JDK動(dòng)態(tài)代理

wangbjun 發(fā)布于2019-08-15 10:39 / 601人閱讀

摘要：用戶在調(diào)用被動(dòng)態(tài)代理過的對(duì)象的方法時(shí)，調(diào)用的請(qǐng)求會(huì)被自動(dòng)發(fā)給代理對(duì)象實(shí)現(xiàn)了接口的方法，由方法來實(shí)現(xiàn)對(duì)請(qǐng)求的統(tǒng)一處理。動(dòng)態(tài)代理實(shí)現(xiàn)切面代替該語句塊一定會(huì)在前面執(zhí)行。

用戶在調(diào)用被動(dòng)態(tài)代理過的對(duì)象的方法時(shí)，調(diào)用的請(qǐng)求會(huì)被自動(dòng)發(fā)給代理對(duì)象（實(shí)現(xiàn)了InvocationHandler接口）的invoke()方法，由invoke()方法來實(shí)現(xiàn)對(duì)請(qǐng)求的統(tǒng)一處理。

源碼分析

Java.lang.reflect.Proxy

public class Proxy implements java.io.Serializable {
private static final Class[] constructorParams =  { InvocationHandler.class };
private static final WeakCache[], Class>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
......

    @CallerSensitive
    public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h)
        throws IllegalArgumentException
    {
        Objects.requireNonNull(h);

        final Class[] intfs = interfaces.clone();    //淺克隆
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        //生成代理類的class
        Class cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //getConstructor(InvocationHandler.class) 獲取代理類的構(gòu)造器
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //代理類實(shí)例化返回
            return cons.newInstance(new Object[]{h});
        }  
    ......
}

 

private static Class getProxyClass0(ClassLoader loader, Class... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // key為WeakReference，若緩存中存在相同key，則返回ProxyClass，若不存在，則用ProxyClassFactory創(chuàng)建ProxyClass。
        return proxyClassCache.get(loader, interfaces);  //java.lang.reflect.WeakCache 這個(gè)內(nèi)核分析？
    }
......
 }

private static final class ProxyClassFactory implements BiFunction[], Class> {
        //?所有代理類ProxyClass名字的前綴?
        private static final String proxyClassNamePrefix = "$Proxy";

        // 用計(jì)數(shù)器生成代理類ProxyClass的名字
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class apply(ClassLoader loader, Class[] interfaces) {

            Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//對(duì)每一個(gè)interface Class進(jìn)行驗(yàn)證
            for (Class intf : interfaces) {
                /*
                 * Verify that the class loader resolves the name of this
                 * interface to the same Class object.
                 */
                Class interfaceClass = null;
                try {
                    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()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                /*
                 * Verify that this interface is not a duplicate.
  * 用IdentityHashMap來驗(yàn)證interface Class的唯一性，因?yàn)镮dentityHashMap的key比較是==，即比較內(nèi)存地址。
                 */
                if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
                    throw new IllegalArgumentException(
                        "repeated interface: " + interfaceClass.getName());
                }
            }

            String proxyPkg = null;      
            int accessFlags = Modifier.PUBLIC | Modifier.FINAL;

            // 對(duì)于非公共接口，ProxyClass的包名設(shè)置為接口的包名??  
            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");
                    }
                }
            }

            //?對(duì)于公共接口，ProxyClass的包名為com.sun.proxy??
            if (proxyPkg == null) {
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            long num = nextUniqueNumber.getAndIncrement();
            //ProxyClass的完全限定名
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            //由c++寫的ProxyGenerator生成代理類的二進(jìn)制字節(jié)碼
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                //?根據(jù)二進(jìn)制字節(jié)碼返回相應(yīng)的ProxyClass??
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {               
                throw new IllegalArgumentException(e.toString());
            }
        }
}

$Proxy.class

以下是反編譯$Proxy0.class：

public?final?class?$Proxy0?extends?Proxy?implements?MyInterface1{??
??private?static?Method?m1;??
??private?static?Method?m3;??
??private?static?Method?m0;??
??private?static?Method?m2;??
?
??public?$Proxy0(InvocationHandler?paramInvocationHandler)?{??
????super(paramInvocationHandler);??
??}??
??
??public?final?boolean?equals(Object?paramObject)?{??
????try?{??
??????return?((Boolean)this.h.invoke(this,?m1,?new?Object[]?{?paramObject?})).booleanValue();??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??public?final?void?sayHello(String?paramString)?{??
????try?{??
??????this.h.invoke(this,?m3,?new?Object[]?{?paramString?});??
??????return;??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
???catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??

??
??public?final?int?hashCode()?{??
????try?{??
??????return?((Integer)this.h.invoke(this,?m0,?null)).intValue();??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??public?final?String?toString()?{??
????try?{??
????    return?(String)this.h.invoke(this,?m2,?null);??
????}??
????catch?(Error|RuntimeException?localError)?{??
??????throw?localError;??
????}??
????catch?(Throwable?localThrowable)?{??
??????throw?new?UndeclaredThrowableException(localThrowable);??
????}??
??}??
??
??static?{??
????try?{??
??????m1?=?Class.forName("java.lang.Object").getMethod("equals",?new?Class[]?{?Class.forName("java.lang.Object")?});??
??????m3?=?Class.forName("com.mikan.proxy.MyInterface1").getMethod("sayHello",?new?Class[]?{?Class.forName("java.lang.String")?});??
??????m0?=?Class.forName("java.lang.Object").getMethod("hashCode",?new?Class[0]);??
????m2?=?Class.forName("java.lang.Object").getMethod("toString",?new?Class[0]);??
??????return;??
????}??
????catch?(NoSuchMethodException?localNoSuchMethodException)?{??
??????throw?new?NoSuchMethodError(localNoSuchMethodException.getMessage());????}??
????catch?(ClassNotFoundException?localClassNotFoundException)?{??
??????throw?new?NoClassDefFoundError(localClassNotFoundException.getMessage());??
????}??
??}??
}?

$Proxy0.class的總結(jié)：

對(duì)于要代理沒有接口的類，則必須繼承該沒有接口的類，從而覆蓋其方法，但由于java不能多繼承，這里代理類已經(jīng)繼承了Proxy類了，不能再繼承其他的類，所以JDK的動(dòng)態(tài)代理不支持對(duì)實(shí)現(xiàn)類的代理，只支持接口的代理。

構(gòu)造方法的入?yún)镮nvocationHandler，這說明該代理類是實(shí)現(xiàn)了InvocationHandler接口的類的代理類，而InvocationHandler實(shí)現(xiàn)類再代理真正的實(shí)體類。

靜態(tài)代碼塊會(huì)初始化真正實(shí)體類的Method對(duì)象，以及Object類的equals、hashCode、toString的Method對(duì)象。

總結(jié)：
Proxy 會(huì)從內(nèi)部類WeakCache中根據(jù)ClassLoader 和interfaces獲取ProxyClass（若緩存中存在相同key，則返回ProxyClass，若不存在，則用ProxyClassFactory創(chuàng)建ProxyClass（由ProxyGenerator生成二進(jìn)制碼，再將寫成*.class文件））。

AOP JDK動(dòng)態(tài)代理實(shí)現(xiàn)

切面InvocationHandler：

public?Object?invoke(Object?proxy,?Method?method,?Object[]?args)?throws?Throwable?{??
????//?BeforeAdvice??
????Object?retVal?=?null;??
????try?{??
????????retVal?=?method.invoke(target,?args);??//?AroundAdvice?代替該語句????? 
????????//?AfterReturningAdvice??
????}??
????catch?(Throwable?e)?{??
????????//?AfterThrowingAdvice??
????}??
????finally?{??
????    //finally塊一定會(huì)在return前面執(zhí)行。
????????//?AfterAdvice??
????}??
????return?retVal;??
}??