jvm介绍 day4

获取ClassLoader的途径

1. 获取当前类的ClassLoader
   clazz.getCLassLoader();
2. 获取当前线程上下文的ClassLoader
   Thread.currentThread().getContextClassLoader()
3. 获得系统的ClassLoader
   ClassLoader.getSystemClassLoader()
4. 获得调用者的ClassLoader
   DirverManager.getCallerClassLoader()

/**
 * @author NingXioaoming
 * @createTime 2019/12/16 13:56
 * @description
 */
public class MyTest14 {
    public static void main(String[] args) {
         ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();

        System.out.println(contextClassLoader);


        final Class<MyTest14> clazz = MyTest14.class;
        System.out.println(clazz.getClassLoader());//由系统类加载sun.misc.Launcher$AppClassLoader@18b4aac2
        final Class clazz1 = String.class;
        System.out.println(clazz1.getClassLoader()); //由根类或者启动类加载的，null    因为String位于rt.jar包中
    }
}

数组对是在程序运行期间由jvm创建的而不是由类加载器加载的，调用数组的getClassLoader得到的结果是数组中对象所返回的结果，如果数组中的元素是原生类型（int，double）的话调用getClassLoader是没有类加载器的

自定义类加载器（[loader1]）和系统类加载器（sun.misc.Launcher$AppClassLoader@18b4aac2）的区别

package com.mobius.vision.jvm.classloader;

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

/**
 * @author NingXioaoming
 * @createTime 2019/12/21 10:43
 * @description
 */
public class MyTest16 extends ClassLoader {

    private String classLoaderName;
    private final String fileExtension = ".class";

    private String path;

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

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

    public MyTest16(ClassLoader parent,String classLoaderName){
        super(parent);//显示指定该类加载器的父加载器
        this.classLoaderName = classLoaderName;
//        System.out.println("nihap111");
    }

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

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

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

        className = className.replace(".","\\");

        try {
            is = new FileInputStream(new File(this.path+className+this.fileExtension));
            baos = new ByteArrayOutputStream();
            int ch;
            while (-1 !=(ch = is.read())){
                baos.write(ch);
            }
            data = baos.toByteArray();
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            try {
                is.close();
                baos.close();
            }catch (Exception e){
                e.printStackTrace();
            }
        }



        return data;
    }

/*    public static void test(ClassLoader classLoader) throws Exception{

        final Class<?> clazz = classLoader.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
        final Object object = clazz.newInstance();
        System.out.println(object);
//        System.out.println(object.getClass().getClassLoader());
//        System.out.println(clazz.getClassLoader());
    }*/

    public static void main(String[] args) throws Exception {
        final MyTest16 loader1 = new MyTest16("loader1");
        loader1.setPath("C:\\Users\\user\\Desktop\\");
        final Class<?> clazz = loader1.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
        System.out.println("class:"+clazz.hashCode());
        final Object object = clazz.newInstance();
        System.out.println(object.getClass().getClassLoader());

//        test(loader1);
        System.out.println("************************");
        final MyTest16 loader2 = new MyTest16("loader2");
        loader2.setPath("C:\\Users\\user\\Desktop\\");
        final Class<?> clazz2 = loader2.loadClass("com.mobius.vision.jvm.classloader.MyTest1");
        System.out.println("class:"+clazz2.hashCode());
        final Object object2 = clazz2.newInstance();
        System.out.println(object2.getClass().getClassLoader());

    }
}

自定义类加载器输出的结果

• hashCode值是不同的 说明加载了两次 类被实例化了两次。

findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader1
class:325040804
[loader1]
************************
findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader2
class:621009875
[loader2]

Process finished with exit code 0

系统类加载器的输出结果

• 两次输出相同的hashCode值 说明两次第二次new出来的实例还是上一个实例化出来的对象
• 出现这个原因是因为 在加载类的时候 调用了loadClass方法，这个方法首先就是执行findLoaderClass(String s)：这个方法就是寻找已经被加载的类来检查这个类是否已经被加载过了。

class:2133927002
sun.misc.Launcher$AppClassLoader@18b4aac2
************************
class:2133927002
sun.misc.Launcher$AppClassLoader@18b4aac2

Process finished with exit code 0

将代码中第二次new的时候改成MyTest16 loader2 = new MyTest16(loader1,"loader2");变一个构造方法 将loader1的类加载器作为loader2的父加载器进行加载的输出结果

• hashCode值相同
• 由于类加载的机制是双亲委托机制，所以

findClass invoked:com.mobius.vision.jvm.classloader.MyTest1
class loader name:loader1
class:325040804
[loader1]
************************
class:325040804
[loader1]