jvm雙親委派機制之打破雙親委派機制

在上一篇博客中介紹了一下如何書寫自定義加載器.傳送門
這一篇要介紹的就是自定義類加載器來打破雙親委派機制.
思路就是加載類的時候不走parent屬性來找,直接從指定的classpath中找

package com.bonc.jvm;

import java.io.FileInputStream;
import java.lang.reflect.Method;

/**
 * @Classname MyClassLoadTEst
 * @Description TODO
 * @Date 2020/6/14 16:31
 * @Created by sz
 */
public class MyClassLoadTest {

    static  class  MyClassLoad  extends  ClassLoader{

        private  String classPath;


        public MyClassLoad(String classPath){
            this.classPath=classPath;
        }

        protected Class<?> loadClass(String name, boolean resolve)
                throws ClassNotFoundException
        {
            synchronized (getClassLoadingLock(name)) {
                // First, check if the class has already been loaded
                Class<?> c = findLoadedClass(name);
                if (c == null) {
                    long t0 = System.nanoTime();
                    //只有自己寫的類走自己寫的類加載器，原來的類走自己類加載機制 這行代碼非常關鍵
                    if (!name.startsWith("com.bonc")){
                       c = super.loadClass(name,resolve);
                    }
                    if (c == null) {
                        // If still not found, then invoke findClass in order
                        // to find the class.
                        long t1 = System.nanoTime();
                        c = findClass(name);

                        // this is the defining class loader; record the stats
                        sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                        sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                        sun.misc.PerfCounter.getFindClasses().increment();
                    }
                }
                if (resolve) {
                    resolveClass(c);
                }
                return c;
            }
        }

        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name
                    + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;
        }
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                //defineClass將一個字節數組轉爲Class對象，這個字節數組是class文件讀取後最終的字節 數組。
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        MyClassLoad classLoader = new MyClassLoad("D:/tmp");
        Class clazz = classLoader.loadClass("com.bonc.jvm.User1");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("print", null);
        method.invoke(obj,null);
        System.out.println(clazz.getClassLoader().getClass().getName());
        System.out.println(clazz.getClassLoader().getParent().getClass().getName());
        System.out.println(clazz.getClassLoader().getParent().getParent().getClass().getName());
    }

}

代碼運行效果

在不打破雙親委派機制之前的代碼執行第一個的類加載器是AppClassLoader而不是自定義類加載器.因爲你的classpath目錄下已經存在一個Use1了.
那麼我們是否能夠重寫一個java.lang.String呢

package com.bonc.jvm;

import java.io.FileInputStream;
import java.lang.reflect.Method;

/**
 * @Classname MyClassLoadTEst
 * @Description TODO
 * @Date 2020/6/14 16:31
 * @Created by sz
 */
public class MyClassLoadTest {

    static  class  MyClassLoad  extends  ClassLoader{

        private  String classPath;


        public MyClassLoad(String classPath){
            this.classPath=classPath;
        }

        protected Class<?> loadClass(String name, boolean resolve)
                throws ClassNotFoundException
        {
            synchronized (getClassLoadingLock(name)) {
                // First, check if the class has already been loaded
                Class<?> c = findLoadedClass(name);
                if (c == null) {
                    long t0 = System.nanoTime();

                    if (c == null) {
                        // If still not found, then invoke findClass in order
                        // to find the class.
                        long t1 = System.nanoTime();
                        c = findClass(name);

                        // this is the defining class loader; record the stats
                        sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                        sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                        sun.misc.PerfCounter.getFindClasses().increment();
                    }
                }
                if (resolve) {
                    resolveClass(c);
                }
                return c;
            }
        }

        private byte[] loadByte(String name) throws Exception {
            name = name.replaceAll("\\.", "/");
            FileInputStream fis = new FileInputStream(classPath + "/" + name
                    + ".class");
            int len = fis.available();
            byte[] data = new byte[len];
            fis.read(data);
            fis.close();
            return data;
        }
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            try {
                byte[] data = loadByte(name);
                //defineClass將一個字節數組轉爲Class對象，這個字節數組是class文件讀取後最終的字節 數組。
                return defineClass(name, data, 0, data.length);
            } catch (Exception e) {
                e.printStackTrace();
                throw new ClassNotFoundException();
            }
        }
    }

    public static void main(String[] args) throws Exception {
        MyClassLoad classLoader = new MyClassLoad("D:/tmp");
        Class clazz = classLoader.loadClass("java.lang.String");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("print", null);
        method.invoke(obj,null);
        System.out.println(clazz.getClassLoader().getClass().getName());
        System.out.println(clazz.getClassLoader().getParent().getClass().getName());
        System.out.println(clazz.getClassLoader().getParent().getParent().getClass().getName());
    }

}

運行結果

結論:java爲了保護自己的核心類庫被篡權,核心的包下面的類不能被重寫