BAEL-4302 : How can I list all classes loaded in a specific class loader

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/ClassLoaderType.java

@@ -0,0 +1,6 @@
+package com.baeldung.loadedclasslisting;
+
+public enum ClassLoaderType {
+
+    SYSTEM, EXTENSION, BOOTSTRAP, CUSTOM
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/Launcher.java

@@ -0,0 +1,51 @@
+package com.baeldung.loadedclasslisting;
+
+import java.lang.reflect.Method;
+import java.util.Arrays;
+
+import com.baeldung.loadedclasslisting.customLoader.ClassLoaderInfo;
+import com.baeldung.loadedclasslisting.customLoader.CustomClassLoader;
+
+public class Launcher {
+
+    private static ClassLoader customClassLoader;
+
+    public static void main(String[] args) {
+        
+        printClassesLoadedBy(ClassLoaderType.BOOTSTRAP);
+
+        printClassesLoadedBy(ClassLoaderType.SYSTEM);
+
+        printClassesLoadedBy(ClassLoaderType.EXTENSION);
+
+        printClassesLoadedBy(ClassLoaderType.CUSTOM);
+    }
+
+    private static void printClassesLoadedBy(ClassLoaderType classLoaderType) {
+        Class<?>[] classes;
+        if (classLoaderType.equals(ClassLoaderType.CUSTOM)) {
+            customClassLoader = customClassLoading();
+            classes = ListLoadedClassesAgent.listLoadedClasses(customClassLoader);
+        } else {
+            classes = ListLoadedClassesAgent.listLoadedClasses(classLoaderType);
+        }
+        Arrays.asList(classes)
+            .forEach(clazz -> System.out.println(
+              classLoaderType + " ClassLoader : " + clazz.getCanonicalName()));
+    }
+    
+    private static CustomClassLoader customClassLoading() {
+        CustomClassLoader customClassLoader = new CustomClassLoader();
+        Class<?> c;
+        try {
+            c = customClassLoader.findClass(ClassLoaderInfo.class.getName());
+            Object ob = c.getDeclaredConstructor()
+                .newInstance();
+            Method md = c.getMethod("printClassLoaders");
+            md.invoke(ob);
+        } catch (Exception e) {
+            e.printStackTrace();
+        }
+        return customClassLoader;
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/ListLoadedClassesAgent.java

@@ -0,0 +1,46 @@
+package com.baeldung.loadedclasslisting;
+
+import java.lang.instrument.Instrumentation;
+
+public class ListLoadedClassesAgent {
+
+    private static Instrumentation instrumentation;
+
+    public static void premain(String agentArgs, Instrumentation instrumentation) {
+        ListLoadedClassesAgent.instrumentation = instrumentation;
+    }
+
+    public static Class<?>[] listLoadedClasses(ClassLoaderType classLoaderType) {
+        if (instrumentation == null) {
+            throw new IllegalStateException(
+              "ListLoadedClassesAgent is not initialized.");
+        }
+        return instrumentation.getInitiatedClasses(
+          getClassLoader(classLoaderType));
+    }
+
+    public static Class<?>[] listLoadedClasses(ClassLoader classLoader) {
+        if (instrumentation == null) {
+            throw new IllegalStateException(
+              "ListLoadedClassesAgent is not initialized.");
+        }
+        return instrumentation.getInitiatedClasses(classLoader);
+    }
+
+    private static ClassLoader getClassLoader(ClassLoaderType classLoaderType) {
+        ClassLoader classLoader = null;
+        switch (classLoaderType) {
+        case SYSTEM:
+            classLoader = ClassLoader.getSystemClassLoader();
+            break;
+        case EXTENSION:
+            classLoader = ClassLoader.getSystemClassLoader().getParent();
+            break;
+        case BOOTSTRAP:
+            break;
+        default:
+            break;
+        }
+        return classLoader;
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/MANIFEST.MF

@@ -0,0 +1 @@
+Premain-Class: com.baeldung.loadedclasslisting.ListLoadedClassesAgent

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/customLoader/ClassLoaderInfo.java

@@ -0,0 +1,12 @@
+package com.baeldung.loadedclasslisting.customLoader;
+
+import java.util.ArrayList;
+
+public class ClassLoaderInfo {
+
+    public void printClassLoaders() throws ClassNotFoundException {
+
+        System.out.println("Classloader of this class:" + ClassLoaderInfo.class.getClassLoader());
+        System.out.println("Classloader of ArrayList:" + ArrayList.class.getClassLoader());
+    }
+}

core-java-modules/core-java-jvm-2/src/main/java/com/baeldung/loadedclasslisting/customLoader/CustomClassLoader.java

@@ -0,0 +1,32 @@
+package com.baeldung.loadedclasslisting.customLoader;
+
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.io.IOException;
+import java.io.InputStream;
+
+public class CustomClassLoader extends ClassLoader {
+
+    @Override
+    public Class<?> findClass(String name) throws ClassNotFoundException {
+        byte[] b = loadClassFromFile(name);
+        return defineClass(name, b, 0, b.length);
+    }
+
+    private byte[] loadClassFromFile(String fileName) {
+        InputStream inputStream = getClass().getClassLoader()
+            .getResourceAsStream(fileName.replace('.', File.separatorChar) + ".class");
+        byte[] buffer;
+        ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
+        int nextValue = 0;
+        try {
+            while ((nextValue = inputStream.read()) != -1) {
+                byteStream.write(nextValue);
+            }
+        } catch (IOException e) {
+            e.printStackTrace();
+        }
+        buffer = byteStream.toByteArray();
+        return buffer;
+    }
+}