使用不同的加载器在JVM中两次加载一个类 [英] Loading a class twice in JVM using different loaders

问题描述

关于类加载的概念，我有一个问题。如何在JVM中两次加载.class文件。我也在写摘录的代码摘录，以完成此操作。.

I have one question regarding concepts of class loading. How to load a .class file twice in JVM. I am also writing an excerpt of code that I have written to accomplish this..

1）加载程序1的代码

1) Loader 1 code

public class MyClassLoader extends ClassLoader {

    public MyClassLoader(){
        super(MyClassLoader.class.getClassLoader());
    }

    public Class loadClass(String classname){
        try {
            return super.loadClass(classname);
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }
}

2）加载程序2代码

public class AnotherClassLoader extends ClassLoader {

    public AnotherClassLoader(){
        super(AnotherClassLoader.class.getClassLoader());
    }

    public Class loadClass(String classname){
        try {
            return super.loadClass(classname);
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }
}

3）现在，我正在使用这两个不同的类加载器。我想操作classA1 == newClassA应该返回false。这是代码：

3) Now i am loading a class named A using this two different class loaders. I suppose the operation classA1==newClassA should return false. Here is the code:

public static void main(String[] args) {
        MyClassLoader loader1 = new MyClassLoader();
        AnotherClassLoader newLoader = new AnotherClassLoader();
            System.out.println("Load with Custom Class Loader instance");
            Class classA1 = loader1.loadClass("com.hitesh.coreJava.A");
            System.out.println("Class Loader:::"+classA1.getClassLoader());
            Class newClassA = newLoader.loadClass("com.hitesh.coreJava.A");
            System.out.println("Class Loader:::"+newClassA.getClassLoader());
            System.out.println(classA1==newClassA);
            System.out.println(classA1.hashCode() + " , " + newClassA.hashCode());

    }

4）执行以上代码的结果：

4) Result of executing above code:

使用自定义类加载器实例加载
类加载器::: sun.misc.Launcher$AppClassLoader@11b86e7
类加载器:: ：sun.misc.Launcher$AppClassLoader@11b86e7
true
1641745，1641745

Load with Custom Class Loader instance Class Loader:::sun.misc.Launcher$AppClassLoader@11b86e7 Class Loader:::sun.misc.Launcher$AppClassLoader@11b86e7 true 1641745 , 1641745

请您解释一下

推荐答案

尝试一下

public class Test1 {

    static class TestClassLoader1 extends ClassLoader {

        @Override
        public Class<?> loadClass(String name) throws ClassNotFoundException {
            if (!name.equals("Test1")) {
                return super.loadClass(name);
            }
            try {
                InputStream in = ClassLoader.getSystemResourceAsStream("Test1.class");
                byte[] a = new byte[10000];
                int len  = in.read(a);
                in.close();
                return defineClass(name, a, 0, len);
            } catch (IOException e) {
                throw new ClassNotFoundException();
            }
        }
    }


    public static void main(String[] args) throws Exception {
        Class<?> c1 = new TestClassLoader1().loadClass("Test1");
        Class<?> c2 = new TestClassLoader1().loadClass("Test1");
        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c1 == c2);
    }
}

输出

class Test1
class Test1
false

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