思路

先上一张图

对目标对象进行代理，本质其实是底层动态生成一个类加载到 jvm 中，该类将实现原目标对象的所有接口，重写所有方法，通过动态代理扩展功能，其实也可以理解为是通过重写扩展功能，下面就通过这个思路手写一个动态代理

实现

jdk动态生成代理对象步骤

1. 创建目标对象
2. 目标对象通过反射获取所有接口
3. 动态生成Java源代码，实现所有接口
4. 通过IO流生成Java文件
5. 使用JavaCompiler类将Java文件编译成class文件
6. 将生成的新类加载到jvm
7. 获取该类构造器创建代理对象

MyInvocationHandler接口

package com.cml.proxy;import java.lang.reflect.Method;/*** @author yys* @version 1.0*/
public interface MyInvocationHandler {/*** 该方法进行加强原方法，需要被实现* @param proxy     生成的代理对象* @param method    方法对象* @param args      方法需要的参数* @return          返回Method执行的返回结果* @throws Throwable 抛出所有错误和异常*/Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}

MyClassLoader

package com.cml.proxy;import java.io.*;
import java.util.Objects;/*** @author yys* @version 1.0*/
public class MyClassLoader extends ClassLoader{private final File classPathFile;public MyClassLoader(){String classPath = Objects.requireNonNull(MyClassLoader.class.getResource("")).getPath();this.classPathFile = new File(classPath);}/*** 通过类名查找Class对象，必须重写，原方法默认抛出ClassNotFoundException异常* @param name* @return*/@Overrideprotected Class<?> findClass(String name) {String className = MyClassLoader.class.getPackage().getName() + "." + name;if(classPathFile  != null){File classFile = new File(classPathFile,name.replaceAll("\\.","/") + ".class");if(classFile.exists()){try {//将class文件转为字节数组byte[] bytes = file2ByteArray(classFile);//将字节数组转为Class对象返回return defineClass(className,bytes,0,bytes.length);} catch (Exception e) {e.printStackTrace();}}}return null;}/*** File对象转字节数组* @param file* @return* @throws Exception*/private byte[] file2ByteArray(File file) throws Exception {FileInputStream in = new FileInputStream(file);ByteArrayOutputStream out = new ByteArrayOutputStream();byte [] buff = new byte[1024];int len;while ((len = in.read(buff)) != -1){out.write(buff,0,len);}return out.toByteArray();}
}

MyProxy

package com.cml.proxy;import javax.tools.JavaCompiler;
import javax.tools.JavaFileObject;
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;
import java.util.Objects;/*** @author yys* @version 1.0*/
public class MyProxy {private static final String LN = "\r\n";public static Object newProxyInstance(MyClassLoader classLoader, Class<?>[] interfaces, MyInvocationHandler h) {try {//1.动态生成java源代码，字符串拼接String src = generateSrc(interfaces);System.out.println(src);//2.IO流生成java文件String filePath = Objects.requireNonNull(MyProxy.class.getResource("")).getPath();File f = new File(filePath + "$MyProxy0.java");System.out.println(f);FileWriter fw = new FileWriter(f);fw.write(src);fw.flush();fw.close();//3.编译java文件JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();StandardJavaFileManager manage = compiler.getStandardFileManager(null, null, null);Iterable<? extends JavaFileObject> iterable = manage.getJavaFileObjects(f);JavaCompiler.CompilationTask task = compiler.getTask(null, manage, null, null, null, iterable);task.call();manage.close();//4.加载到JVMClass<?> myProxyClass = classLoader.findClass("$MyProxy0");//5.获取动态生成的类的构造器Constructor<?> c = myProxyClass.getConstructor(MyInvocationHandler.class);//加载完class文件删除生成的java源文件f.delete();//6.返回构造的代理对象return c.newInstance(h);} catch (Exception e) {e.printStackTrace();}return null;}/*** 拼接java源代码*/private static String generateSrc(Class<?>[] interfaces) {StringBuilder sb = new StringBuilder();sb.append(MyProxy.class.getPackage()).append(";").append(LN);sb.append("import ").append(interfaces[0].getName()).append(";").append(LN);sb.append("import java.lang.reflect.*;" + LN);sb.append("public class $MyProxy0 implements ").append(interfaces[0].getName()).append("{").append(LN);sb.append("MyInvocationHandler h;" + LN);sb.append("public $MyProxy0(MyInvocationHandler h) { " + LN);sb.append("this.h = h;");sb.append("}" + LN);for (Method m : interfaces[0].getMethods()) {Class<?>[] params = m.getParameterTypes();StringBuilder paramNames = new StringBuilder();StringBuilder paramValues = new StringBuilder();StringBuilder paramClasses = new StringBuilder();for (int i = 0; i < params.length; i++) {Class<?> clazz = params[i];String type = clazz.getName();//参数变量名String paramName = "var" + i;//参数列表拼接类型和变量名paramNames.append(type).append(" ").append(paramName);//变量名拼接作为Object数组，作为invoke的参数paramValues.append(paramName);//拼接参数类型作为Class数组，用于反射查找对应的方法对象paramClasses.append(clazz.getName()).append(".class");if (i < params.length - 1) {paramNames.append(",");paramClasses.append(",");paramValues.append(",");}}sb.append("public ").append(m.getReturnType().getName()).append(" ").append(m.getName()).append("(").append(paramNames).append(") {").append(LN).append("try{" + LN).append("Method m = ").append(interfaces[0].getName()).append(".class.getMethod(\"").append(m.getName()).append("\",new Class[]{").append(paramClasses).append("});").append(LN).append(hasReturnValue(m.getReturnType()) ? "return (" + m.getReturnType().getName() + ")" : "").append("this.h.invoke(this,m,new Object[]{").append(paramValues).append("})").append(";").append(LN).append("}catch(Error er) {throw er; }").append(LN).append("catch(Throwable e){").append(LN).append("throw new UndeclaredThrowableException(e);" + LN).append("}").append(LN).append("}");}sb.append("}" + LN);return sb.toString();}/*** 判断方法是否有返回值* @param clazz* @return*/private static boolean hasReturnValue(Class<?> clazz) {return clazz != void.class;}}

测试

定义接口InterfaceOne

package com.cml.test;public interface InterfaceOne {String show(String s1, String s2);String hello(String s1);
}

接口实现类

package com.cml.test;public class ImplOne implements InterfaceOne {@Overridepublic String show(String s1, String s2) {System.out.println("InterfaceOne >> ImplOne >> show");return s1 + s2;}@Overridepublic String hello(String s1) {return s1;}}

定义通知类

package com.cml.test;import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;/*** 此类做通知* @author yys**/
public class Aspect {public void before(){System.out.println("前置通知");}public void afterReturning(Object ret){System.out.println("后置通知, 返回值为-->" + ret);}public void afterThrowing(Throwable e){System.out.println("抛出异常通知 ，异常信息： " + e.getMessage());}public void after(){System.out.println("最终通知：finally里面的代码,发生异常也会执行。。。");}public Object around(Object target, Method method, Object[] args) throws InvocationTargetException, IllegalAccessException {System.out.println("环绕通知      前");Object ret = method.invoke(target, args);System.out.println("环绕通知      后");return ret;}
}

实现MyInvocationHandler接口

package com.cml.test;import com.cml.proxy.MyClassLoader;
import com.cml.proxy.MyInvocationHandler;
import com.cml.proxy.MyProxy;import java.lang.reflect.Method;/*** @author yys* @version 1.0*/
public class MyDynamicProxy implements MyInvocationHandler {private Object target;/*** 通知类*/private Aspect aspect;//设置通知类public void setAspect(Aspect aspect) {this.aspect = aspect;}/*** 此方法用于创建代理对象* @param target 传入一个被代理的对象* @return 返回代理对象*/public Object createProxyInstance(Object target) {this.target = target;return MyProxy.newProxyInstance(new MyClassLoader(), target.getClass().getInterfaces(), this);}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {Object ret = null;try {aspect.before();ret = aspect.around(target, method, args);aspect.afterReturning(ret);} catch (Exception e) {aspect.afterThrowing(e);} finally {aspect.after();}return ret;}
}

执行

package com.cml.test;/*** @author yys* @version 1.0*/
public class Test {public static void main(String[] args) {MyDynamicProxy myDynamicProxy = new MyDynamicProxy();myDynamicProxy.setAspect(new Aspect());InterfaceOne interfaceOne = (InterfaceOne) myDynamicProxy.createProxyInstance(new ImplOne());interfaceOne.show("show", "cml");System.out.println(interfaceOne.getClass());System.out.println(interfaceOne.getClass().getSuperclass());}
}

结果

动态生成的类长这个样子