代理模式與它在源碼中的運用

時間 2019-12-05

標籤代理模式源碼運用简体版

原文原文鏈接

代理是指一個包裝了真實訪問對象的類，以便控制對真實類的訪問java

訪問流程以下spring

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();
    }
}
複製代碼

此時RealSubject做爲代理對象的一個屬性字段，在運行以前就會生成RealSubject的字節碼文件，這種方式也稱做靜態代理segmentfault

動態代理

被代理的類在運行時動態生成的，編譯的時候並無生成RealSubject緩存

使用JDK實現動態代理

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

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;
    }
}

複製代碼

客戶端經過使用java.lang.reflect.Proxy自行建立代理,而後調用目標方法便可app

public class Client {
    public static void main(String[] args) {
    //classloader會被用來驗證是否能夠加載傳入的接口，
     SubjectInterface proxy = (SubjectInterface) Proxy.newProxyInstance(SubjectInterface.class.getClassLoader()
     , new Class[]{SubjectInterface.class}
     , new SubjectInvocationHandler(new RealSubject()));
     proxy.hi();
 }
複製代碼

訪問流程以下ide

生成代理proxy class階段

代理對象會在內部緩存，若是沒有緩存則會由ProxyClassFactory新生成。
首先會作接口校驗，好比是否能夠從提供的classLoader獲取接口函數

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");
    }
複製代碼

驗證完畢後，會讀取接口的class文件使用的是 ProxyGenerator.generateProxyClass,能夠看到它會對整個的class文件的字節作讀取ui

private byte[] generateClassFile() {
 ...
 dout.writeInt(0xCAFEBABE);
 ...
}
複製代碼

最後調用native方法生成代理對象，並存入緩存this

獲取proxy的構造函數

構造函數指定的參數就是InvocationHandler

建立實例

調用構造函數，傳入自定義的invocationHandler,自今生成了一個proxy實例,且實例自己會實現傳入的接口,代碼實例生成的對象以下所示

public final class $Proxy0 extends Proxy implements SubjectInterface {
    ...
       public final void hi() throws  {
        try {
           //這裏的h即Proxy中的InvocationHandler，也就是用戶自定義的InvocationHanlder
           //這個this對象表明的也就是 $Proxy0 自己
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    ...
}
複製代碼

從JVM dump下動態代理的類

使用CGlib動態代理

引入CGlib以後，執行以下代碼便可

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();
    }
}
複製代碼

動態代理在spring aop中的運用

spring中默認使用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)) {
			    //對於接口使用的是JDK
				return new JdkDynamicAopProxy(config);
			}
			//其他使用Cglib做爲動態代理的實現
			return new ObjenesisCglibAopProxy(config);
		}
		else {
			return new JdkDynamicAopProxy(config);
		}
	}
複製代碼

JdkDynamicAopProxy

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

@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);
	}

複製代碼

當有須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)) {
    //這裏會執行AfterReturningAdviceInterceptor、AspectJAfterThrowingAdvice等等方法
    	return dm.interceptor.invoke(this);
    }
    ...
}
複製代碼

CglibAopProxy

它實現了AopProxy，核心的生成代理的方式以下

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;
}
...
}
複製代碼

以DynamicAdvisedInterceptor爲例，它就實現了MethodInterceptor

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
    ...
    //一樣在此處去運行具體的Advice
    retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
    ...
}
複製代碼