代理模式與它在原始碼中的運用

爬蜥發表於2019-01-27
模式原始碼

代理是指一個包裝了真實訪問物件的類，以便控制對真實類的訪問

代理模式與它在原始碼中的運用

訪問流程如下

代理模式與它在原始碼中的運用

public interface SubjectInterface {
    void hi();
}


public class RealSubject implements SubjectInterface {
    @Override
    public void hi() {
        System.out.print("hi");
    }
}

public class SubjectProxy implements SubjectInterface{
    private RealSubject r;
    public SubjectProxy() {
        r=new RealSubject();
    }

    @Override
    public void hi() {
        System.out.println("proxy");
        r.hi();
    }
}

public class Client {
    public static void main(String[] args) {
       SubjectInterface subject = new SubjectProxy();
        subject.hi();
    }
}
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此時RealSubject作為代理物件的一個屬性欄位，在執行之前就會生成RealSubject的位元組碼檔案，這種方式也稱作靜態代理

動態代理

被代理的類在執行時動態生成的，編譯的時候並沒有生成RealSubject

使用JDK實現動態代理

jdk實現動態代理必須有實現介面InvocationHandler的處理類，用於執行被代理類的方法

public class SubjectInvocationHandler implements InvocationHandler {
    private Object myproxy;

    public SubjectInvocationHandler(Object proxy) {
        this.myproxy = proxy;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("invocation handler");
        method.invoke(myproxy,args);
        return null;
    }
}

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客戶端通過使用java.lang.reflect.Proxy自行建立代理,然後呼叫目標方法即可

public class Client {
    public static void main(String[] args) {
    //classloader會被用來驗證是否可以載入傳入的介面，
     SubjectInterface proxy = (SubjectInterface) Proxy.newProxyInstance(SubjectInterface.class.getClassLoader()
     , new Class[]{SubjectInterface.class}
     , new SubjectInvocationHandler(new RealSubject()));
     proxy.hi();
 }
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訪問流程如下

代理模式與它在原始碼中的運用

生成代理proxy class階段

代理物件會在內部快取，如果沒有快取則會由ProxyClassFactory新生成。
首先會做介面校驗，比如是否可以從提供的classLoader獲取介面

    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");
    }
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驗證完畢後，會讀取介面的class檔案使用的是 ProxyGenerator.generateProxyClass,可以看到它會對整個的class檔案的位元組做讀取

private byte[] generateClassFile() {
 ...
 dout.writeInt(0xCAFEBABE);
 ...
}
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最後呼叫native方法生成代理物件，並存入快取

獲取proxy的建構函式

建構函式指定的引數就是InvocationHandler

建立例項

呼叫建構函式，傳入自定義的invocationHandler,自此生成了一個proxy例項,且例項本身會實現傳入的介面,程式碼例項生成的物件如下所示

public final class $Proxy0 extends Proxy implements SubjectInterface {
    ...
       public final void hi() throws  {
        try {
           //這裡的h即Proxy中的InvocationHandler，也就是使用者自定義的InvocationHanlder
           //這個this物件代表的也就是 $Proxy0 本身
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    ...
}
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從JVM dump下動態代理的類

使用CGlib動態代理

引入CGlib之後，執行如下程式碼即可

public class Client {
    public static void main(String[] args) {
        Enhancer enhancer = new Enhancer();
        enhancer.setSuperclass(RealSubject.class);
        enhancer.setCallback(new MethodInterceptor() {
            @Override
            public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
                System.out.println("cglib:"+method.getName());
                methodProxy.invokeSuper(o, objects);
                return o;
            }
        });
        RealSubject realSubject = (RealSubject)enhancer.create();
        realSubject.hi();
    }
}
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動態代理在spring aop中的運用

spring中預設使用DefaultAopProxyFactory

	public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
		if (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config)) {
			Class<?> targetClass = config.getTargetClass();
			if (targetClass == null) {
				throw new AopConfigException("TargetSource cannot determine target class: " +
						"Either an interface or a target is required for proxy creation.");
			}
			if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
			    //對於介面使用的是JDK
				return new JdkDynamicAopProxy(config);
			}
			//其餘使用Cglib作為動態代理的實現
			return new ObjenesisCglibAopProxy(config);
		}
		else {
			return new JdkDynamicAopProxy(config);
		}
	}
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JdkDynamicAopProxy

它實現了InvocationHandler和AopProxy。AopProxy主要是負責提供getProxy,實現為

@Override
	public Object getProxy() {
		return getProxy(ClassUtils.getDefaultClassLoader());
	}

	@Override
	public Object getProxy(@Nullable ClassLoader classLoader) {
		if (logger.isTraceEnabled()) {
			logger.trace("Creating JDK dynamic proxy: " + this.advised.getTargetSource());
		}
		Class<?>[] proxiedInterfaces = AopProxyUtils.completeProxiedInterfaces(this.advised, true);
		findDefinedEqualsAndHashCodeMethods(proxiedInterfaces);
		return Proxy.newProxyInstance(classLoader, proxiedInterfaces, this);
	}

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當有需要執行的方法的時候，則是執行invoke

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
 ...
 	invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
	// Proceed to the joinpoint through the interceptor chain.
	retVal = invocation.proceed();
 ...
}

public Object proceed() throws Throwable {
    ...
    if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
    //這裡會執行AfterReturningAdviceInterceptor、AspectJAfterThrowingAdvice等等方法
    	return dm.interceptor.invoke(this);
    }
    ...
}
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CglibAopProxy

它實現了AopProxy，核心的生成代理的方式如下

public Object getProxy(@Nullable ClassLoader classLoader) {
 ...
 Enhancer enhancer = createEnhancer();
 if (classLoader != null) {
		enhancer.setClassLoader(classLoader);
		if (classLoader instanceof SmartClassLoader &&
				((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
			enhancer.setUseCache(false);
		}
	}
	enhancer.setSuperclass(proxySuperClass);
	enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
	enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
	enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));

	Callback[] callbacks = getCallbacks(rootClass);
	Class<?>[] types = new Class<?>[callbacks.length];
	for (int x = 0; x < types.length; x++) {
		types[x] = callbacks[x].getClass();
	}
	// fixedInterceptorMap only populated at this point, after getCallbacks call above
	enhancer.setCallbackFilter(new ProxyCallbackFilter(
			this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
	enhancer.setCallbackTypes(types);

	// Generate the proxy class and create a proxy instance.
	return createProxyClassAndInstance(enhancer, callbacks);
 ...
}

private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
...
Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);
...
Callback[] mainCallbacks = new Callback[] {
		aopInterceptor,  // for normal advice
		targetInterceptor,  // invoke target without considering advice, if optimized
		new SerializableNoOp(),  // no override for methods mapped to this
		targetDispatcher, this.advisedDispatcher,
		new EqualsInterceptor(this.advised),
		new HashCodeInterceptor(this.advised)
};
...
else {
	callbacks = mainCallbacks;
}
...
}
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以DynamicAdvisedInterceptor為例，它就實現了MethodInterceptor

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
    ...
    //同樣在此處去執行具體的Advice
    retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
    ...
}
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通常可以使用Proxy的情況

  1. 虛代理：根據需要建立開銷很大的物件，只有用到時才建立
  2. 保護代理：控制對原始物件的訪問
  3. 智慧指引：在訪問物件時附加了一些操作，比如物件沒有引用時釋放它
  4. 遠端代理：為一個物件在不同的地址空間提供區域性代表

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