之前雖然會用JDK的動態(tài)代理，但是有些問題卻一直沒有搞明白。比如說：InvocationHandler的invoke方法是由誰來調(diào)用的，代理對象是怎么生成的，直到前幾個(gè)星期才把這些問題全部搞明白了。?
??? 廢話不多說了，先來看一下JDK的動態(tài)是怎么用的。?

Java代碼??

package?dynamic.proxy;???

??

import?java.lang.reflect.InvocationHandler;??

import?java.lang.reflect.Method;??

import?java.lang.reflect.Proxy;??

??

/**?

?*?實(shí)現(xiàn)自己的InvocationHandler?

?*?@author?zyb?

?*?@since?2012-8-9?

?*?

?*/??

public?class?MyInvocationHandler?implements?InvocationHandler?{??

??????

????//?目標(biāo)對象???

????private?Object?target;??

??????

????/**?

?????*?構(gòu)造方法?

?????*?@param?target?目標(biāo)對象??

?????*/??

????public?MyInvocationHandler(Object?target)?{??

????????super();??

????????this.target?=?target;??

????}??

??

??

????/**?

?????*?執(zhí)行目標(biāo)對象的方法?

?????*/??

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

??????????

????????//?在目標(biāo)對象的方法執(zhí)行之前簡單的打印一下??

????????System.out.println("------------------before------------------");??

??????????

????????//?執(zhí)行目標(biāo)對象的方法??

????????Object?result?=?method.invoke(target,?args);??

??????????

????????//?在目標(biāo)對象的方法執(zhí)行之后簡單的打印一下??

????????System.out.println("-------------------after------------------");??

??????????

????????return?result;??

????}??

??

????/**?

?????*?獲取目標(biāo)對象的代理對象?

?????*?@return?代理對象?

?????*/??

????public?Object?getProxy()?{??

????????return?Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),???

????????????????target.getClass().getInterfaces(),?this);??

????}??

}??

??

package?dynamic.proxy;??

??

/**?

?*?目標(biāo)對象實(shí)現(xiàn)的接口，用JDK來生成代理對象一定要實(shí)現(xiàn)一個(gè)接口?

?*?@author?zyb?

?*?@since?2012-8-9?

?*?

?*/??

public?interface?UserService?{??

??

????/**?

?????*?目標(biāo)方法??

?????*/??

????public?abstract?void?add();??

??

}??

??

package?dynamic.proxy;???

??

/**?

?*?目標(biāo)對象?

?*?@author?zyb?

?*?@since?2012-8-9?

?*?

?*/??

public?class?UserServiceImpl?implements?UserService?{??

??

????/*?(non-Javadoc)?

?????*?@see?dynamic.proxy.UserService#add()?

?????*/??

????public?void?add()?{??

????????System.out.println("--------------------add---------------");??

????}??

}??

??

package?dynamic.proxy;???

??

import?org.junit.Test;??

??

/**?

?*?動態(tài)代理測試類?

?*?@author?zyb?

?*?@since?2012-8-9?

?*?

?*/??

public?class?ProxyTest?{??

??

????@Test??

????public?void?testProxy()?throws?Throwable?{??

????????//?實(shí)例化目標(biāo)對象??

????????UserService?userService?=?new?UserServiceImpl();??

??????????

????????//?實(shí)例化InvocationHandler??

????????MyInvocationHandler?invocationHandler?=?new?MyInvocationHandler(userService);??

??????????

????????//?根據(jù)目標(biāo)對象生成代理對象??

????????UserService?proxy?=?(UserService)?invocationHandler.getProxy();??

??????????

????????//?調(diào)用代理對象的方法??

????????proxy.add();??

??????????

????}??

}??

執(zhí)行結(jié)果如下：?
------------------before------------------?
--------------------add---------------?
-------------------after------------------?

?? 用起來是很簡單吧，其實(shí)這里基本上就是AOP的一個(gè)簡單實(shí)現(xiàn)了，在目標(biāo)對象的方法執(zhí)行之前和執(zhí)行之后進(jìn)行了增強(qiáng)。Spring的AOP實(shí)現(xiàn)其實(shí)也是用了Proxy和InvocationHandler這兩個(gè)東西的。?

??? 用起來是比較簡單，但是如果能知道它背后做了些什么手腳，那就更好不過了。首先來看一下JDK是怎樣生成代理對象的。既然生成代理對象是用的Proxy類的靜態(tài)方newProxyInstance，那么我們就去它的源碼里看一下它到底都做了些什么？?

Java代碼??

/**?

?*?loader:類加載器?

?*?interfaces:目標(biāo)對象實(shí)現(xiàn)的接口?

?*?h:InvocationHandler的實(shí)現(xiàn)類?

?*/??

public?static?Object?newProxyInstance(ClassLoader?loader,??

??????????????????????Class<?>[]?interfaces,??

??????????????????????InvocationHandler?h)??

????throws?IllegalArgumentException??

????{??

????if?(h?==?null)?{??

????????throw?new?NullPointerException();??

????}??

??

????/*?

?????*?Look?up?or?generate?the?designated?proxy?class.?

?????*/??

????Class?cl?=?getProxyClass(loader,?interfaces);??

??

????/*?

?????*?Invoke?its?constructor?with?the?designated?invocation?handler.?

?????*/??

????try?{??

????????????//?調(diào)用代理對象的構(gòu)造方法（也就是$Proxy0(InvocationHandler?h)）??

????????Constructor?cons?=?cl.getConstructor(constructorParams);??

????????????//?生成代理類的實(shí)例并把MyInvocationHandler的實(shí)例傳給它的構(gòu)造方法??

????????return?(Object)?cons.newInstance(new?Object[]?{?h?});??

????}?catch?(NoSuchMethodException?e)?{??

????????throw?new?InternalError(e.toString());??

????}?catch?(IllegalAccessException?e)?{??

????????throw?new?InternalError(e.toString());??

????}?catch?(InstantiationException?e)?{??

????????throw?new?InternalError(e.toString());??

????}?catch?(InvocationTargetException?e)?{??

????????throw?new?InternalError(e.toString());??

????}??

????}??

?? 我們再進(jìn)去getProxyClass方法看一下?

Java代碼??

public?static?Class<?>?getProxyClass(ClassLoader?loader,???

?????????????????????????????????????????Class<?>...?interfaces)??

????throws?IllegalArgumentException??

????{??

????//?如果目標(biāo)類實(shí)現(xiàn)的接口數(shù)大于65535個(gè)則拋出異常（我XX，誰會寫這么NB的代碼啊？）??

????if?(interfaces.length?>?65535)?{??

????????throw?new?IllegalArgumentException("interface?limit?exceeded");??

????}??

??

????//?聲明代理對象所代表的Class對象（有點(diǎn)拗口）??

????Class?proxyClass?=?null;??

??

????String[]?interfaceNames?=?new?String[interfaces.length];??

??

????Set?interfaceSet?=?new?HashSet();???//?for?detecting?duplicates??

??

????//?遍歷目標(biāo)類所實(shí)現(xiàn)的接口??

????for?(int?i?=?0;?i?<?interfaces.length;?i++)?{??

??????????

????????//?拿到目標(biāo)類實(shí)現(xiàn)的接口的名稱??

????????String?interfaceName?=?interfaces[i].getName();??

????????Class?interfaceClass?=?null;??

????????try?{??

????????//?加載目標(biāo)類實(shí)現(xiàn)的接口到內(nèi)存中??

????????interfaceClass?=?Class.forName(interfaceName,?false,?loader);??

????????}?catch?(ClassNotFoundException?e)?{??

????????}??

????????if?(interfaceClass?!=?interfaces[i])?{??

????????throw?new?IllegalArgumentException(??

????????????interfaces[i]?+?"?is?not?visible?from?class?loader");??

????????}??

??

????????//?中間省略了一些無關(guān)緊要的代碼?.......??

??????????

????????//?把目標(biāo)類實(shí)現(xiàn)的接口代表的Class對象放到Set中??

????????interfaceSet.add(interfaceClass);??

??

????????interfaceNames[i]?=?interfaceName;??

????}??

??

????//?把目標(biāo)類實(shí)現(xiàn)的接口名稱作為緩存（Map）中的key??

????Object?key?=?Arrays.asList(interfaceNames);??

??

????Map?cache;??

??????

????synchronized?(loaderToCache)?{??

????????//?從緩存中獲取cache??

????????cache?=?(Map)?loaderToCache.get(loader);??

????????if?(cache?==?null)?{??

????????//?如果獲取不到，則新建地個(gè)HashMap實(shí)例??

????????cache?=?new?HashMap();??

????????//?把HashMap實(shí)例和當(dāng)前加載器放到緩存中??

????????loaderToCache.put(loader,?cache);??

????????}??

??

????}??

??

????synchronized?(cache)?{??

??

????????do?{??

????????//?根據(jù)接口的名稱從緩存中獲取對象??

????????Object?value?=?cache.get(key);??

????????if?(value?instanceof?Reference)?{??

????????????proxyClass?=?(Class)?((Reference)?value).get();??

????????}??

????????if?(proxyClass?!=?null)?{??

????????????//?如果代理對象的Class實(shí)例已經(jīng)存在，則直接返回??

????????????return?proxyClass;??

????????}?else?if?(value?==?pendingGenerationMarker)?{??

????????????try?{??

????????????cache.wait();??

????????????}?catch?(InterruptedException?e)?{??

????????????}??

????????????continue;??

????????}?else?{??

????????????cache.put(key,?pendingGenerationMarker);??

????????????break;??

????????}??

????????}?while?(true);??

????}??

??

????try?{??

????????//?中間省略了一些代碼?.......??

??????????

????????//?這里就是動態(tài)生成代理對象的最關(guān)鍵的地方??

????????byte[]?proxyClassFile?=?ProxyGenerator.generateProxyClass(??

????????????proxyName,?interfaces);??

????????try?{??

????????????//?根據(jù)代理類的字節(jié)碼生成代理類的實(shí)例??

????????????proxyClass?=?defineClass0(loader,?proxyName,??

????????????proxyClassFile,?0,?proxyClassFile.length);??

????????}?catch?(ClassFormatError?e)?{??

????????????throw?new?IllegalArgumentException(e.toString());??

????????}??

????????}??

????????//?add?to?set?of?all?generated?proxy?classes,?for?isProxyClass??

????????proxyClasses.put(proxyClass,?null);??

??

????}???

????//?中間省略了一些代碼?.......??

??????

????return?proxyClass;??

????}??

進(jìn)去ProxyGenerator類的靜態(tài)方法generateProxyClass，這里是真正生成代理類class字節(jié)碼的地方。?

Java代碼??

public?static?byte[]?generateProxyClass(final?String?name,??

???????????????????????????????????????????Class[]?interfaces)??

???{??

???????ProxyGenerator?gen?=?new?ProxyGenerator(name,?interfaces);??

????//?這里動態(tài)生成代理類的字節(jié)碼，由于比較復(fù)雜就不進(jìn)去看了??

???????final?byte[]?classFile?=?gen.generateClassFile();??

??

????//?如果saveGeneratedFiles的值為true，則會把所生成的代理類的字節(jié)碼保存到硬盤上??

???????if?(saveGeneratedFiles)?{??

???????????java.security.AccessController.doPrivileged(??

???????????new?java.security.PrivilegedAction<Void>()?{??

???????????????public?Void?run()?{??

???????????????????try?{??

???????????????????????FileOutputStream?file?=??

???????????????????????????new?FileOutputStream(dotToSlash(name)?+?".class");??

???????????????????????file.write(classFile);??

???????????????????????file.close();??

???????????????????????return?null;??

???????????????????}?catch?(IOException?e)?{??

???????????????????????throw?new?InternalError(??

???????????????????????????"I/O?exception?saving?generated?file:?"?+?e);??

???????????????????}??

???????????????}??

???????????});??

???????}??

??

????//?返回代理類的字節(jié)碼??

???????return?classFile;??

???}??

現(xiàn)在，JDK是怎樣動態(tài)生成代理類的字節(jié)的原理已經(jīng)一目了然了。?

好了，再來解決另外一個(gè)問題，那就是由誰來調(diào)用InvocationHandler的invoke方法的。要解決這個(gè)問題就要看一下JDK到底為我們生成了一個(gè)什么東西。用以下代碼可以獲取到JDK為我們生成的字節(jié)碼并寫到硬盤中。?

Java代碼??

package?dynamic.proxy;???

??

import?java.io.FileOutputStream;??

import?java.io.IOException;??

??

import?sun.misc.ProxyGenerator;??

??

/**?

?*?代理類的生成工具?

?*?@author?zyb?

?*?@since?2012-8-9?

?*/??

public?class?ProxyGeneratorUtils?{??

??

????/**?

?????*?把代理類的字節(jié)碼寫到硬盤上?

?????*?@param?path?保存路徑?

?????*/??

????public?static?void?writeProxyClassToHardDisk(String?path)?{??

????????//?第一種方法，這種方式在剛才分析ProxyGenerator時(shí)已經(jīng)知道了??

????????//?System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles",?true);??

??????????

????????//?第二種方法??

??????????

????????//?獲取代理類的字節(jié)碼??

????????byte[]?classFile?=?ProxyGenerator.generateProxyClass("$Proxy11",?UserServiceImpl.class.getInterfaces());??

??????????

????????FileOutputStream?out?=?null;??

??????????

????????try?{??

????????????out?=?new?FileOutputStream(path);??

????????????out.write(classFile);??

????????????out.flush();??

????????}?catch?(Exception?e)?{??

????????????e.printStackTrace();??

????????}?finally?{??

????????????try?{??

????????????????out.close();??

????????????}?catch?(IOException?e)?{??

????????????????e.printStackTrace();??

????????????}??

????????}??

????}??

}??

??

package?dynamic.proxy;???

??

import?org.junit.Test;??

??

/**?

?*?動態(tài)代理測試類?

?*?@author?zyb?

?*?@since?2012-8-9?

?*?

?*/??

public?class?ProxyTest?{??

??

????@Test??

????public?void?testProxy()?throws?Throwable?{??

????????//?實(shí)例化目標(biāo)對象??

????????UserService?userService?=?new?UserServiceImpl();??

??????????

????????//?實(shí)例化InvocationHandler??

????????MyInvocationHandler?invocationHandler?=?new?MyInvocationHandler(userService);??

??????????

????????//?根據(jù)目標(biāo)對象生成代理對象??

????????UserService?proxy?=?(UserService)?invocationHandler.getProxy();??

??????????

????????//?調(diào)用代理對象的方法??

????????proxy.add();??

??????????

????}??

??????

????@Test??

????public?void?testGenerateProxyClass()?{??

????????ProxyGeneratorUtils.writeProxyClassToHardDisk("F:/$Proxy11.class");??

????}??

}??

通過以上代碼，就可以在F盤上生成一個(gè)$Proxy.class文件了，現(xiàn)在用反編譯工具來看一下這個(gè)class文件里面的內(nèi)容。?

Java代碼??

//?Decompiled?by?DJ?v3.11.11.95?Copyright?2009?Atanas?Neshkov??Date:?2012/8/9?20:11:32??

//?Home?Page:?http://members.fortunecity.com/neshkov/dj.html??http://www.neshkov.com/dj.html?-?Check?often?for?new?version!??

//?Decompiler?options:?packimports(3)???

??

import?dynamic.proxy.UserService;??

import?java.lang.reflect.*;??

??

public?final?class?$Proxy11?extends?Proxy??

????implements?UserService??

{??

??

????//?構(gòu)造方法，參數(shù)就是剛才傳過來的MyInvocationHandler類的實(shí)例??

????public?$Proxy11(InvocationHandler?invocationhandler)??

????{??

????????super(invocationhandler);??

????}??

??

????public?final?boolean?equals(Object?obj)??

????{??

????????try??

????????{??

????????????return?((Boolean)super.h.invoke(this,?m1,?new?Object[]?{??

????????????????obj??

????????????})).booleanValue();??

????????}??

????????catch(Error?_ex)?{?}??

????????catch(Throwable?throwable)??

????????{??

????????????throw?new?UndeclaredThrowableException(throwable);??

????????}??

????}??

??

????/**?

?????*?這個(gè)方法是關(guān)鍵部分?

?????*/??

????public?final?void?add()??

????{??

????????try??

????????{??

????????????//?實(shí)際上就是調(diào)用MyInvocationHandler的public?Object?invoke(Object?proxy,?Method?method,?Object[]?args)方法，第二個(gè)問題就解決了??

????????????super.h.invoke(this,?m3,?null);??

????????????return;??

????????}??

????????catch(Error?_ex)?{?}??

????????catch(Throwable?throwable)??

????????{??

????????????throw?new?UndeclaredThrowableException(throwable);??

????????}??

????}??

??

????public?final?int?hashCode()??

????{??

????????try??

????????{??

????????????return?((Integer)super.h.invoke(this,?m0,?null)).intValue();??

????????}??

????????catch(Error?_ex)?{?}??

????????catch(Throwable?throwable)??

????????{??

????????????throw?new?UndeclaredThrowableException(throwable);??

????????}??

????}??

??

????public?final?String?toString()??

????{??

????????try??

????????{??

????????????return?(String)super.h.invoke(this,?m2,?null);??

????????}??

????????catch(Error?_ex)?{?}??

????????catch(Throwable?throwable)??

????????{??

????????????throw?new?UndeclaredThrowableException(throwable);??

????????}??

????}??

??

????private?static?Method?m1;??

????private?static?Method?m3;??

????private?static?Method?m0;??

????private?static?Method?m2;??

??

????//?在靜態(tài)代碼塊中獲取了4個(gè)方法：Object中的equals方法、UserService中的add方法、Object中的hashCode方法、Object中toString方法??

????static???

????{??

????????try??

????????{??

????????????m1?=?Class.forName("java.lang.Object").getMethod("equals",?new?Class[]?{??

????????????????Class.forName("java.lang.Object")??

????????????});??

????????????m3?=?Class.forName("dynamic.proxy.UserService").getMethod("add",?new?Class[0]);??

????????????m0?=?Class.forName("java.lang.Object").getMethod("hashCode",?new?Class[0]);??

????????????m2?=?Class.forName("java.lang.Object").getMethod("toString",?new?Class[0]);??

????????}??

????????catch(NoSuchMethodException?nosuchmethodexception)??

????????{??

????????????throw?new?NoSuchMethodError(nosuchmethodexception.getMessage());??

????????}??

????????catch(ClassNotFoundException?classnotfoundexception)??

????????{??

????????????throw?new?NoClassDefFoundError(classnotfoundexception.getMessage());??

????????}??

????}??

}??

好了，到目前為止，前面 的兩個(gè)問題都已經(jīng)知道回事了，現(xiàn)在再用JDK動態(tài)代理的時(shí)候就不只會用而已了，真正的達(dá)到了“知其然，知其所以然”的目的。。。???

就寫到這了，累死了。。??

原文地址：http://rejoy.iteye.com/blog/1627405

轉(zhuǎn)載于:https://www.cnblogs.com/davidwang456/p/4045412.html

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