上文：jdk-Launcher源码学习

背景

上文说过，jdk是通过双亲委派机制实现类的加载，但是这个加载效率及场景存在弊端，所以本文借鉴tomcat的实现方式去打破双亲委派机制实现自定义类加载器来模似tomcat的类加载方式。

实现方式

实现思想：首先继承ClassLoader，然后通过classLoader进行重写findClass实现。

package com.classloader.simulate;import com.encryption.demo.EncryptionUtils;
import org.apache.commons.lang.StringUtils;import java.io.File;
import java.io.FileInputStream;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;/*** @author hong* @version 1.0* @description: 模似打破双亲委派机制* @date 2022/4/16 18:50*/
public class SimulateTomcatClassLoader  {static class MySelfClassLoader extends ClassLoader{/*** 类路劲**/private String classPath;public MySelfClassLoader(String classPath) {this.classPath = classPath;}private byte[] loadByClassName(String className) throws Exception {if (StringUtils.isEmpty(className)) {throw new Exception("类名不能为空！");}//替换路劲格式className = className.replaceAll("\\.", "/");//获取文件信息FileInputStream fis = new FileInputStream(classPath + "/" + className + ".class");//获取长度int len = fis.available();//文件信息byte[] data = new byte[len];//读写文件信息fis.read(data);fis.close();return data;}@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {try {byte[] data = loadByClassName(name);return defineClass(name, data, 0, data.length);} catch (Exception e) {e.printStackTrace();throw new ClassNotFoundException();}}/*** 重写类加载方法，在这里打破双亲委派* @param name* @param resolve* @return* @throws ClassNotFoundException*/@Overrideprotected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {synchronized (getClassLoadingLock(name)) {// First, check if the class has already been loadedClass<?> c = findLoadedClass(name);if (c == null) {// If still not found, then invoke findClass in order// to find the class.long t1 = System.nanoTime();//这里重写逻辑 如果不是自定义类开头就用默认类加载器，如果是就有自定义类加载器加载if(!name.startsWith("com.classloader")){c = this.getParent().loadClass(name);}else{c = findClass(name);}// this is the defining class loader; record the stats
// sun.misc.PerfCounter.getParentDelegationTime().addTime(t1);sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);sun.misc.PerfCounter.getFindClasses().increment();}
// }if (resolve) {resolveClass(c);}return c;}}}public void test(){System.out.println("测试类！");}public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {//获取项目路劲File file = new File(EncryptionUtils.class.getResource("/").getPath());MySelfClassLoader mySelfClaasLoader = new MySelfClassLoader(file.getPath());Class<?> loadClass = mySelfClaasLoader.loadClass("com.classloader.simulate.SimulateTomcatClassLoader");//获取实例对象Object obj = loadClass.newInstance();//获取方法列表Method[] declaredMethods = loadClass.getDeclaredMethods();System.out.println(declaredMethods);//获取方法Method method = loadClass.getMethod("test", null);method.invoke(obj,null);System.out.println(loadClass.getClassLoader().getClass().getName());;}
}

结果

测试类！
com.classloader.simulate.SimulateTomcatClassLoader$MySelfClassLoader

最后

很多容器都是打破Jvm的双亲委派机制来实现的，比如tomcat，如果不打破的话根据无法部署多个项目，所以打破双亲委派机制也是一种业务场景的需要。而通过了解和实现打破双亲委派机制来学习tomcat的实现，有效提升对jvm底层的实现了解。

参考文章：

https://www.jianshu.com/p/7706a42ba200

https://cwiki.apache.org/confluence/display/tomcat/FAQ

https://blog.csdn.net/lisheng5218/article/details/111475536