[java] Java - Get a list of all Classes loaded in the JVM

I would like to get a list of all the classes belonging to a certain package as well as all of their children. The classes may or may not be already loaded in the JVM.

You might be able to get a list of classes that are loaded through the classloader but this would not include classes you haven't loaded yet but are on your classpath.

To get ALL classes on your classpath you have to do something like your second solution. If you really want classes that are currently "Loaded" (in other words, classes you have already referenced, accessed or instantiated) then you should refine your question to indicate this.

This program will prints all the classes with its physical path. use can simply copy this to any JSP if you need to analyse the class loading from any web/application server.

import java.lang.reflect.Field;
import java.util.Vector;

public class TestMain {

    public static void main(String[] args) {
        Field f;
        try {
            f = ClassLoader.class.getDeclaredField("classes");
            f.setAccessible(true);
            ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
            Vector<Class> classes =  (Vector<Class>) f.get(classLoader);

            for(Class cls : classes){
                java.net.URL location = cls.getResource('/' + cls.getName().replace('.',
                '/') + ".class");
                System.out.println("<p>"+location +"<p/>");
            }
        } catch (Exception e) {

            e.printStackTrace();
        }
    }
}

It's not a programmatic solution but you can run

java -verbose:class ....

and the JVM will dump out what it's loading, and from where.

[Opened /usr/java/j2sdk1.4.1/jre/lib/rt.jar]
[Opened /usr/java/j2sdk1.4.1/jre/lib/sunrsasign.jar]
[Opened /usr/java/j2sdk1.4.1/jre/lib/jsse.jar]
[Opened /usr/java/j2sdk1.4.1/jre/lib/jce.jar]
[Opened /usr/java/j2sdk1.4.1/jre/lib/charsets.jar]
[Loaded java.lang.Object from /usr/java/j2sdk1.4.1/jre/lib/rt.jar]
[Loaded java.io.Serializable from /usr/java/j2sdk1.4.1/jre/lib/rt.jar]
[Loaded java.lang.Comparable from /usr/java/j2sdk1.4.1/jre/lib/rt.jar]
[Loaded java.lang.CharSequence from /usr/java/j2sdk1.4.1/jre/lib/rt.jar]
[Loaded java.lang.String from /usr/java/j2sdk1.4.1/jre/lib/rt.jar]

See here for more details.

using the Reflections library, it's easy as:

Reflections reflections = new Reflections("my.pkg", new SubTypesScanner(false));

That would scan all classes in the url/s that contains my.pkg package.

• the false parameter means - don't exclude the Object class, which is excluded by default.
• in some scenarios (different containers) you might pass the classLoader as well as a parameter.

So, getting all classes is effectively getting all subtypes of Object, transitively:

Set<String> allClasses = 
    reflections.getStore().getSubTypesOf(Object.class.getName());

(The ordinary way reflections.getSubTypesOf(Object.class) would cause loading all classes into PermGen and would probably throw OutOfMemoryError. you don't want to do it...)

If you want to get all direct subtypes of Object (or any other type), without getting its transitive subtypes all in once, use this:

Collection<String> directSubtypes = 
    reflections.getStore().get(SubTypesScanner.class).get(Object.class.getName());

An alternative approach to those described above would be to create an external agent using java.lang.instrument to find out what classes are loaded and run your program with the -javaagent switch:

import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;

public class SimpleTransformer implements ClassFileTransformer {

    public SimpleTransformer() {
        super();
    }

    public byte[] transform(ClassLoader loader, String className, Class redefiningClass, ProtectionDomain domain, byte[] bytes) throws IllegalClassFormatException {
        System.out.println("Loading class: " + className);
        return bytes;
    }
}

This approach has the added benefit of providing you with information about which ClassLoader loaded a given class.

There are multiple answers to this question, partly due to ambiguous question - the title is talking about classes loaded by the JVM, whereas the contents of the question says "may or may not be loaded by the JVM".

Assuming that OP needs classes that are loaded by the JVM by a given classloader, and only those classes - my need as well - there is a solution (elaborated here) that goes like this:

import java.net.URL;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Vector;

public class CPTest {

    private static Iterator list(ClassLoader CL)
        throws NoSuchFieldException, SecurityException,
        IllegalArgumentException, IllegalAccessException {
        Class CL_class = CL.getClass();
        while (CL_class != java.lang.ClassLoader.class) {
            CL_class = CL_class.getSuperclass();
        }
        java.lang.reflect.Field ClassLoader_classes_field = CL_class
                .getDeclaredField("classes");
        ClassLoader_classes_field.setAccessible(true);
        Vector classes = (Vector) ClassLoader_classes_field.get(CL);
        return classes.iterator();
    }

    public static void main(String args[]) throws Exception {
        ClassLoader myCL = Thread.currentThread().getContextClassLoader();
        while (myCL != null) {
            System.out.println("ClassLoader: " + myCL);
            for (Iterator iter = list(myCL); iter.hasNext();) {
                System.out.println("\t" + iter.next());
            }
            myCL = myCL.getParent();
        }
    }

}

One of the neat things about it is that you can choose an arbitrary classloader you want to check. It is however likely to break should internals of classloader class change, so it is to be used as one-off diagnostic tool.

From Oracle doc you can use -Xlog option that has a possibility to write into file.

java -Xlog:class+load=info:classloaded.txt

I'd also suggest you write a -javagent agent, but use the getAllLoadedClasses method instead of transforming any classes.

To synchronize with your client code (Normal Java code), create a socket and communicate with the agent through it. Then you can trigger a "list all classes" method whenever you need.

One way if you already know the package top level path is to use OpenPojo

final List<PojoClass> pojoClasses = PojoClassFactory.getPojoClassesRecursively("my.package.path", null);

Then you can go over the list and perform any functionality you desire.

Run your code under a JRockit JVM, then use JRCMD <PID> print_class_summary

This will output all loaded classes, one on each line.