JVM 自定義類加載器

時間 2019-12-08

標籤 jvm 自定義加載欄目 Java 简体版

原文原文鏈接

1、建立自定義類加載器java

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(); //將系統類加載器當作該類加載器的父加載器
        this.classLoadName = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //顯示指定該類加載器的父加載器器
        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);

    }
}

　　打印結果jvm

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

2、完善上一個實例建立的類加載器ide

命名空間：測試

每一個類加載器都有本身的命名空間，命名空間由該加載器及全部父加載器所加載的類組成。ui

在同一個命名空間中，不會出現類的完整名字（包括類的包名）相同的兩個類。this

在不一樣的命名空間中，有可能會出現類的完整名字（包括類的包名）相同的兩個類。spa

public class MyTest16  extends  ClassLoader{

    private String className;

    //目錄
     private String path;

    private final String fileExtension = ".class";

    public MyTest16(String classLoadName){
        super(); //將系統類加載器當作該類加載器的父加載器
        this.className = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //顯示指定該類加載器的父加載器器
        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/"); // 將 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件移動到 D:/temp/com/example/jvm/classloader目錄下
        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);
 
    }



}

　　將 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader MyTest1文件移動到 D:/temp/com/example/jvm/classloader目錄下code

打印結果：blog

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 是兩個實例，構成了兩個不一樣的命名空間。ssl

此時使用的是自定義類加載器。兩個類的hascode值是不同的。

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

打印的結果：

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

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

　　兩個列的hasCode值是同樣的，是同一個值。使用的類加載器是APP類加載器。由於測試，父加載器會加載D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader的MyTest1.class 文件，加載到了。自定義類加載器就不須要在加載了。

3、在二的基礎上進行改造

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;

    //目錄
     private String path;

    private final String fileExtension = ".class";

    public MyTest16(String classLoadName){
        super(); //將系統類加載器當作該類加載器的父加載器
        this.className = classLoadName;
    }

    public MyTest16(ClassLoader parent, String classLoadName){
        super(parent); //顯示指定該類加載器的父加載器器
        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/"); //刪除 將 D:/workspace/study/ jvm_demo/build/classes/java/main/com/example/jvm/classloader  MyTest1文件移動到 D:/temp/com/example/jvm/classloader目錄下
        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);

    }



}

　　刪除工程裏的MyTest1.class，保留D:\temp 目錄下的MyTest1.class 文件。

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

輸出結果以下：

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由自定義loader1加載，

loader2委託loader1加載，loader1已經加載過了MyTest類，因此loader2不須要加載了。

4、在三的基礎上進行改造

 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();



    }

　刪除工程下的的MyTest1.class,保留D:\temp 下的MyTest1.class文件，　打印結果

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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