手写实现JDK的动态代理

Person接口

package com.zhoucong.proxy.jdk;

public interface Person {

//    寻找真爱
    void findlove();

}

人物实现类

package com.zhoucong.proxy.jdk;

public class Zhangsan implements Person{

    @Override
    public void findlove() {
        System.out.println("我叫张三，性别女，我找对象的要求如下：\n");
        System.out.println("高富帅");
        System.out.println("有房有车");
        System.out.println("身高180cm以上，体重70kg");
    }

}

自定义InvocationHandler接口

package com.zhoucong.custom;

import java.lang.reflect.Method;

public interface GPInvocationHandler {

    public Object invoke(Object proxy, Method method, Object[] args)
            throws Throwable;

}

代理类

package com.zhoucong.custom;

import java.lang.reflect.Method;

import com.zhoucong.proxy.jdk.Person;

public class GPMeiPo implements GPInvocationHandler{

    private Person target;

//  获取被代理人的个人资料
    public Object getInstance(Person target) throws Exception {
        this.target = target;
        Class clazz = target.getClass();
        System.out.println("被代理对象的class是："+ clazz);
        return GPProxy.newProxyInstance(new GPClassLoader(), clazz.getInterfaces(), this);

    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
         System.out.println("我是媒婆!");
         System.out.println("开始信息海选");
         System.out.println("-------------");
         method.invoke(this.target, args);
         System.out.println("-------------");
         System.out.println("如果合适的话，就准备办事");
         return null;
    }

}

生成代理对象类

package com.zhoucong.custom;

import java.io.File;
import java.io.FileWriter;
import java.lang.reflect.Constructor;
import java.lang.reflect.Method;

import javax.tools.JavaCompiler;
import javax.tools.JavaCompiler.CompilationTask;
import javax.tools.StandardJavaFileManager;
import javax.tools.ToolProvider;

/**
 * 生成代理对象的代码
 *
 * @author ZhouCong
 *
 */
public class GPProxy {

    private static String ln = "\r\n";

    public static Object newProxyInstance(GPClassLoader loader, Class<?>[] interfaces, GPInvocationHandler h)
            throws IllegalArgumentException {
        try {
//        1.生成源代码
        String proxySrc = generateSrc(interfaces[0]);

//        2.将生成的源代码输出到磁盘，保存.java文件
            String path = GPProxy.class.getResource("").getPath();
            File f = new File(path + "$Proxy0.java");
            FileWriter fw = new FileWriter(f);
            fw.write(proxySrc);
            fw.flush();
            fw.close();

//        3.编译源代码，并且生成.class文件
        JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
        StandardJavaFileManager manager = compiler.getStandardFileManager(null, null, null);
        Iterable iterable = manager.getJavaFileObjects(f);
        CompilationTask task = compiler.getTask(null, manager, null, null, null, iterable);
        task.call();
        manager.close();

//        4.将class文件中的内容，动态加载到JVM中
        Class<?> proxyClass = loader.findClass("$Proxy0");
        Constructor<?> constructor = proxyClass.getConstructor(GPInvocationHandler.class);
        f.delete(); //删除生成的Java文件

//        5.返回被代理后的代理对象
        return constructor.newInstance(h);

        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 生成代码
     * @param interfaces
     * @return
     */
    private static String generateSrc(Class<?> interfaces) {

        StringBuffer src = new StringBuffer();
        src.append("package com.zhoucong.custom;" + ln);
        src.append("import java.lang.reflect.Method;" + ln);
        src.append("public class $Proxy0 implements " + interfaces.getName() + "{" + ln);
        src.append("GPInvocationHandler h;" + ln);
        src.append("public $Proxy0(GPInvocationHandler h){" + ln);
        src.append("this.h = h;" + ln);
        src.append("}" + ln);

        for (Method m : interfaces.getMethods()) {
            src.append("public " + m.getReturnType().getName() + " " + m.getName() + "() {" + ln);
            src.append("try {" + ln);
            src.append("Method m = " + interfaces.getName() + ".class.getMethod(\"" + m.getName() +"\",new Class[]{});" + ln);
            src.append("this.h.invoke(this,m,null);" + ln);
            src.append("} catch(Throwable e){e.printStackTrace();}" + ln);
            src.append("}" + ln);
        }

        src.append("}");
        return src.toString();
    };
}

自定义ClassLoader类

package com.zhoucong.custom;

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

/**
 * 代码生成、编译、重新动态load到JVM中
 * @author ZhouCong
 *
 */
public class GPClassLoader extends ClassLoader{

    private File baseDir;

    public GPClassLoader() {
        String path = GPClassLoader.class.getResource("").getPath();
        this.baseDir = new File(path);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        String className = GPClassLoader.class.getPackage().getName() + "." + name;
        if(baseDir != null) {
            File classFile = new File(baseDir,name.replaceAll("\\.", "/")+".class");
            if(classFile.exists()) {
                FileInputStream in = null;
                ByteArrayOutputStream out = null;
                try {
                    in = new FileInputStream(classFile);
                     out = new ByteArrayOutputStream();
                    byte [] buff = new byte[1024];
                    int len;
                    while((len = in.read(buff)) != -1) {
                        out.write(buff,0,len);
                    }
                    return defineClass(className,out.toByteArray(),0,out.size());

                }catch (Exception e) {
                    e.printStackTrace();
                }finally {

                    if(null != in) {
                        try {
                            in.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                    if(null != out) {
                        try {
                            out.close();
                        } catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
//                    删除class文件
                    classFile.delete();
                }
            }

        }

        return null;
    }

}

运行类

package com.zhoucong.proxy.jdk;

import com.zhoucong.custom.GPMeiPo;

public class TestFindLove {

    public static void main(String[] args) {

        try {

            Person obj = (Person) new GPMeiPo().getInstance(new Zhangsan());
            System.out.println(obj.getClass());
            obj.findlove();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

原文地址：https://www.cnblogs.com/itzhoucong/p/12144339.html