Java - Inject java agent in to running jvm

Question

Basically, I am trying to write something that lists every class loaded by the JVM. What I wrote works, but it only works for the jvm it is running on. I crafted a java agent to dynamically inject into another JVM, but then realized I don't actually know how to inject it. How do I actually send this agent into another JVM? Is it possible?

Cultivator

You can use jvm-attach, no JDK needed – Salenasalene

Answer 1

Agents can be injected with HotSpot Attach API.
Run the following snippet with $JAVA_HOME/lib/tools.jar on the class path.

    VirtualMachine vm = VirtualMachine.attach(PID);
    try {
        vm.loadAgent(agentJar);
    } finally {
        vm.detach();
    }

Alternatively you may do this with my command-line jattach utility:

$ jattach PID load instrument false /path/to/agent.jar

Note that in order to support dynamic attach, your Java agent should have agentmain method and Agent-Class property in MANIFEST.MF.

Comestible

I have a small problem with the agents. I am assuming that it's the most standard practice to pack your instrumentation agent separately from your target JVM. When my agentmain() is called I tried to find a JVM class by calling Class.forName("pkg.name") but it is always returning NoClassDefFoundError. I have posted the question here (#46523555. I am slightly confused why the agent, even when attached to the VM, cannot refer to its classes. – Systaltic

Alternativelly you can use jvm-attach which is a wrapper for jattach to make instrumentation programatically, no tools.jar or JDK needed – Salenasalene

Answer 2

Dynamic agents need to declare an agentmain(String, Instrumentation) method which is executed upon attachment within the target VM. You can use the tools.jar dependency which is (until Java 9) only included in a JDK but not a JRE. You can however bundle your agent program with a JDK and attach to JVMs from there.

The biggest pitfall is that the API differs for different VMs; you can however use a library like byte-buddy-agent which contains different implementations for different VMs. An attachment can be done using:

ByteBuddyAgent.attach("my.jar", "my-pid");

This attaches the agent contained in my.jar onto the Java process with id my-id.

Hesitancy

Yes. The OS process id. – Hesitancy

I attempted to do this, and under testing it appears my agentmain method is never being called. – Cultivator

On further inspection, the agentmain method is only being called if I attach to the JVM it is running on. – Cultivator

It is called by the target vm. – Hesitancy

Basically what I am trying to do is get an instance of the Instrumentation. If the target vm calls agentmain, does that make getting an instance of a different JVM's instrumentation impossible? – Cultivator

That is impossible! You could however proxy the interface and run some form of receiver on the other VM that executes the commands. – Hesitancy

Did you use a JDK? – Hesitancy

@RafaelWinterhalter does this handle webapp classloading delegation? When you go inside the agent, classloading is not the same as your webapplication classloading....So you have a delegation mechanism working in there? – Systaltic

@RafaelWinterhalter How can I create 'File' of my agent jar? – Transpire

You have to create a jar, just like for any Java program. – Hesitancy

Answer 3

As far as I understand from the comment, you are interested in something that can inspect remote JVM from within another Java process. If it is the case, then you need a Serviceability Agent rather than Java Agent.

Serviceability Agent API allows you to attach to another JVM process, to read its memory, to reconstruct VM structures and to inspect remote objects in reflection-like manner.

Here is a sample tool to list all classes loaded by a remote JVM:

import sun.jvm.hotspot.runtime.VM;
import sun.jvm.hotspot.tools.Tool;

public class ListRemoteClasses extends Tool {

    public static void main(String[] args) {
        new ListRemoteClasses().execute(args);
    }

    @Override
    public void run() {
        VM.getVM().getSystemDictionary().classesDo(klass -> {
            String className = klass.getName().asString().replace('/', '.');
            System.out.println(className);
        });
    }
}

How to run it:

java -cp $JAVA_HOME/lib/sa-jdi.jar:. ListRemoteClasses PID

Comestible

Answer 4

It's hard to provide assistance without looking at the content that you've written but this is just to notify that there is a class named as Instrumentation interface (public interface Instrumentation) from java.lang.instrument package that provides services needed to instrument Java programming language code.

One such method provided by this class is getInitiatedClasses which returns an array containing all the classes that are loaded.

Look at the documentation here

getInitiatedClasses

Class[] getInitiatedClasses(ClassLoader loader)
Returns an array of all classes for which loader is an initiating loader. If the supplied loader is null, classes initiated by the bootstrap class loader are returned.

Parameters: loader - the loader whose initiated class list will be returned Returns: an array containing all the classes for which loader is an initiating loader, zero-length if there are none

Kado

I am aware of this. The problem is that the jar is already running and I need to get a javaagent to connect with it. – Cultivator