详解Java的类加载机制及热部署的原理

 package com.ys.classLoader; public class FindClassLoaderTest { public static void main(String[] args) { ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); System.out.println(classLoader); System.out.println(classLoader.getParent()); System.out.println(classLoader.getParent().getParent()); } } 

 package com.ys.classLoader; public class LoadClassTest { public static void main(String[] args) throws Exception { ClassLoader classLoader = LoadClassTest.class.getClassLoader(); System.out.println(classLoader); //使用ClassLoader.loadClass()来加载类，不会执行初始化代码 //classLoader.loadClass("com.ys.classLoader.Test1"); //使用Class.forName()来加载类，会默认执行初始化代码 //Class.forName("com.ys.classLoader.Test1"); //使用Class.forName()来加载类，并指定类加载器，不会执行初始化代码 Class.forName("com.ys.classLoader.Test1", false, classLoader); } } 

 package com.ys.classLoader; public class Test1 { static { System.out.println("静态初始化代码执行了...."); } }

 public Class loadClass(String name)throws ClassNotFoundException { return loadClass(name, false); } protected synchronized Class loadClass(String name, boolean resolve)throws ClassNotFoundException { // 首先判断该类型是否已经被加载 Class c = findLoadedClass(name); if (c == null) { //如果没有被加载，就委托给父类加载或者委派给启动类加载器加载 try { if (parent != null) { //如果存在父类加载器，就委派给父类加载器加载 c = parent.loadClass(name, false); } else { //如果不存在父类加载器，就检查是否是由启动类加载器加载的类，通过调用本地方法native Class findBootstrapClass(String name) c = findBootstrapClass0(name); } } catch (ClassNotFoundException e) { // 如果父类加载器和启动类加载器都不能完成加载任务，才调用自身的加载功能 c = findClass(name); } } if (resolve) { resolveClass(c); } return c; } 

 package com.classloader; import com.Application; import com.Test; import java.io.File; import java.io.FileInputStream; import java.io.InputStream; import java.util.ArrayList; import java.util.List; public class MyClassLoader extends ClassLoader { //目的 让缓存里面永远能返回一个Class对象 这样就不需要走父类加载器了 //在构造方法里面加载类  loadClass //项目的根路径 public String rootPath; //所有需要由我这个类加载器加载的类存在这个集合 public List clazzs; //两个classloader  一个是负责加载 需要被热部署的代码的 //一个是加载系统的一些类的 //classPaths: 需要被热部署的加载器去加载的目录 public MyClassLoader(String rootPath, String... classPaths) throws Exception { this.rootPath = rootPath; this.clazzs = new ArrayList<>(); for (String classPath : classPaths) { scanClassPath(new File(classPath)); } } //扫描项目里面传进来的一些class public void scanClassPath(File file) throws Exception { if (file.isDirectory()) { for (File file1 : file.listFiles()) { scanClassPath(file1); } } else { String fileName = file.getName(); String filePath = file.getPath(); String endName = fileName.substring(fileName.lastIndexOf(".") + 1); if (endName.equals("class")) { //现在我们加载到的是一个Class文件 //如何吧一个Class文件 加载成一个Class对象？？？？ InputStream inputStream = new FileInputStream(file); byte[] bytes = new byte[(int) file.length()]; inputStream.read(bytes); //文件名转类名 String className = fileNameToClassName(filePath); //类名 defineClass(className, bytes, 0, bytes.length); clazzs.add(className); //loadClass 是从当前ClassLoader里面去获取一个Class对象 } } } //文件名转类名，这儿操作系统不同可能文件的分隔符不一样，所以这儿需要注意，笔者用的Mac。Windows操作需要自行修改。 public String fileNameToClassName(String filePath) { String className = filePath.replace(rootPath, "").replaceAll("/", "."); className = className.substring(1, className.lastIndexOf(".")); return className; } @Override public Class loadClass(String name) throws ClassNotFoundException { Class loadClass = findLoadedClass(name); //第一情况 这个类 不需要由我们加载 //第二种情况 这个类需要由我们加载 但是 确实加载不到 if (loadClass == null) { if (!clazzs.contains(name)) { loadClass = getSystemClassLoader().loadClass(name); } else { throw new ClassNotFoundException("没有加载到类"); } } return loadClass; } //当文件被修改的时候再进行热部署 public static void main(String[] args) throws Exception { //双亲委派机制 Application.run(MyClassLoader.class); } } 

 package com; import com.classloader.MyClassLoader; import org.apache.commons.io.monitor.FileAlterationMonitor; import org.apache.commons.io.monitor.FileAlterationObserver; import java.io.File; public class Application { public static String rootPath; public static void run(Class clazz) throws Exception { String rootPath = MyClassLoader.class.getResource("/").getPath().replaceAll("%20", " "); rootPath = new File(rootPath).getPath(); Application.rootPath = rootPath; MyClassLoader myClassLoader = new MyClassLoader(rootPath, rootPath + "/com"); //用我们自己的类加载器加载程序入口 startFileListener(rootPath); start0(myClassLoader); } //开启文件监听器 public static void startFileListener(String rootPath) throws Exception { FileAlterationObserver fileAlterationObserver = new FileAlterationObserver(rootPath); fileAlterationObserver.addListener(new FileListener()); FileAlterationMonitor fileAlterationMonitor = new FileAlterationMonitor(500); fileAlterationMonitor.addObserver(fileAlterationObserver); fileAlterationMonitor.start(); //要实现文件监听：  写一个线程 去定时监听某个路径下所有的文件 //如果文件发生改动 就回调监听器 } //新的classLoader public static void start() { System.out.println("启动我们的应用程序"); new Test().test(); } public static void stop() { System.out.println("程序退出"); //告诉jvm需要gc了 System.gc(); //告诉jvm可以清除对象引用 System.runFinalization(); } public static void start0(MyClassLoader classLoader) throws Exception { //全盘委托的机制，通过自定义的类加载加载，然后start方法中的new Test()也会通过自定义的类加载器加载 Class aClass = classLoader.loadClass("com.Application"); aClass.getMethod("start").invoke(aClass.newInstance()); } } 

 package com; import com.classloader.MyClassLoader; import org.apache.commons.io.monitor.FileAlterationListenerAdaptor; import java.io.File; public class FileListener extends FileAlterationListenerAdaptor { @Override public void onFileChange(File file) { if (file.getName().indexOf(".class") != -1) { try { MyClassLoader myClassLoader = new MyClassLoader(Application.rootPath, Application.rootPath + "/com"); Application.stop(); Application.start0(myClassLoader); } catch (Exception e) { e.printStackTrace(); } } } } 

 package com; public class Test { public void test() { System.out.println("当前版本:version 1.0"); System.out.println(this.getClass().getClassLoader()); } } 

 package com.ys.classloader; import java.io.*; public class MyClassLoader extends ClassLoader { private String root; protected Class findClass(String name) throws ClassNotFoundException { byte[] classData = loadClassData(name); if (classData == null) { throw new ClassNotFoundException(); } else { return defineClass(name, classData, 0, classData.length); } } private byte[] loadClassData(String className) { String fileName = root + File.separatorChar + className.replace('.', File.separatorChar) + ".class"; try { InputStream ins = new FileInputStream(fileName); ByteArrayOutputStream baos = new ByteArrayOutputStream(); int bufferSize = 1024; byte[] buffer = new byte[bufferSize]; int length = 0; while ((length = ins.read(buffer)) != -1) { baos.write(buffer, 0, length); } return baos.toByteArray(); } catch (IOException e) { e.printStackTrace(); } return null; } public String getRoot() { return root; } public void setRoot(String root) { this.root = root; } public static void main(String[] args)  { MyClassLoader classLoader = new MyClassLoader(); classLoader.setRoot("/User/king/temp"); Class testClass = null; try { testClass = classLoader.loadClass("com.ys.classloader.Test2"); Object object = testClass.newInstance(); System.out.println(object.getClass().getClassLoader()); } catch (ClassNotFoundException e) { e.printStackTrace(); } catch (InstantiationException e) { e.printStackTrace(); } catch (IllegalAccessException e) { e.printStackTrace(); } } }