／var／logmarcus chiu

Java - Java Agent (Loading Agent)

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see: Java - Instrumentation API & Java Agents

to use a Java Agent, you must load in 1 of the following ways:

  • static load - makes use of the premain to load the agent using -javaagent option
  • dynamic load - makes use of the agentmain to load the agent into the JVM using the Java Attach API

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:java-agent-here.jar -jar application.jar

IMPORTANT always put the –javaagent parameter before the –jar parameter

Dynamic Load

create an AgentLoader class that dynamically loads our agent to the application

VirtualMachine jvm = VirtualMachine.attach(jvmPid);
jvm.loadAgent(agentFile.getAbsolutePath());
jvm.detach();

In the application main:

public class Launcher {
    public static void main(String[] args) throws Exception {
        if(args[0].equals("StartMyAtmApplication")) {
            new MyAtmApplication().run(args);
        } else if(args[0].equals("LoadAgent")) {
            new AgentLoader().run(args);
        }
    }
}

Starting the Application:

  • java -jar application.jar StartMyAtmApplication

Attaching Java Agent (after the first operation, we attach the java agent to our JVM):

  • java -jar application.jar LoadAgent