Java Instrumentation


Java Instrumentation API provides the ability to add byte-code to existing compiled Java classes.

Java Instrumentation will give a demonstration of how powerful Java is. Most importantly, this power can be realized by a developer for innovative means.

  • For example using Java instrumentation, we can access a class that is loaded by the Java classloader from the JVM and modify its bytecode by inserting our custom code, all these done at runtime.

  • Don’t worry about security, these are governed by the same security context applicable for Java classes and respective classloaders.

Key Components

  • Agent – is a jar file containing agent and transformer class files.

  • Agent Class – A java class file, containing a method named premain.

  • Manifest – file containing the Premain-Class property.

  • Transformer – A Java class file implementing the interface ClassFileTransformer.

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

Instrumentation Activity Sequence


Loading a Java Agent

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

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

Launcher -> args: StartMyAtmApplication 2 7 8

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.

VirtualMachine jvm = VirtualMachine.attach(jvmPid);
  • Starting the Application: java -jar application.jar StartMyAtmApplication

  • Attaching Java Agent: java -jar application.jar LoadAgent

    • Launcher -> args: LoadAgent

  • Check Application Logs


  • 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

Creating a Java Agent

  1. Create the premain and agentmain Methods: Premain-transformClass(String className, Instrumentation instrumentation)

  2. Defining Our ClassFileTransformer: AtmTransformer

  3. Creating an Agent Manifest File

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