深入理解JDK动态代理原理，使用javassist动手写一个动态代理框架

一、动手实现一个动态代理框架

1、初识javassist

Java操纵字节码，最底层一般是使用ASM进行操作的，但是ASM上手难度很大，我们可以使用javassist对字节码进行操作，相对来说简单一些。

下面这个实例，就是我们使用javassist对接口动态生成一个实现类：

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;public class JavassistDemo {public static void main(String[] args) throws Exception {TestService proxy = createProxy();proxy.sayHello("zhangsan"); // hello:zhangsan}/*** 生成一个TestService的实现类*/public static TestService createProxy() throws Exception {// javassist 底层是ASM，ASM底层是编辑JVM指令码ClassPool classPool = new ClassPool();// 添加classLoaderclassPool.appendSystemPath();// 1.创建一个类CtClass class1 = classPool.makeClass("TestServiceImpl");class1.addInterface(classPool.get(TestService.class.getName()));// 2.创建一个方法CtMethod satHelloMethod = CtNewMethod.make(CtClass.voidType, // void返回值"sayHello", // 方法名new CtClass[]{classPool.get(String.class.getName())}, // 方法参数new CtClass[0], // 异常类型"{System.out.println(\"hello:\"+$1);}", // 方法体内容，$1表示第一个参数class1 // 指定类);class1.addMethod(satHelloMethod);// 3.实例化这个对象Class aClass = classPool.toClass(class1);// 强制转换return (TestService) aClass.newInstance();}public interface TestService {void sayHello(String name);}
}

2、使用javassist实现一个动态代理框架

import javassist.*;
import java.lang.reflect.Method;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.stream.Collectors;public class Javassist3Demo {public static void main(String[] args) throws Exception {TestService proxy = createProxy(TestService.class, new InvocationHandler() {@Overridepublic Object invoke(String methodName, Object[] args) {// 根据方法判断逻辑if(methodName.equals("sayHello3")) {System.out.println("hello" + args[0]);return "aa";} else {System.out.println("hello2" + args[0]);return "aa2";}}});proxy.sayHello("zhangsan"); // hellozhangsanproxy.sayHello2("zz", 1);System.out.println(proxy.sayHello3("qq"));}static int count = 0;public static <T> T createProxy(Class<T> classInterface, InvocationHandler handler) throws Exception {ClassPool classPool = new ClassPool();classPool.appendSystemPath();// 1.创建一个类CtClass impl = classPool.makeClass("$Proxy" + count ++);impl.addInterface(classPool.get(classInterface.getName()));// 2.impl类中 添加属性handlerCtField fie = CtField.make("public com.mydemo.Javassast3Demo.InvocationHandler handler=null;", impl);impl.addField(fie);// 有返回值类型和无返回值类型的源码String src = "return ($r)this.handler.invoke(\"%s\", $args);"; // $args获取所有参数String voidSrc = "this.handler.invoke(\"%s\",$args);";for (Method method : classInterface.getMethods()) {CtClass returnType = classPool.get(method.getReturnType().getName());String name = method.getName();CtClass[] parameters = toCtClass(classPool, method.getParameterTypes());CtClass[] errors = toCtClass(classPool, method.getExceptionTypes());// 2.创建一个方法CtMethod newMethod = CtNewMethod.make(returnType, // 返回值name, // 方法名parameters, // 方法参数errors, // 异常类型method.getReturnType().equals(Void.class) ? String.format(voidSrc, method.getName()) : String.format(src, method.getName()), // 方法体内容impl // 指定类);impl.addMethod(newMethod);}// 生成字节码（辅助学习用）//byte[] bytes = impl.toBytecode();//Files.write(Paths.get(System.getProperty("user.dir") + "/target/" + impl.getName() + ".class"), bytes);// 3.实例化这个对象Class aClass = classPool.toClass(impl);T t = (T) aClass.newInstance();aClass.getField("handler").set(t, handler); // 初始化赋值// 强制转换return t;}private static CtClass[] toCtClass(ClassPool pool, Class[] classes) {return Arrays.stream(classes).map(c -> {try {return pool.get(c.getName());} catch (NotFoundException e) {throw new RuntimeException(e);}}).collect(Collectors.toList()).toArray(new CtClass[0]);}public interface InvocationHandler {Object invoke(String methodName, Object args[]);}public class InvocationHandlerImpl implements InvocationHandler {@Overridepublic Object invoke(String methodName, Object[] args) {System.out.println("hello");return null;}}public interface TestService {void sayHello(String name);void sayHello2(String name, Integer id);String sayHello3(String name);}
}

这样，我们只要实现InvocationHandler 接口，就可以自定义代理类的核心逻辑了，我们对生成的代理类进行反编译：

public class $Proxy0 implements TestService {public InvocationHandler handler = null;public String sayHello3(String var1) {return (String)this.handler.invoke("sayHello3", new Object[]{var1});}public void sayHello(String var1) {this.handler.invoke("sayHello", new Object[]{var1});}public void sayHello2(String var1, Integer var2) {this.handler.invoke("sayHello2", new Object[]{var1, var2});}public $Proxy0() {}
}

实现了我们的接口，并且重写了接口的所有方法，最终执行的是InvocationHandler的invoke方法。

这也正是JDK动态代理的基本思想。

二、JDK动态代理

1、编码实现

public interface EchoService {String echo(String message) throws NullPointerException;
}public class DefaultEchoService implements EchoService {@Overridepublic String echo(String message) {return "[ECHO] " + message;}
}import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;/*** JDK动态代理实例*/
public class JDKDynamicProxyDemo {public static void main(String[] args) {ClassLoader classLoader = Thread.currentThread().getContextClassLoader();// 真实的对象DefaultEchoService realObj = new DefaultEchoService();// 代理的对象Object proxy = Proxy.newProxyInstance(classLoader, new Class[]{EchoService.class}, new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {System.out.println("动态前置");Object obj = null;if (EchoService.class.isAssignableFrom(method.getDeclaringClass())) {// 执行真实方法obj = method.invoke(realObj, args);}System.out.println("动态后置");return obj;}});EchoService echoService = (EchoService) proxy;System.out.println(echoService.echo("Hello,World"));}
}

我们发现，使用JDK动态代理，基本逻辑和我们上面使用javassist手写的工具类差不多，只不过JDK动态代理底层是使用更复杂的方式实现的，我们这里取巧，使用javassist实现。

注意！这里需要DefaultEchoService 实现EchoService接口，代理的其实是EchoService接口而不是DefaultEchoService 类。

动态代理对原代码没有侵入性，通常可以动态加载。

2、基本原理

为什么 Proxy.newProxyInstance 会生成新的字节码？

ClassLoader classLoader = Thread.currentThread().getContextClassLoader();Object proxy = Proxy.newProxyInstance(classLoader, new Class[]{EchoService.class}, new InvocationHandler() {@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {return null;}
});System.out.println(proxy.getClass());// com.sun.proxy.$Proxy0Object proxy2 = Proxy.newProxyInstance(classLoader, new Class[]{Comparable.class}, (proxy1, method, args1) -> {return null;});System.out.println(proxy2.getClass());// com.sun.proxy.$Proxy1

上面代码我们会发现，Java动态代理每生成一个代理，它的class总是com.sun.proxy包下的$Proxy*，从0开始累加，它是如何实现的呢？

我们来分析一下Proxy的newProxyInstance方法：

// java.lang.reflect.Proxy#newProxyInstance
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)throws IllegalArgumentException
{Objects.requireNonNull(h);// 对象克隆final Class<?>[] intfs = interfaces.clone();final SecurityManager sm = System.getSecurityManager();if (sm != null) {checkProxyAccess(Reflection.getCallerClass(), loader, intfs);}/** Look up or generate the designated proxy class.*/// 先从缓存获取（见 （1））Class<?> cl = getProxyClass0(loader, intfs);/** Invoke its constructor with the designated invocation handler.*/try {if (sm != null) {checkNewProxyPermission(Reflection.getCallerClass(), cl);}// 获取代理对象的构造方法，带着InvocationHandler参数的构造方法final Constructor<?> cons = cl.getConstructor(constructorParams);final InvocationHandler ih = h;if (!Modifier.isPublic(cl.getModifiers())) {AccessController.doPrivileged(new PrivilegedAction<Void>() {public Void run() {cons.setAccessible(true);return null;}});}// 返回Proxy对象return cons.newInstance(new Object[]{h});} catch (IllegalAccessException|InstantiationException e) {throw new InternalError(e.toString(), e);} catch (InvocationTargetException e) {Throwable t = e.getCause();if (t instanceof RuntimeException) {throw (RuntimeException) t;} else {throw new InternalError(t.toString(), t);}} catch (NoSuchMethodException e) {throw new InternalError(e.toString(), e);}
}

（1）getProxyClass0方法

在getProxyClass0方法中，从proxyClassCache缓存中获取了这个代理类：

// java.lang.reflect.Proxy#getProxyClass0
private static Class<?> getProxyClass0(ClassLoader loader,Class<?>... interfaces) {if (interfaces.length > 65535) {throw new IllegalArgumentException("interface limit exceeded");}// If the proxy class defined by the given loader implementing// the given interfaces exists, this will simply return the cached copy;// otherwise, it will create the proxy class via the ProxyClassFactoryreturn proxyClassCache.get(loader, interfaces);
}

而proxyClassCache在初始化时，自动创建了KeyFactory和ProxyClassFactory

private static final WeakCache<ClassLoader, Class<?>[], Class<?>>proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

ProxyClassFactory的核心方法apply，隐藏着代理接口的创建逻辑：

// java.lang.reflect.Proxy.ProxyClassFactory
private static final class ProxyClassFactoryimplements BiFunction<ClassLoader, Class<?>[], Class<?>>
{// prefix for all proxy class namesprivate static final String proxyClassNamePrefix = "$Proxy";// next number to use for generation of unique proxy class namesprivate static final AtomicLong nextUniqueNumber = new AtomicLong();@Overridepublic Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);for (Class<?> intf : interfaces) { // 遍历我们传入的接口数组/** Verify that the class loader resolves the name of this* interface to the same Class object.*/Class<?> interfaceClass = null;try {// 通过classLoader加载我们的接口interfaceClass = Class.forName(intf.getName(), false, loader);} catch (ClassNotFoundException e) {}if (interfaceClass != intf) {throw new IllegalArgumentException(intf + " is not visible from class loader");}/** Verify that the Class object actually represents an* interface.*/if (!interfaceClass.isInterface()) {// 只能代理接口，非接口直接抛异常throw new IllegalArgumentException(interfaceClass.getName() + " is not an interface");}/** Verify that this interface is not a duplicate.*/if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {throw new IllegalArgumentException("repeated interface: " + interfaceClass.getName());}}String proxyPkg = null;     // package to define proxy class inint accessFlags = Modifier.PUBLIC | Modifier.FINAL;/** Record the package of a non-public proxy interface so that the* proxy class will be defined in the same package.  Verify that* all non-public proxy interfaces are in the same package.*/for (Class<?> intf : interfaces) {int flags = intf.getModifiers();if (!Modifier.isPublic(flags)) {accessFlags = Modifier.FINAL;String name = intf.getName();int n = name.lastIndexOf('.');String pkg = ((n == -1) ? "" : name.substring(0, n + 1));if (proxyPkg == null) {proxyPkg = pkg;} else if (!pkg.equals(proxyPkg)) {throw new IllegalArgumentException("non-public interfaces from different packages");}}}if (proxyPkg == null) { // 包名就是com.sun.proxy// if no non-public proxy interfaces, use com.sun.proxy packageproxyPkg = ReflectUtil.PROXY_PACKAGE + ".";}/** Choose a name for the proxy class to generate.*/long num = nextUniqueNumber.getAndIncrement();String proxyName = proxyPkg + proxyClassNamePrefix + num; // 依次递增/** Generate the specified proxy class.*/// 代理类生成器，返回字节数组，就是字节码byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);try {// classLoader加载类，是一个native方法，返回一个Class对象return defineClass0(loader, proxyName,proxyClassFile, 0, proxyClassFile.length);} catch (ClassFormatError e) {/** A ClassFormatError here means that (barring bugs in the* proxy class generation code) there was some other* invalid aspect of the arguments supplied to the proxy* class creation (such as virtual machine limitations* exceeded).*/throw new IllegalArgumentException(e.toString());}}
}

（2）总结

vm options参数设置-Dsun.misc.ProxyGenerator.saveGeneratedFiles=true，就可以把生成的代理类的源码保存在com.sun.proxy目录下面，或者用arthas来查看运行中的类信息。

JDK动态代理生成的代理类，我们通过反编译，发现其实是这个样子的：

package com.sun.proxy;import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import com.demo.EchoService;public final class $Proxy0
extends Proxy
implements EchoService {private static Method m1;private static Method m3;private static Method m2;private static Method m0;public $Proxy0(InvocationHandler invocationHandler) {super(invocationHandler);}static {try {m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));m3 = Class.forName("com.demo.EchoService").getMethod("echo", Class.forName("java.lang.String"));m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);return;}catch (NoSuchMethodException noSuchMethodException) {throw new NoSuchMethodError(noSuchMethodException.getMessage());}catch (ClassNotFoundException classNotFoundException) {throw new NoClassDefFoundError(classNotFoundException.getMessage());}}public final boolean equals(Object object) {try {return (Boolean)this.h.invoke(this, m1, new Object[]{object});}catch (Error | RuntimeException throwable) {throw throwable;}catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}public final String toString() {try {return (String)this.h.invoke(this, m2, null);}catch (Error | RuntimeException throwable) {throw throwable;}catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}public final int hashCode() {try {return (Integer)this.h.invoke(this, m0, null);}catch (Error | RuntimeException throwable) {throw throwable;}catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}public final String echo(String string) throws NullPointerException {try {return (String)this.h.invoke(this, m3, new Object[]{string});}catch (Error | RuntimeException throwable) {throw throwable;}catch (Throwable throwable) {throw new UndeclaredThrowableException(throwable);}}
}

InvocationHandler就是Proxy.newProxyInstance传入的最后一个参数。

当调用代理对象的方法时，会执行InvocationHandler的invoke方法。

注意，JDK生成的代理类的包名不总是com.sun.proxy,只有当接口为Public时是这样的，当接口为非public时，生成的代理类与接口所在包名相同。

写在后面

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