JVM 自定义类加载器

一、创建自定义类加载器

package com.example.jvm.classloader;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;

/**
 * Created by Think on 2019/6/9.
 */
public class MyTest16  extends  ClassLoader{

    private String classLoadName;

    private final String fileExtension = ".class";

    public MyTest16(String classLoadName){
        super(); //将系统类加载器当做该类加载器的父加载器
        this.classLoadName = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //显示指定该类加载器的父加载器器
        this.classLoadName = classLoadName;
    }

    @Override
    public String toString() {
        return "[" + this.classLoadName + "]";
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] data = this.loadClassData(classLoadName);
        return  this.defineClass(classLoadName,data, 0, data.length);
    }

    private byte[] loadClassData(String name){
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        try{
            this.classLoadName = this.classLoadName.replace(".","//");
            is = new FileInputStream(new File(name + this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch = 0;
            while ( -1 != (ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();

        }catch (Exception ex){
            ex.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception ex){
                ex.printStackTrace();
            }
        }

        return  data;

    }

    public static void main(String[] args)  throws Exception{
        MyTest16 loader1 = new MyTest16("loader1");
        test(loader1);
    }


    public static  void test(ClassLoader classLoader) throws Exception {
        Class<?> clazz = classLoader.loadClass("com.example.jvm.classloader.MyTest1");
        Object object = clazz.newInstance();
        System.out.println(object);

    }
}

　　打印结果

com.example.jvm.classloader.MyTest1@1540e19d

　　

二、完善上一个实例创建的类加载器

命名空间：

每个类加载器都有自己的命名空间，命名空间由该加载器及所有父加载器所加载的类组成。

在同一个命名空间中，不会出现类的完整名字（包括类的包名）相同的两个类。

在不同的命名空间中，有可能会出现类的完整名字（包括类的包名）相同的两个类。

public class MyTest16  extends  ClassLoader{

    private String className;

    //目录
     private String path;

    private final String fileExtension = ".class";

    public MyTest16(String classLoadName){
        super(); //将系统类加载器当做该类加载器的父加载器
        this.className = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //显示指定该类加载器的父加载器器
        this.className = classLoadName;
    }

    public void setPath(String path) {
        this.path = path;
    }

    @Override
    public String toString() {
        return "[" + this.className + "]";
    }

    @Override
    protected Class<?> findClass(String clasName) throws ClassNotFoundException {
        System.out.println("findClass invoked:" + clasName);
        System.out.println("class loader name: " + this.className);
        byte[] data = this.loadClassData(clasName);
        return  this.defineClass(clasName,data, 0, data.length);
    }

    private byte[] loadClassData(String className){
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        try{
            className = className.replace(".","//");
            System.out.println("className11:" +this.className);
            is = new FileInputStream(new File(this.path + className + this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch = 0;
            while ( -1 != (ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();

        }catch (Exception ex){
            ex.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception ex){
                ex.printStackTrace();
            }
        }

        return  data;

    }

    public static void main(String[] args)  throws Exception{
        MyTest16 loader1 = new MyTest16("loader1");
        //loader1.setPath("D:/workspace/study/ jvm_demo/build/classes/java/main/");
        loader1.setPath("D:/temp/"); // 将 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件移动到 D:/temp/com/example/jvm/classloader目录下
        Class<?> clazz = loader1.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz.hashCode());
        Object object = clazz.newInstance();
        System.out.println(object);
        //System.out.println(object.getClass().getClassLoader());


        MyTest16 loader2 = new MyTest16("loader2");
        loader2.setPath("D:/temp/");
        Class<?> clazz2 = loader2.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class2:" + clazz2.hashCode());
        Object object2 = clazz2.newInstance();
        System.out.println(object2);
 
    }



}

　　将 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件移动到 D:/temp/com/example/jvm/classloader目录下

打印结果：

findClass invoked:com.example.jvm.classloader.MyTest1
class loader name: loader1
className11:loader1
class:21685669
com.example.jvm.classloader.MyTest1@7f31245a

findClass invoked:com.example.jvm.classloader.MyTest1
class loader name: loader2
className11:loader2
class2:1173230247
com.example.jvm.classloader.MyTest1@330bedb4

　　1）loader1 和loader2 是两个实例，构成了两个不同的命名空间。

此时使用的是自定义类加载器。两个类的hascode值是不一样的。

2）如果将D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件不移除，

打印的结果：

class:356573597
com.example.jvm.classloader.MyTest1@677327b6

class2:356573597
com.example.jvm.classloader.MyTest1@14ae5a5

　　两个列的hasCode值是一样的，是同一个值。使用的类加载器是APP类加载器。 因为测试，父加载器会加载D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader的MyTest1.class 文件，加载到了。自定义类加载器就不需要在加载了。

三、在二的基础上进行改造

package com.example.jvm.classloader;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;

/**
 * Created by Think on 2019/6/9.
 */
public class MyTest16  extends  ClassLoader{

    private String className;

    //目录
     private String path;

    private final String fileExtension = ".class";

    public MyTest16(String classLoadName){
        super(); //将系统类加载器当做该类加载器的父加载器
        this.className = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //显示指定该类加载器的父加载器器
        this.className = classLoadName;
    }

    public void setPath(String path) {
        this.path = path;
    }

    @Override
    public String toString() {
        return "[" + this.className + "]";
    }

    @Override
    protected Class<?> findClass(String clasName) throws ClassNotFoundException {
        System.out.println("findClass invoked:" + clasName);
        System.out.println("class loader name: " + this.className);
        byte[] data = this.loadClassData(clasName);
        return  this.defineClass(clasName,data, 0, data.length);
    }

    private byte[] loadClassData(String className){
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        try{
            className = className.replace(".","//");
            //System.out.println("className:" +this.className);
            is = new FileInputStream(new File(this.path + className + this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch = 0;
            while ( -1 != (ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();

        }catch (Exception ex){
            ex.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception ex){
                ex.printStackTrace();
            }
        }

        return  data;

    }

    public static void main(String[] args)  throws Exception{
        MyTest16 loader1 = new MyTest16("loader1");
        //loader1.setPath("D:/workspace/study/ jvm_demo/build/classes/java/main/");
        loader1.setPath("D:/temp/"); //删除 将 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件移动到 D:/temp/com/example/jvm/classloader目录下
        Class<?> clazz = loader1.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz.hashCode());
        Object object = clazz.newInstance();
        System.out.println(object);
        System.out.println();

        MyTest16 loader2 = new MyTest16(loader1,"loader2");
        loader2.setPath("D:/temp/");
        Class<?> clazz2 = loader2.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz2.hashCode());
        Object object2 = clazz2.newInstance();
        System.out.println(object2);

    }



}

　　删除工程里的MyTest1.class，保留D: emp 目录下的MyTest1.class 文件。

设置MyTest16 loader2 = new MyTest16(loader1,"loader2");

输出结果如下：

findClass invoked:com.example.jvm.classloader.MyTest1
class loader name: loader1
class:21685669
com.example.jvm.classloader.MyTest1@7f31245a

class:21685669
com.example.jvm.classloader.MyTest1@6d6f6e28

　　hashCode值都是21685669

 MyTest1由自定义loader1加载，

loader2委托loader1加载，loader1已经加载过了MyTest类，所以loader2不需要加载了。

四、在三的基础上进行改造

 public static void main(String[] args)  throws Exception{
        MyTest16 loader1 = new MyTest16("loader1");
        //loader1.setPath("D:/workspace/study/ jvm_demo/build/classes/java/main/");
        loader1.setPath("D:/temp/"); 
        Class<?> clazz = loader1.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz.hashCode());
        Object object = clazz.newInstance();
        System.out.println(object);
        System.out.println();

        MyTest16 loader2 = new MyTest16(loader1,"loader2");
        loader2.setPath("D:/temp/");
        Class<?> clazz2 = loader2.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz2.hashCode());
        Object object2 = clazz2.newInstance();
        System.out.println(object2);
        System.out.println();

        MyTest16 loader3 = new MyTest16(loader2,"loader3");
        loader3.setPath("D:/temp/");
        Class<?> clazz3 = loader3.loadClass("com.example.jvm.classloader.MyTest1"); //
        System.out.println("class:" + clazz3.hashCode());
        Object object3 = clazz3.newInstance();
        System.out.println(object3);
        System.out.println();



    }

　删除工程下的的MyTest1.class,保留D: emp 下的MyTest1.class文件，　打印结果

findClass invoked:com.example.jvm.classloader.MyTest1
class loader name: loader1
class:21685669
com.example.jvm.classloader.MyTest1@7f31245a

class:21685669
com.example.jvm.classloader.MyTest1@6d6f6e28

class:21685669
com.example.jvm.classloader.MyTest1@135fbaa4

　　

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原文地址：https://www.cnblogs.com/linlf03/p/10995208.html