Merge pull request #3589 from raksha-rao/BAEL-1540-classloader

Bael 1540 classloader

core-java/src/main/java/com/baeldung/classloader/CustomClassLoader.java

@@ -0,0 +1,29 @@
+package com.baeldung.classloader;
+
+import java.io.*;
+
+public class CustomClassLoader extends ClassLoader {
+
+
+    public Class getClass(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;
+    }
+}

core-java/src/main/java/com/baeldung/classloader/PrintClassLoader.java

@@ -0,0 +1,22 @@
+package com.baeldung.classloader;
+
+import com.sun.javafx.util.Logging;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.MalformedURLException;
+import java.net.URL;
+import java.net.URLConnection;
+import java.util.ArrayList;
+
+public class PrintClassLoader {
+
+    public void printClassLoaders() throws ClassNotFoundException {
+
+        System.out.println("Classloader of this class:" + PrintClassLoader.class.getClassLoader());
+        System.out.println("Classloader of Logging:" + Logging.class.getClassLoader());
+        System.out.println("Classloader of ArrayList:" + ArrayList.class.getClassLoader());
+
+    }
+}

core-java/src/test/java/com/baeldung/classloader/CustomClassLoaderTest.java

@@ -0,0 +1,23 @@
+package com.baeldung.classloader;
+
+import org.junit.Test;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+
+public class CustomClassLoaderTest {
+
+    @Test
+    public void customLoader() throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
+
+        CustomClassLoader customClassLoader = new CustomClassLoader();
+        Class<?> c = customClassLoader.getClass(PrintClassLoader.class.getName());
+
+        Object ob = c.newInstance();
+
+        Method md = c.getMethod("printClassLoaders");
+        md.invoke(ob);
+
+    }
+
+}

core-java/src/test/java/com/baeldung/classloader/PrintClassLoaderTest.java

@@ -0,0 +1,14 @@
+package com.baeldung.classloader;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class PrintClassLoaderTest {
+    @Test(expected = ClassNotFoundException.class)
+    public void givenAppClassLoader_whenParentClassLoader_thenClassNotFoundException() throws Exception {
+        PrintClassLoader sampleClassLoader = (PrintClassLoader) Class.forName(PrintClassLoader.class.getName()).newInstance();
+        sampleClassLoader.printClassLoaders();
+        Class.forName(PrintClassLoader.class.getName(), true, PrintClassLoader.class.getClassLoader().getParent());
+    }
+}