Example of how to instrument the java class files on class loading. You can change(instrument) the byte code without changing the java source file and recompiling.
Package the agent jar file.
mvn clean package
Find the packaged jar file in target directory and add it to the java command line options. For example.
java -javaagent:/Users/liuzhengyang/Code/opensource/github/javaagent/target/javaagent-1.0-SNAPSHOT-jar-with-dependencies.jar Test
Most processing happens in com.lzy.javaagent.AgentMain, you can change it and add your own logic.
Have Fun!