JVM 类加载器命名空间深度解析与实例分析

时间：2019-06-15 20:25:03 阅读：108 评论：0 收藏：0 [点我收藏+]

一、创建Sample

1、创建实例

public class MyPerson {

    private MyPerson myPerson;

    public void  setMyPerson(Object obj){
        this.myPerson = (MyPerson)obj;
    }
}

2、创建测试类

public class MyTest20 {

    public static void main(String[] args) throws Exception {
        MyTest16 loader1 = new MyTest16("loader1");
        MyTest16 loader2 = new MyTest16("loader2");

        Class<?> clazz1 = loader1.loadClass("com.example.jvm.classloader.MyPerson");

        Class<?> clazz2 = loader2.loadClass("com.example.jvm.classloader.MyPerson");

        System.out.println( clazz1 == clazz2);

        Object object1 = clazz1.newInstance();
        Object object2 = clazz2.newInstance();

        Method method = clazz1.getMethod("setMyPerson", Object.class);
        method.invoke(object1, object2);



    }
}

3、MyTest16类和之前的一致

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;

    }

  


}

View Code

打印结果

true

二、修改Sample

在一的基础上修改代码，设置path

loader1.setPath("D:/temp/");

loader2.setPath("D:/temp/");

public class MyTest21 {

    public static void main(String[] args) throws Exception {
        MyTest16 loader1 = new MyTest16("loader1");
        MyTest16 loader2 = new MyTest16("loader2");

        loader1.setPath("D:/temp/");
        loader2.setPath("D:/temp/");

        Class<?> clazz1 = loader1.loadClass("com.example.jvm.classloader.MyPerson");

        Class<?> clazz2 = loader2.loadClass("com.example.jvm.classloader.MyPerson");

        System.out.println( clazz1 == clazz2);

        Object object1 = clazz1.newInstance();
        Object object2 = clazz2.newInstance();

        Method method = clazz1.getMethod("setMyPerson", Object.class);
        method.invoke(object1, object2);



    }
}

　　然后将class文件所在的build下的com文件夹拷贝到D:\temp 下，删除build下的MyPerson.class 文件

打印结果：

findClass invoked:com.example.jvm.classloader.MyPerson
class loader name: loader1
findClass invoked:com.example.jvm.classloader.MyPerson
class loader name: loader2
false
Exception in thread "main" java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.example.jvm.classloader.MyTest21.main(MyTest21.java:27)
Caused by: java.lang.ClassCastException: com.example.jvm.classloader.MyPerson cannot be cast to com.example.jvm.classloader.MyPerson
	at com.example.jvm.classloader.MyPerson.setMyPerson(MyPerson.java:11)
	... 5 more

　　结果分析，loader1和loader2分别加载了MyPerson，分别给MyPerson分配了内存空间，如下图：

技术图片