Java ClassLoader更改 [英] Java ClassLoader change

问题描述

我有一些课 A ：

public class A {
    public A(String str) {
        System.out.println("Create A instance: " + str);
    }

    public void methodA() {
        System.out.println("#methodA1()");
    }
}

我的类加载器实现：

public class MyClassLoader extends ClassLoader {
    public MyClassLoader() {    
        super();
    }

    @Override
    public synchronized Class<?> loadClass(String name) 
            throws ClassNotFoundException {

        System.out.println("Load: " + name);

        return super.loadClass(name);
    }
}

现在我尝试更改当前的默认类加载器thread：

And now I try to change default class loader in current thread:

import java.util.ArrayList;
import java.util.List;

public class ChangeLoaderTest {
    public static void main(String[] args) {
        // Save class loader so that we can restore later.
        ClassLoader oldLoader = Thread.currentThread().getContextClassLoader();

        MyClassLoader newLoader = new MyClassLoader();
        try {
            // Set new classloader.
            Thread.currentThread().setContextClassLoader(newLoader);

            // My class.
            A a = new A("1");
            a.methodA();

            // Standard Java class.
            List<Integer> list = new ArrayList<Integer>();
            list.add(2);
            list.add(3);
        } finally {
            // Restore.
            Thread.currentThread().setContextClassLoader(oldLoader);
        }
    }
}

和 ChangeLoaderTest 输出：

Create A instance: 1
#methodA1()

没有人

Load: ...

为什么？如何将 ClassLoader 更改为某个帖子？

Why? How I can change ClassLoader into some thread?

推荐答案

作为Marko Topolnik指出上下文类加载器供框架使用。要自己使用类加载器，必须调用 loadClass（somepackage.A），然后使用反射API创建A的新实例（ Class.newInstance（））。

As Marko Topolnik points out the context classloader is for use by frameworks. To use the classloader yourself you have to call loadClass("somepackage.A") and then use the reflection API to create a new instance of A (Class.newInstance()).

由于调用代码不知道，您无法直接在源代码中使用A或其方法A - 它使用不同的类加载器。可以由普通类加载器加载的A的接口或基类可用于避免反射。

You wont be able to use A or its methods in your source directly since the calling code does not know A - it uses a different classloader. An interface or baseclass of A that can be loaded by the normal classloader can be used to avoid reflection.

interface AIF{
        void someMethod();
 }
class A implements AIF{
      public void someMethod(){}
 }


public void test(){
     MyLoader loader = new MyLoader();
     Class cla = loader.loadClass("A");
     AIF a = (AIF) cla.newInstance();
     a.someMethod();

 }

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