Sundararajan's Weblog

  • Java
    February 20, 2006

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

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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 -
function loadAgentPath(pid, agent, options) {
// attach to the given pid
var vm = href="http://download.java.net/jdk6/docs/guide/attach/spec/com/sun/tools/attach/VirtualMachine.html">VirtualMachine.attach(pid);
// load native library specified by path. optionally, pass options
// to the agent
if (options == undefined) {
} else {
vm.loadAgentPath(agent, options);
// detach the VM!
// 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

Agent_OnAttach(JavaVM \*vm, char \*options, void \*reserved) {
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.

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Comments ( 2 )
  • Jesse Glick Monday, February 20, 2006
    If you're playing with JavaScript + attach-on-demand, take a look at
  • A. Sundararajan Monday, February 20, 2006
    Hi Jesse Glick: Yes, I've read your post on running scripts on a running JVM -- where you use attach-on-demand to "inject" scripts into a running JVM. Nice work! (how about a java.net project of your idea?). Here, I tried to show how users can do "exploratory programming" with JavaScript - to learn new Java APIs such as attach-on-demand. BTW, thanks for your comment!
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