自定义类加载器并打破双亲委派机制

前言

今天我们来自定义一个类加载器

第一步

继承ClassLoader类

public class MyClassLoader  extends  ClassLoader

第二步

重写findClass方法

public Class<?> findClass(String name) throws ClassNotFoundException{
        try {
            byte[] data = loadByte(name);
            return  defineClass(name, data ,0,data.length);
        }catch (Exception e){
            e.printStackTrace();
            throw new ClassNotFoundException();
        }
    }

其中defineClass类是核心，需要传入类名以及类的路径data，所以需要对类进行一个类路径的获取

private byte[] loadByte(String name) throws Exception{
        name = name.replaceAll("\\.","/");
        FileInputStream fis = new FileInputStream("/" + name + ".class");
        int len = fis.available();
        byte[] data = new byte[len];
        fis.read(data);
        fis.close();
        return data;
    }

最后写一个main方法

public static void main(String[] args)  throws Exception{
        MyClassLoaderTest.MyClassLoader classLoader = new MyClassLoaderTest.MyClassLoader("D:\test");
        Class clazz = classLoader.loadClass("User");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("sout",null);
        method.invoke(obj,null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }

其中我们将User类的class文件放到D:\test文件夹下，其中User类中有一个sout方法，方法中输出一句话证明我们调用成功了，总代码如下

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

public class MyClassLoader  extends  ClassLoader{


    private byte[] loadByte(String name) throws Exception{
        name = name.replaceAll("\\.","/");
        FileInputStream fis = new FileInputStream("/" + name + ".class");
        int len = fis.available();
        byte[] data = new byte[len];
        fis.read(data);
        fis.close();
        return data;
    }
    public Class<?> findClass(String name) throws ClassNotFoundException{
        try {
            byte[] data = loadByte(name);
            return  defineClass(name, data ,0,data.length);
        }catch (Exception e){
            e.printStackTrace();
            throw new ClassNotFoundException();
        }
    }

    public static void main(String[] args)  throws Exception{
        MyClassLoaderTest.MyClassLoader classLoader = new MyClassLoaderTest.MyClassLoader("D:\test");
        Class clazz = classLoader.loadClass("User");
        Object obj = clazz.newInstance();
        Method method = clazz.getDeclaredMethod("sout",null);
        method.invoke(obj,null);
        System.out.println(clazz.getClassLoader().getClass().getName());
    }
}

运行结果如下

但是每一次都会使用自定义的类加载器的父类去加载文件，所以我们要打破双亲委派机制，我们找到jdk自带的类ClassLoader下的loadClass方法，将他复制到我们的项目中

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();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                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;
        }
    }

其中这几句代码的作用就是实现了双亲委派机制，很明显他的意思是如果父加载器不为空就调用父类加载器，如果父类加载器为空就使用引导类BootstrapClass加载器加载，我们将这端代码删除就可以打破双亲委派机制了。