代理模式與它在原始碼中的運用
摘要：
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
訪問流程如下
public interface SubjectInterface {
void hi();
}
public class RealSubject implem...
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
訪問流程如下
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
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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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
- 虛代理：根據需要建立開銷很大的物件，只有用到時才建立
- 保護代理：控制對原始物件的訪問
- 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
- 遠端代理：為一個物件在不同的地址空間提供區域性代表