A. Sundararajan's Weblog

My experiments with attach-on-demand (scripting way!)

As highlighted already in many places, Mustang (java SE 6) has support for Scripting (javax.script API) and includes JavaScript engine in it. There are many ways to use scripting - one very important way is exploratory programming. You can play with new Java APIs without having to write complete Java programs (i.e., avoid "compile" step in the edit/compile/run cycle). You can use jrunscript - command line script shell to explore Java APIs.

Elsewhere I mentioned about attach-on-demand API in Mustang - the facility to load java.lang.instrument (or JVM TI native agents) into running JVM(s). We'll how we can load an agent into running JVM using JavaScript. There is a demo JVM TI agent in $JDK_HOME/demo/jvmti/heapViewer. This agent prints heap histogram whenever SIGQUIT is sent to the process or when the Java process exits. Now, we'll attempt to load this agent "on demand".

File: attach.js

// package where attach API lives - 
importPackage(Packages.com.sun.tools.attach);

function loadAgentPath(pid, agent, options) {
    // attach to the given pid
    var vm = VirtualMachine.attach(pid);
    // load native library specified by path. optionally, pass options
    // to the agent
    if (options == undefined) {
       vm.loadAgentPath(agent);
    } else {
       vm.loadAgentPath(agent, options);
    }
    // detach the VM!
    vm.detach();
}

// call the above function using command line arguments to script
loadAgentPath(arguments[0], arguments[1], arguments[2]);

I tried to the above script with jrunscript using the command:

    jrunscript attach.js 13525 $JDK_HOME/demo/jvmti/lib/libheapViewer.so

where 13525 is a Java process running on my machine and JDK_HOME is the directory where Mustang JDK is installed on my machine. I got the following error:

script error in file attach.js : sun.org.mozilla.javascript.internal.WrappedException: Wrapped com.sun.tools.attach.AgentLoadException: Failed to load agent library (attach.js#10) in attach.js at line number 10

Why? It seems that heapViewer agent has not been written to support attach-on-demand. i.e., this agent has to be loaded on start-up. It seems that Agent_OnAttach function is missing in heapViewer. So what do we do? well, we can easily add one. The source for heapViewer is in the directory $JDK_HOME/demo/jvmti/heapViewer/src. I looked at heapViewer.c and it does miss Agent_OnAttach function. For experimentation purpose, I just added

JNIEXPORT jint JNICALL
Agent_OnAttach(JavaVM *vm, char *options, void *reserved) {
   printf("attached..\n");
   return Agent_OnLoad(vm, options, reserved);
}

in heapViewer.c and recompiled it (I had to copy sample.makefile.txt ad Makefile and do some editing to get correct GNU makefile). After that, I started a simple Java program and tried the above mentioned jrunscript attach.js ... command. It worked! This time, the Java process printed "attached..." and whenver I pressed Ctrl-\ (SIGQUIT) histogram was printed. Also, when the Java process exited, histogram was printed as expected.

---