手写实现JDK动态代理

水深无声 2021-07-24 14:13 349阅读 0赞

**一 参考**

[https://www.jianshu.com/p/af271ad4e056][https_www.jianshu.com_p_af271ad4e056]

**二 代码位置**

[https://gitee.com/cakin24/javatest/tree/master/src/main/java/GPProxy][https_gitee.com_cakin24_javatest_tree_master_src_main_java_GPProxy]

**三 代码**

1 Customer

package GPProxy;
    
    public class Customer implements Person{
        public void findLove(){
            System.out.println("高富帅");
            System.out.println("身高180cm");
            System.out.println("有6块腹肌");
        }
    }

2 CustomMeipo

package GPProxy;
    
    import java.lang.reflect.Method;
    public class CustomMeipo implements  GPInvocationHandler {
    
        //被代理的对象，把引用给保存下来
        private Person target;
    
        public Object getInstance(Person target) throws Exception{
            this.target = target;
            Class<?> clazz = target.getClass();
    
            //下半截，老师深入底层来给大家讲解字节码是如何重组的
            //用来生成一个新的对象（字节码重组来实现）
            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：我要给你找对象，现在已经拿到你的需求");
            System.out.println("开始物色");
            method.invoke(this.target,args);
            System.out.println("如果合适的话，就准备办事");
            return  null;
        }
    }

3 CustomPorxyTest

package GPProxy;
    
    public class CustomPorxyTest {
        public static void main(String[] args) {
            try {
                Person obj = (Person)new CustomMeipo().getInstance(new Customer());
                System.out.println(obj.getClass());
                obj.findLove();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

4 GPClassLoader

package GPProxy;
    
    import java.io.ByteArrayOutputStream;
    import java.io.File;
    import java.io.FileInputStream;
    import java.io.IOException;
    
    public class GPClassLoader extends ClassLoader{
        private File classPathFile;
        public GPClassLoader(){
            String classPath = GPClassLoader.class.getResource("").getPath();
            this.classPathFile = new File(classPath);
        }
    
        @Override
        protected Class<?> findClass(String name) throws ClassNotFoundException {
            String className = GPClassLoader.class.getPackage().getName() + "." + name;
    
            if(classPathFile != null){
                File classFile = new File(classPathFile,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(out != null){
                            try {
                                out.close();
                            } catch (IOException e) {
                                e.printStackTrace();
                            }
                        }
                    }
                }
            }
            return null;
        }
    }

5 GPInvocationHandler

package GPProxy;
    
    import java.lang.reflect.Method;
    public interface GPInvocationHandler {
        public Object invoke( Object proxy, Method method, Object[] args) throws Throwable;
    }

6 GPProxy

package GPProxy;
    
    import javax.tools.JavaCompiler;
    import javax.tools.StandardJavaFileManager;
    import javax.tools.ToolProvider;
    import java.io.File;
    import java.io.FileWriter;
    import java.lang.reflect.Constructor;
    import java.lang.reflect.Method;
    
    public class GPProxy {
        public static final String ln = "\r\n";
        public static Object newProxyInstance(GPClassLoader classLoader,Class<?> [] interfaces,GPInvocationHandler h){
            try {
                //1、动态生成源代码.java文件
                String src = generateSrc(interfaces);
    
                //2、Java文件输出磁盘
                String filePath = GPProxy.class.getResource("").getPath();
                System.out.println(filePath);
                File f = new File(filePath + "$Proxy0.java");
                FileWriter fw = new FileWriter(f);
                fw.write(src);
                fw.flush();
                fw.close();
    
                //3、把生成的.java文件编译成.class文件
                JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
                StandardJavaFileManager manage = compiler.getStandardFileManager(null,null,null);
                Iterable iterable = manage.getJavaFileObjects(f);
    
                JavaCompiler.CompilationTask task = compiler.getTask(null,manage,null,null,null,iterable);
                task.call();
                manage.close();
    
                //4、编译生成的.class文件加载到JVM中来
                Class proxyClass =  classLoader.findClass("$Proxy0");
                Constructor c = proxyClass.getConstructor(GPInvocationHandler.class);
    //          f.delete();
    
                //5、返回字节码重组以后的新的代理对象
                return c.newInstance(h);
            }catch (Exception e){
                e.printStackTrace();
            }
            return null;
        }
    
        private static String generateSrc(Class<?>[] interfaces){
            StringBuffer sb = new StringBuffer();
            sb.append("package GPProxy;" + ln);
    //        sb.append("import GPProxy.*;" + ln);
            sb.append("import GPProxy.Person;" + ln);
            sb.append("import java.lang.reflect.Method;" + ln);
            sb.append("public class $Proxy0 implements " + interfaces[0].getName() + "{" + ln);
            sb.append("GPInvocationHandler h;" + ln);
            sb.append("public $Proxy0(GPInvocationHandler h) { " + ln);
            sb.append("this.h = h;");
            sb.append("}" + ln);
            for (Method m : interfaces[0].getMethods()){
                sb.append("public " + m.getReturnType().getName() + " " + m.getName() + "() {" + ln);
                sb.append("try{" + ln);
                sb.append("Method m = " + interfaces[0].getName() + ".class.getMethod(\"" + m.getName() + "\",new Class[]{});" + ln);
                sb.append("this.h.invoke(this,m,null);" + ln);
                sb.append("}catch(Throwable e){" + ln);
                sb.append("e.printStackTrace();" + ln);
                sb.append("}");
                sb.append("}");
            }
            sb.append("}" + ln);
            return sb.toString();
        }
    }

7 Person

package GPProxy;
    
    public interface Person {
        public void findLove();
    }

**四 测试结果**

/F:/Javatest/target/classes/GPProxy/
    F:\Javatest\target\classes\GPProxy\$Proxy0.java:4: 错误: 找不到符号
    public class $Proxy0 implements GPProxy.Person{
                                           ^
      符号:   类 Person
      位置: 类 GPProxy
    F:\Javatest\target\classes\GPProxy\$Proxy0.java:10: 错误: 找不到符号
    Method m = GPProxy.Person.class.getMethod("findLove",new Class[]{});
                      ^
      符号:   类 Person
      位置: 类 GPProxy
    2 个错误
    class GPProxy.$Proxy0
    我是媒婆：我要给你找对象，现在已经拿到你的需求
    开始物色
    高富帅
    身高180cm
    有6块腹肌
    如果合适的话，就准备办事

**五 分析**

测试结果有两个错误，但不影响结果，这两个错误主要出现在生成的代码上，生成代码如下：

package GPProxy;
    
    import GPProxy.Person;
    import java.lang.reflect.Method;
    
    public class $Proxy0 implements GPProxy.Person {   // GPProxy.Person改为Person可规避这个错误
        GPInvocationHandler h;
    
        public $Proxy0( GPInvocationHandler h ) {
            this.h = h;
        }
    
        public void findLove() {
            try {
                // // GPProxy.Person改为Person可规避这个错误
                Method m = GPProxy.Person.class.getMethod("findLove", new Class[]{});
                this.h.invoke(this, m, null);
            } catch (Throwable e) {
                e.printStackTrace();
            }
        }
    }

将GPProxy.Person手动修改为Person可规避错误。

[https_www.jianshu.com_p_af271ad4e056]: https://www.jianshu.com/p/af271ad4e056
[https_gitee.com_cakin24_javatest_tree_master_src_main_java_GPProxy]: https://gitee.com/cakin24/javatest/tree/master/src/main/java/GPProxy