Is there a simple way of obtaining all object instances of a specific class in Java

The debugger in Eclipse can show you all the instances of a class, so I looked around Eclipse's sources. Eclipse uses the Java Debug Wire Protocol, which allows you (since Java 6) to look up all the instances of the requested class. If you want to go down this path, grab a copy of Eclipse sources and check out the instances method of org.eclipse.jdi.internal.ReferenceTypeImpl.

A simpler way is to use the Java Debug Interface. Note the ReferenceType.instances method.

I still haven't figured out how to use JDI to connect to a running process and how to obtain an instance of ReferenceType. The JDK contains several examples, so I'm sure it's doable.

When I read this I was thinking that there must be SOME way to get this kind of info, since java profilers exist. Maybe this will help: http://java.sun.com/j2se/1.4.2/docs/guide/jvmpi/jvmpi.html. It describes the interface between the JVM and a profiler agent. But if you were actually looking to write this in Java you may be out of luck.

Specifically, check out this function:

jint (*EnableEvent)(jint event_type, void *arg);

    Called by the profiler agent to enable notification of a particular type of event. Apart from event_type, the profiler may also pass an argument that provides additional information specific to the given event type.

    All events are disabled when the VM starts up. Once enabled, an event stays enabled until it is explicitly disabled.

    This function returns JVMPI_NOT_AVAILABLE if event_type is JVMPI_EVENT_HEAP_DUMP, JVMPI_EVENT_MONITOR_DUMP or JVMPI_EVENT_OBJECT_DUMP. The profiler agent must use the RequestEvent function to request these events.

    Arguments:

        event_type  - type of event, JVMPI_EVENT_CLASS_LOAD etc.
        arg     - event specific argument.

    Returns:

        JVMPI_SUCCESS   enable succeeded.
        JVMPI_FAIL  enable failed.
        JVMPI_NOT_AVAILABLE     support for enabling the given event_type is not available.