In Java, is it possible to know whether a class has already been loaded?

You can use the findLoadedClass(String) method in ClassLoader. It returns null if the class is not loaded.

One way to do this would be to write a Java agent using the instrumentation API. This would allow you to record the loading of classes by the JVM.

public class ClassLoadedAgent implements ClassFileTransformer { private static ClassLoadedAgent AGENT = null; /** Agent "main" equivalent */ public static void premain(String agentArguments, Instrumentation instrumentation) { AGENT = new ClassLoadedAgent(); for (Class<?> clazz : instrumentation.getAllLoadedClasses()) { AGENT.add(clazz); } instrumentation.addTransformer(AGENT); } private final Map<ClassLoader, Set<String>> classMap = new WeakHashMap<ClassLoader, Set<String>>(); private void add(Class<?> clazz) { add(clazz.getClassLoader(), clazz.getName()); } private void add(ClassLoader loader, String className) { synchronized (classMap) { System.out.println("loaded: " + className); Set<String> set = classMap.get(loader); if (set == null) { set = new HashSet<String>(); classMap.put(loader, set); } set.add(className); } } private boolean isLoaded(String className, ClassLoader loader) { synchronized (classMap) { Set<String> set = classMap.get(loader); if (set == null) { return false; } return set.contains(className); } } @Override public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException { add(loader, className); return classfileBuffer; } public static boolean isClassLoaded(String className, ClassLoader loader) { if (AGENT == null) { throw new IllegalStateException("Agent not initialized"); } if (loader == null || className == null) { throw new IllegalArgumentException(); } while (loader != null) { if (AGENT.isLoaded(className, loader)) { return true; } loader = loader.getParent(); } return false; } } 

META-INF/MANIFEST.MF:

Manifest-Version: 1.0 Premain-Class: myinstrument.ClassLoadedAgent 

The downside is that you have to load the agent when you start the JVM:

java -javaagent:myagent.jar ....etcetera 

If you're in control of the source of the classes for which you are interested in whether they are loaded or not (which I doubt, but you don't state in your question), then you could register your load in a static initializer.

public class TestLoaded { public static boolean loaded = false; public static void main(String[] args) throws ClassNotFoundException { System.out.println(loaded); ClassToTest.reportLoaded(); System.out.println(loaded); } static class ClassToTest { static { System.out.println("Loading"); TestLoaded.loaded = true; } static void reportLoaded() { System.out.println("Loaded"); } } } 

Output:

false Loading Loaded true 

I had a similar problem recently, where I suspected that classes were being loaded (presumably by way of -classpath or something similar) by my users that conflicted with classes I was loading later on into my own classloader.

After trying a few of the things mentioned here, the following seemed to do the trick for me. I'm not sure if it works for every circumstance, it may only work for java classes loaded from jar files.

InputStream is = getResourceAsStream(name); 

Where name is the path to the class file such as com/blah/blah/blah/foo.class.

getResourceAsStream returned null when the class had not been loaded into my class loader, or the system class loader, and returned non-null when the class had already been loaded.