类加载的过程
加载-》连接-》初始化
三种类加载器
AppClassLoader-> ExtClassLoader-> BootStrapClassLoader
- 启动类加载器(Bootstrap ClassLoader)
jre/lib/*jar、rt.jar、jce.jar
- 扩展类加载器(Extension ClassLoader)
jre/lib/ext/*.jar
- 应用程序类加载器(Application ClassLoader)
classpath
demo
package com.jysemel.java.basic.claasloader;public class ClassLoaderDemo {public static void main(String[] args) {//父子关系 AppClassLoader-> ExtClassLoader-> BootStrapClassLoaderClassLoader demo = ClassLoaderDemo.class.getClassLoader();System.out.println("demo " + demo);System.out.println("demo parent " + demo.getParent());//BootStrapClassLoader 引导类加载器，由C++ 实现，无法获取System.out.println("BootStrapClassLoader " + demo.getParent().getParent());//String,Integer 等基础均由 BootStrapClassLoader 加载ClassLoader c1 = String.class.getClassLoader();System.out.println("c1 " + c1);ClassLoader c2 = Integer.class.getClassLoader();System.out.println("c2 " + c2);}
}package com.jysemel.java.basic.claasloader;public class ClassLoaderDemo1 {public static void main(String[] args) {//BootStrapClassLoaderSystem.out.println("BootStrapClassLoader " + System.getProperty("sun.boot.class.path"));//ExtClassLoaderSystem.out.println("ExtClassLoader " + System.getProperty("java.ext.dirs"));//AppClassLoaderSystem.out.println("AppClassLoader " + System.getProperty("java.class.path"));}
}
向上委托查找、向下委托加载
保护底层类不会被应用程序覆盖
如何打破双亲委派
常见原因
容器部署多个应用可能依赖不同版本三方类库，保证每个应用类库独立和相互隔离 支持
打破方式
通过spi机制，使用ServiceLoader.load去加载
通过自定义类加载器，继承classloader，重写loadclass方法
demo
spi
package com.jysemel.java.basic.claasloader;import com.jysemel.java.basic.claasloader.spi.SpiService;import java.util.ServiceLoader;public class SpiDemo {public static void main(String[] args) {ServiceLoader<SpiService> serviceLoader = ServiceLoader.load(SpiService.class);System.out.println("加载到的 spiService 实现：");for (SpiService spiService : serviceLoader) {spiService.execute();}}
}package com.jysemel.java.basic.claasloader.spi;public interface SpiService {void execute();}package com.jysemel.java.basic.claasloader.spi;public class SpiServiceImpl1 implements SpiService {@Overridepublic void execute() {System.out.println("SpiServiceImpl1 执行，当前线程: " + Thread.currentThread().getName());}
}package com.jysemel.java.basic.claasloader.spi;public class SpiServiceImpl2 implements SpiService {@Overridepublic void execute() {System.out.println("SpiServiceImpl2 执行，当前线程: " + Thread.currentThread().getName());}
}重写loadClass()
package com.jysemel.java.basic.claasloader.loadclass;import java.io.*;public class MyClassLoader extends ClassLoader {@Overridepublic Class<?> loadClass(String name) throws ClassNotFoundException {System.out.println("MyClassLoader 尝试加载类: " + name);// 1. 检查类是否已被加载Class<?> loadedClass = findLoadedClass(name);if (loadedClass != null) {System.out.println("类已加载，直接返回: " + name);return loadedClass;}// 2. 尝试使用父加载器加载（双亲委派）try {Class<?> parentLoaded = super.loadClass(name);System.out.println("父加载器成功加载: " + name);return parentLoaded;} catch (ClassNotFoundException e) {System.out.println("父加载器无法加载: " + name);}// 3. 如果父加载器失败，自己尝试加载（本例未实现，直接抛出异常）// 实际中可以在这里添加 findClass 逻辑throw new ClassNotFoundException("类 " + name + " 找不到");}
}package com.jysemel.java.basic.claasloader.loadclass;public class Test {public static void main(String[] args) throws Exception {// 创建自定义类加载器实例MyClassLoader loader = new MyClassLoader();// 使用自定义加载器加载一个存在的类（如 java.lang.String）Class<?> stringClass = loader.loadClass("java.lang.String");System.out.println("加载 String 的类加载器: " + stringClass.getClassLoader());System.out.println("----------------------------");// 加载一个不存在的类（触发异常）try {Class<?> unknownClass = loader.loadClass("com.example.Unknown");} catch (ClassNotFoundException e) {System.out.println("捕获异常: " + e.getMessage());}}
}jdk9之后变化
ExtClassLoader 被PlatformClassLoader取代