Class.forName(String className)使用哪个类加载器?

不看源码，以为Class.forName(String className)使用的是系统类加载器，看了源码才知道不是这么回事。

 public static Class<?> forName(String className)                 throws ClassNotFoundException {        return forName0(className, true, ClassLoader.getCallerClassLoader());    }

通过 ClassLoader.getCallerClassLoader()获取类加载器：

 // Returns the invoker's class loader, or null if none.    // NOTE: This must always be invoked when there is exactly one intervening    // frame from the core libraries on the stack between this method's    // invocation and the desired invoker.    static ClassLoader getCallerClassLoader() {        // NOTE use of more generic Reflection.getCallerClass()        Class caller = Reflection.getCallerClass(3);        // This can be null if the VM is requesting it        if (caller == null) {            return null;        }        // Circumvent security check since this is package-private        return caller.getClassLoader0();    }

看第一行注释，返回的是调用者的类加载器。显然，调用者的类加载器不一定是系统类加载器，比如我们使用了自定义类加载器。看下面的例子：

User.java

package org.zzj;public class User {}

UserService.java

package org.zzj;public class UserService {public void add() {try {System.out.println(Class.forName("org.zzj.User").getClassLoader());} catch (ClassNotFoundException e) {e.printStackTrace();}}}

ClassForNameTest.java

package org.zzj;import java.io.IOException;import java.io.InputStream;import java.lang.reflect.Method;public class ClassForNameTest {public static void main(String[] args) throws Exception {System.out.println(Class.forName("org.zzj.User").getClassLoader());MyClassLoader classLoader = new MyClassLoader();Class<?> clazz = classLoader.loadClass("org.zzj.UserService");Method method = clazz.getMethod("add");method.invoke(clazz.newInstance());}}class MyClassLoader extends ClassLoader {@Overridepublic Class<?> loadClass(String name) throws ClassNotFoundException {String fileName = name.substring(name.lastIndexOf(".") + 1) + ".class";InputStream in = getClass().getResourceAsStream(fileName);if (in == null) {return super.loadClass(name);}byte[] b = null;try {b = new byte[in.available()];in.read(b);in.close();} catch (IOException e) {e.printStackTrace();}return defineClass(name, b, 0, b.length);}}

输出：

sun.misc.Launcher$[e-mail]@14318bb

两次加载使用的不是同一个类加载器，而是调用者的类加载器。