ie不加载jre_Java类加载过程

Simple[] simples &＃61; new Simple[10];

static {try {System.out.println("static code will be inviked");TimeUnit.SECONDS.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}}public static void main(String[] args) {IntStream.range(0,5).forEach(i -> new Thread(DeadLock::new));}

System.out.println("Bootstrap" &＃43; String.class.getClassLoader());null
System.out.println(System.getProperty("sun.boot.class.path"));/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/resources.jar:/Library/Java/JavaVirtualMachines/jdk1.8.0_181.jdk/Contents/Home/jre/lib/rt.jar

System.out.println(System.getProperty("java.ext.dirs"));

System.out.println(System.getProperty("java.class.path"));
System.out.println(DeadLock.class.getClassLoader());

sun.misc.Launcher$AppClassLoader&＃64;18b4aac2

package thread.chapt1;public class MyClassLoader extends ClassLoader {private final static Path DEFAULT_CLASS_DIR &＃61; Paths.get("/Users/xuxliu/Ifoods/Java");// 默认的class 路径public MyClassLoader() {super();this.classDir &＃61; DEFAULT_CLASS_DIR;}private final Path classDir;// 指定父类加载器// 允许传入指定路径的 class 路径public MyClassLoader(String classDir, ClassLoader parent) {super(parent);this.classDir &＃61; Paths.get(classDir);}// 指定加载特定的目录public MyClassLoader(String classDir) {super();this.classDir &＃61; Paths.get(classDir);}// 重写父类的 findClass 这是非常重要的&＃64;Overrideprotected Class findClass(String name) throws ClassNotFoundException {// 读取二进制数据byte[] classBytes &＃61; new byte[0];try {classBytes &＃61; this.readClassBytes(name);} catch (IOException e) {e.printStackTrace();}if (classBytes &＃61;&＃61; null || classBytes.length &＃61;&＃61; 0) {throw new ClassNotFoundException("can not load the class");}// 调用 defineClass 方法定义class&＃xff0c;读取class 文件return this.defineClass(name, classBytes, 0, classBytes.length);}// class 文件读入内存private byte[] readClassBytes(String name) throws ClassNotFoundException, IOException {// 包名分隔符专为文件路径String classPath &＃61; name.replace(".", "/");Path classFullPath &＃61; classDir.resolve(Paths.get(classPath) &＃43; name);if (!classFullPath.toFile().exists()){throw new ClassNotFoundException("the class " &＃43; name &＃43; " not found");}try (ByteArrayOutputStream baos &＃61; new ByteArrayOutputStream()) {Files.copy(classFullPath, baos);return baos.toByteArray();}}&＃64;Overridepublic String toString() {return "MyClassLoader";}public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {MyClassLoader myClassLoader &＃61; new MyClassLoader();Class aClass &＃61; myClassLoader.loadClass("thread.chapt1.HelloWorld");System.out.println(aClass.getClassLoader());Object helloWorld &＃61; aClass.newInstance();System.out.println(helloWorld);Method welcome &＃61; aClass.getMethod("welcome");String result &＃61; (String) welcome.invoke(helloWorld);System.out.println(result);}
}上面的代码先对程序进行编译&＃xff0c;然后将编译后的class 文件夹复制到 /Users/xuxliu/Ifoods/Java 路径下
这里还需要删除掉 .java 文件&＃xff0c;因为父委托机制会破坏我们的测试代码
程序在执行过程中&＃xff0c;主要步骤是
1. myClassLoader.loadClass("thread.chapt1.HelloWorld");
2. 执行 readClassBytes 读取 class 文件
3. findClass 找到class
4. 在 findClass 方法中最后执行 defineClass 就定义好了&＃xff0c;HelloWorld 类
5. Object helloWorld &＃61; aClass.newInstance(); 创建 HelloWorld 类对象
6. Method welcome &＃61; aClass.getMethod("welcome"); 反射获取方法
7. String result &＃61; (String) welcome.invoke(helloWorld); 反射执行方法

protected Class loadClass(String name, boolean resolve)throws ClassNotFoundException{synchronized (getClassLoadingLock(name)) {// First, check if the class has already been loadedClass c &＃61; findLoadedClass(name);if (c &＃61;&＃61; null) {long t0 &＃61; System.nanoTime();try {if (parent !&＃61; null) {c &＃61; parent.loadClass(name, false);} else {c &＃61; findBootstrapClassOrNull(name);}} catch (ClassNotFoundException e) {// ClassNotFoundException thrown if class not found// from the non-null parent class loader}if (c &＃61;&＃61; null) {// If still not found, then invoke findClass in order// to find the class.long t1 &＃61; System.nanoTime();c &＃61; findClass(name);// this is the defining class loader; record the statssun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);sun.misc.PerfCounter.getFindClasses().increment();}}if (resolve) {resolveClass(c);}return c;}}

1. 当前类加载器中根据类的全路径名查询是否存在该类&＃xff0c;如果存在则直接返回
2. 如果不存在&＃xff0c;查看当前类是否存在父类加载器&＃xff0c;存在&＃xff0c;则直接调用父类加载器加载类
3. 如果当前父类不存在加载器&＃xff0c;那么直接调用根类加载器对类进行加载
4. 如果当前类的所有父类加载器都没有成功加载class,则尝试调用当前类加载器的findClass 方法加载&＃xff0c;这个方法就是我们定义加载器需要重写的方法

public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {ClassLoader classLoader &＃61; MyClassLoader.class.getClassLoader();// 这个是原来的构造函数
// MyClassLoader myClassLoader &＃61; new MyClassLoader();// 这个是新的指定加载器的构造函数MyClassLoader myClassLoader &＃61; new MyClassLoader("/Users/xuxliu/Ifoods/Java/", classLoader);// 这也也可以破坏&＃xff0c;双委托机制
// MyClassLoader myClassLoader &＃61; new MyClassLoader("/Users/xuxliu/Ifoods/Java/", null);Class aClass &＃61; myClassLoader.loadClass("thread.chapt1.HelloWorld");System.out.println(aClass.getClassLoader());Object helloWorld &＃61; aClass.newInstance();System.out.println(helloWorld);Method welcome &＃61; aClass.getMethod("welcome");String result &＃61; (String) welcome.invoke(helloWorld);System.out.println(result);}