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. 

As explained in the other answers you can do it by using the JDI protocol. It is rather simple: you need to run the JVM in debug mode using

--agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=56855

after that, you can connect to the remote (or local JVM) and list all instances of the specified class. Also, you cannot directly cast remote objects to the real object but you can access to all the fields of the remote object and even cal methods.

Here how you can connect to remote JVM and get VirtualMachine.

    private static VirtualMachine attach(String hostname, String port) throws IOException, IllegalConnectorArgumentsException {
        //getSocketAttaching connector to connect to other JVM using Socket
        AttachingConnector connector = Bootstrap.virtualMachineManager().attachingConnectors()
                .stream().filter(p -> p.transport().name().contains("socket"))
                .findFirst().get();

        //set the arguments for the connector
        Map<String, Argument> arg = connector.defaultArguments();
        arg.get("hostname").setValue(hostname);
        arg.get("port").setValue(port);

        //connect to remote process by socket
        return connector.attach(arg);
    }

After getting VirtualMachine you can get instances of a class using methods classesByName and instances. It return List of ReferenceType:

         VirtualMachine vm = attach("localhost", "56856");

        //get all classes of java.lang.String. There would be only one element.
        List<ReferenceType> classes = vm.classesByName("java.lang.String");

        //get all instances of a classes (set maximum count of instannces to get).
        List<ObjectReference> o = classes.get(0).instances(100000);

        //objectReference holds referenct to remote object. 
        for (ObjectReference objectReference : o) {
            try {
                //show text representation of remote object
                System.out.println(objectReference.toString());
            } catch (com.sun.jdi.ObjectCollectedException e) {
                //the specified object has been garbage collected
                //to avoid this use vm.suspend() vm.resume()
                System.out.println(e);
            }
        }

Here is a working example of a program than runs and connects to itself and list all instances of a java.lang.String. To run an example you need tool.jar from jdk in you classpath.

http://java.sun.com/j2se/1.5.0/docs/guide/jvmti/jvmti.html#IterateOverInstancesOfClass

You can write some native code that obtains the JVMTI pointer and then uses it to iterate over all instances of a given class as shown in the link above. You can call this native code from your Java program. As Eli points out though, there is a high level wrapper for this called Java Debug Interface available from Java 6 onwards, which allows you to make such calls from Java itself without having to muck around with native code.

hope this helps

Ram