Class Loader Magic: How to Write a Java Agent

04 Dec
By Thomas Krieger

For vmlens, a lightweight Java race condition catcher, we are using a Java agent to trace field accesses. Here are the lessons we learned implementing such an agent.

The Start

Create an agent class with a “static public static void premain(String args, Instrumentation inst)” method. Put this class into a jar file with a manifest pointing to the Agent class. The premain method will be called before the main method of the application.

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.9.2
Created-By: 1.8.0_05-b13 (Oracle Corporation)
Built-By: Thomas Krieger
Implementation-Vendor: Anarsoft
Implementation-Title: VMLens Agent
Can-Retransform-Classes: true
Premain-Class: com.anarsoft.trace.agent.Agent
Boot-Class-Path: agent_bootstrap.jar

The MANIFEST.MF file from vmlens.

Class Loader Magic Part 1

The agent class will be loaded by the system class loader, but we have to avoid version conflicts between the classes used by the agent and the application. In particular, the frameworks used in the agent should not be visible to the application classes. So, we use a dedicated URLClassLoader to load all other agent classes:

// remember the currently used classloader
ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();

// Create and set a special URLClassLoader
URLClassLoader classloader = new URLClassLoader(urlList.toArray(new URL[]{}) , null );

// Load and execute the agent
String agentName = "com.anarsoft.trace.agent.runtime.AgentRuntimeImpl";
AgentRuntime agentRuntime  =  (AgentRuntime) classloader.loadClass(agentName).newInstance();

// reset the classloader

Class Loader Magic Part 2

Now we use asm to add our static callbacks methods when a field is accessed. To make sure that the classes are visible in every other class, they have to be loaded by the bootstrap classloader. To do this, they have to be in a Java package and the jar containing them has to be in the boot class path.

package java.anarsoft.trace.agent.bootstrap.callback;

public class FieldAccessCallback {

public static  void getStaticField(int field,int methodId) {


A callback class from vmlens. It has to be in the Java package namespace to be visible in all classes.

Boot-Class-Path: agent_bootstrap.jar

The boot class path entry in the MANIFEST.MF file from vmlens.

VMLens, a lightweight Java race condition catcher, is built as a Java agent. We know that writing Java agents can be a tricky business. So, if you have any questions, just ask them in a comment below.

Java Advent CalendarThis post is part of the Java Advent Calendar, and is licensed under the Creative Commons 3.0 Attribution license. If you like it, please spread the word by sharing, Tweeting, FB, G+, etc!



Class Loader Magic: How to Write a Java Agent

| Java Language| 1,684 views | 0 Comments
About The Author

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

Read previous post:
Use Docker to Create a Multi-Container App Deployed on Multiple Hosts

Docker Machine, Swarm & Compose for Multi-Container and Multi-Host Applications with Couchbase & WildFly This blog will explain how to create...