Java-agent

Simple example of using java instrumentation

Package that provides services to allow instrumentation is java.lang.instrument

The mechanism provides the possibility of modification of byte-code at runtime.

Main components

myapp

Simple hello world application

agent

Application which will modify origin code of myapp.

Contains:

• MyAgent class with premain() method
• MyTransformer class which implements ClassFileTransformer interface and its method transform()

What's going on

The agent connects to the application, integrates into its method, and adds text output after executing the original method

What we need

myapp

• maven-jar-plugin
  • settings for manifest file

agent

• maven-assembly-plugin
  • settings for building jar-with-dependencies
  • settings for premain class in manifest
• javassist dependency (perform operations on the bytecode )

Building results

Two jars with MANIFEST.MF files

myapp

agent

Run

Some hints

If it is needed to get some arguments in premain method (name of the certain class or method which will be updated by agent):