Java Agent Demo

Goal

simple Hello World example, modify the output string 'Hello World! 👀' to 'Bye, Bye! 🖐️' using javassist library.

Prerequisites

openjdk : 11.0.11

Demo

# build helloworld.jar
cd helloworldapp
make jar

• prints 'Hello World! 👀' to console

# run helloworld.jar
cd agent
make

• prints 'Bye, Bye! 🖐️' to console

Structure

├── README.md
├── agent
│   ├── Makefile
│   ├── lib
│   │   └── javassist.jar
│   └── src
│       ├── manifest.mf
│       └── myagent
│           ├── MyAgent.java
│           └── MyTransformer.java
└── helloworldapp
    ├── Makefile
    └── src
        ├── HelloWorld.java
        └── manifest.mf

Instructions

Three jar files are needed for demo

• javassist.jar
• helloworld.jar
• myagent.jarlibrary.

javassist.jar

wget https://github.com/jboss-javassist/javassist/releases/download/rel_3_29_2_ga/javassist.jar

helloworld.jar

• simple java application, print 'Hello World!' to console
• specify the 'Main-class' header in manifest.mf, in provided example, it is theapp.HelloWorld

myagent.jar

• Consists of a entry point class and a transformer class
• Create MyTransformer class implements the interface java.lang.instrument.ClassFileTransformer
• MyTransformer overrides the method transform to modify the bytecode of HelloWorld.class
• Create ClassPool object to load the class file
• Read the class file into a CtClass object using ClassPool and a ByteArrayInputStream created from the classfileBuffer
• Obtain the CtMethod representing the main method from the CtClass object
• Convert the modified CtClass back to bytecode using cc.toBytecode()
• Update the classfileBuffer with the modified bytecode
• specify the 'Premain-Class' header in manifest.mf, in provided example, it is myagent.MyAgent