關于JAVA類加載器。

問題描述

public static <T> T classLoader(String className) throws Exception {ClassLoader myClassLoader = new ClassLoader() { @Override protected Class<?> findClass(String name) throws ClassNotFoundException {try { //獲取類文件名 String fileName = name.substring(name.lastIndexOf('.') + 1) + '.class'; InputStream inputStream = getClass().getResourceAsStream(fileName); if (inputStream == null) {return super.findClass(fileName); } byte[] bytes = new byte[inputStream.available()]; inputStream.close(); return defineClass(name, bytes, 0, bytes.length);} catch (IOException e) { throw new ClassNotFoundException();} }};return (T) myClassLoader.loadClass(className).newInstance(); }public static void main(String[] args) throws Exception {//測試1Object obj2 = classLoader('com.myweb.reflect.classloader.ClassLoaderTest');System.out.println(obj2.getClass());System.out.println(obj2 instanceof com.myweb.reflect.classloader.ClassLoaderTest);//測試2ClassLoader myClassLoader = new ClassLoader() { @Override protected Class<?> findClass(String name) throws ClassNotFoundException {try { //獲取類文件名 String fileName = name.substring(name.lastIndexOf('.') + 1) + '.class'; InputStream inputStream = getClass().getResourceAsStream(fileName); if (inputStream == null) {return super.findClass(fileName); } byte[] bytes = new byte[inputStream.available()]; inputStream.close(); return defineClass(name, bytes, 0, bytes.length);} catch (IOException e) { throw new ClassNotFoundException();} }};Object obj3 = myClassLoader.loadClass('com.myweb.reflect.classloader.ClassLoaderTest');System.out.println(obj3.getClass());System.out.println(obj3 instanceof com.myweb.reflect.classloader.ClassLoaderTest);}

輸出：class com.myweb.reflect.classloader.ClassLoaderTesttrueclass java.lang.Classfalse

為什么兩段相同的代碼，只是一個單獨提取出來，輸出就不一樣了呢？

問題解答

你確定是兩段相同的代碼嗎？第一個代碼段里面有多一句return (T) myClassLoader.loadClass(className).newInstance();