Java - Get a List of All Classes Loaded in the Jvm

How can I list all classes loaded in a specific class loader

Instrumentation.getInitiatedClasses(ClassLoader) may do what you want.

According to the docs:

Returns an array of all classes for which loader is an initiating loader.

I'm not sure what "initiating loader" means though. If that does not give the right result try using the getAllLoadedClasses() method and manually filtering by ClassLoader.

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How to get an instance of Instrumentation

Only the agent JAR (which is separate from the application JAR) can get an instance of the Instrumentation interface. A simple way to make it available to the application is to create an agent JAR containing one class with a premain method that does nothing but save a reference to the Instrumentation instance in the system properties.

Example agent class:

public class InstrumentHook {

    public static void premain(String agentArgs, Instrumentation inst) {
        if (agentArgs != null) {
            System.getProperties().put(AGENT_ARGS_KEY, agentArgs);
        }
        System.getProperties().put(INSTRUMENTATION_KEY, inst);
    }

    public static Instrumentation getInstrumentation() {
        return (Instrumentation) System.getProperties().get(INSTRUMENTATION_KEY);
    }

    // Needn't be a UUID - can be a String or any other object that
    // implements equals().    
    private static final Object AGENT_ARGS_KEY =
        UUID.fromString("887b43f3-c742-4b87-978d-70d2db74e40e");

    private static final Object INSTRUMENTATION_KEY =
        UUID.fromString("214ac54a-60a5-417e-b3b8-772e80a16667");

}

Example manifest:

Manifest-Version: 1.0
Premain-Class: InstrumentHook

The resulting JAR must then be referenced by the application and specified on the command line (with the -javaagent option) when launching the application. It might be loaded twice in different ClassLoaders, but that is not a problem since the system Properties is a per-process singleton.

Example application class

public class Main {
    public static void main(String[] args) {
        Instrumentation inst = InstrumentHook.getInstrumentation();
        for (Class<?> clazz: inst.getAllLoadedClasses()) {
            System.err.println(clazz.getName());
        }
    }
}

Obtaining a list of all Java classes used from all JVM's?

You could use the jcmd command that comes with the JDK. It does not require any special arguments to be past to the JVM upon start.

jcmd 

Will list all of the JVMs that are currently running, along with their pid (process id).

jcmd <pid> GC.class_histogram

will then list every class that is currently loaded in that JVM, along with a count of how many instances.

How to get a list of all classes that are loaded by the bootstrap class loader?

Watch the method getInitiatedClasses:

http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/Instrumentation.html

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