自定义ClassLoader

1.  将Message.java拷贝到 /Users/sunny/test目录下（和NVIDIAClassLoader类MESSAGE_CLASS_PATH指定的路径一致），并编译生成字节码文件(.class)。

Message.java:

package com.nvidia.util;

public class Message {
    public void send() {
        System.out.println("www.nvidia.com");
    }
}

2.编写自定义ClassLoader类。

NVIDIAClassLoader.java:

package com.nvidia.util;

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

public class NVIDIAClassLoader extends ClassLoader{
    private static final String MESSAGE_CLASS_PATH = File.separator +"Users" + File.separator + "sunny" + File.separator + "test" + File.separator+"Message.class";
    public Class<?> loadData(String className) throws Exception{
        byte[] data = this.loadClassData();
        if(data != null) {
            return super.defineClass(className, data, 0, data.length);
        }
        return null;
    }
    private byte[] loadClassData() throws Exception{
        InputStream input = null;
        ByteArrayOutputStream bos = null;
        byte data[] = null;
        try {
            bos = new ByteArrayOutputStream();
            input = new FileInputStream(new File(MESSAGE_CLASS_PATH));
            input.transferTo(bos);
            data = bos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if(input != null) {
                input.close();
            }
            if(bos != null) {
                bos.close();
            }
        }
        return data;
    }
}

3.测试类。

JavaReflectDemo:

package com.nvidia.demo;

import com.nvidia.util.NVIDIAClassLoader;
import java.lang.reflect.Method;

public class JavaReflectDemo {
    public static void main(String[] args) throws Exception {
        NVIDIAClassLoader classloader = new NVIDIAClassLoader();
        Class<?> cls = classloader.loadData("com.nvidia.util.Message");
        //System.out.println(cls);
        Object object = cls.getDeclaredConstructor().newInstance();
        Method method = cls.getDeclaredMethod("send");
        method.invoke(object);
        System.out.println(cls.getClassLoader());
        System.out.println(cls.getClassLoader().getParent());
        System.out.println(cls.getClassLoader().getParent().getParent());
    }
}

运行结果：

www.nvidia.com
com.nvidia.util.NVIDIAClassLoader@5451c3a8
jdk.internal.loader.ClassLoaders$AppClassLoader@55054057
jdk.internal.loader.ClassLoaders$PlatformClassLoader@2c7b84de