Question

I have a java process currently running under a windows shell.

One of the threads responsible for serialisation is blocked indefinitely and as a result important information which is stored in memory is no longer being written to disk.

If I shutdown the process, the information will be lost.

It would be convenient if I could write and compile some new code and have it execute in the same memory space so that the said information could be serialised once more before I shutdown the process.

The process was started using a java -jar command.

With the hotspot VM features, is there any way to achieve this?

Answer 1

You can use the Attach API to attach to a virtual machine. Here’s an article that explains how to use it

Here’s a code example:

String agentJAR = "myAgent.jar";
VirtualMachine vm = VirtualMachine.attach (processid);
vm.loadAgent(agentJAR);

Where the agent is the name of your jar.

The agent jar contains an Agent, which can interface with the JVM using the Instrumentation API.

To create an agent that gets loaded at runtime, you implement an agentmain function like this:

public static void agentmain(String agentArgs, Instrumentation inst); 

or

public static void agentmain(String agentArgs); 

The Instrumentation object is used to modify classes at runtime, which you probably don’t need. But hopefully you can just put whatever code you need to run in agentmain and then use the attach API to run it in the target JVM.

Good luck!!