find the class we want to transform and using the transform method. Also, we add the transformer to the instrumentation engine:

private static void transformClass(
  String className, Instrumentation instrumentation) {
    Class<?> targetCls = null;
    ClassLoader targetClassLoader = null;
    // see if we can get the class using forName
    try {
        targetCls = Class.forName(className);
        targetClassLoader = targetCls.getClassLoader();
        transform(targetCls, targetClassLoader, instrumentation);
    } catch (Exception ex) {
        LOGGER.error("Class [{}] not found with Class.forName");
    // otherwise iterate all loaded classes and find what we want
    for(Class<?> clazz: instrumentation.getAllLoadedClasses()) {
        if(clazz.getName().equals(className)) {
            targetCls = clazz;
            targetClassLoader = targetCls.getClassLoader();
            transform(targetCls, targetClassLoader, instrumentation);
    throw new RuntimeException(
      "Failed to find class [" + className + "]");

private static void transform(
  Class<?> clazz, 
  ClassLoader classLoader,
  Instrumentation instrumentation) {
    AtmTransformer dt = new AtmTransformer(
      clazz.getName(), classLoader);
    instrumentation.addTransformer(dt, true);
    try {
    } catch (Exception ex) {
        throw new RuntimeException(
          "Transform failed for: [" + clazz.getName() + "]", ex);

With this out of the way, let’s define the transformer for MyAtm class.

5.2. Defining Our Transformer

A class transformer must implement ClassFileTransformer and implement the transform method.

We’ll use Javassist to add byte-code to MyAtm class and add a log with the total ATW withdrawal transaction time:

public class AtmTransformer implements ClassFileTransformer {
    public byte[] transform(
      ClassLoader loader, 
      String className, 
      Class<?> classBeingRedefined, 
      ProtectionDomain protectionDomain, 
      byte[] classfileBuffer) {
        byte[] byteCode = classfileBuffer;
        String finalTargetClassName = this.targetClassName
          .replaceAll("\\.", "/"); 
        if (!className.equals(finalTargetClassName)) {
            return byteCode;

        if (className.equals(finalTargetClassName) 
              && loader.equals(targetClassLoader)) {
            LOGGER.info("[Agent] Transforming class MyAtm");
            try {
                ClassPool cp = ClassPool.getDefault();
                CtClass cc = cp.get(targetClassName);
                CtMethod m = cc.getDeclaredMethod(
                  "startTime", CtClass.longType);
                  "startTime = System.currentTimeMillis();");

                StringBuilder endBlock = new StringBuilder();

                m.addLocalVariable("endTime", CtClass.longType);
                m.addLocalVariable("opTime", CtClass.longType);
                  "endTime = System.currentTimeMillis();");
                  "opTime = (endTime-startTime)/1000;");

                  "LOGGER.info(\"[Application] Withdrawal operation completed in:" +
                                "\" + opTime + \" seconds!\");");


                byteCode = cc.toBytecode();
            } catch (NotFoundException | CannotCompileException | IOException e) {
                LOGGER.error("Exception", e);
        return byteCode;

5.3. Creating an Agent Manifest File

Finally, in order to get a working Java agent, we’ll need a manifest file with a couple of attributes.

Hence, we can find the full list of manifest attributes in the Instrumentation Package official documentation.

In the final Java agent jar file, we will add the following lines to the manifest file:

Agent-Class: com.baeldung.instrumentation.agent.MyInstrumentationAgent
Can-Redefine-Classes: true
Can-Retransform-Classes: true
Premain-Class: com.baeldung.instrumentation.agent.MyInstrumentationAgent

Our Java instrumentation agent is now complete. To run it, please refer to Loading a Java Agent section of this article.

6. Conclusion

In this article, we talked about the Java Instrumentation API. We looked at how to load a Java agent into a JVM both statically and dynamically.

We also looked at how we would go about creating our own Java agent from scratch.

As always, the full implementation of the example can be found over on Github.

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