通知是对目标对象方法的增强，而引入可以动态为目标对象实现新的接口，实现对类的增强。

引入的使用

目标类

public class DogService {public void hi() {System.out.println("wangwang");}
}

增强接口

public interface CatService {void eat();
}

增强接口实现类

public class CatServiceImpl implements CatService {@Overridepublic void eat() {System.out.println("cat eat");}
}

切面类

package com.morris.spring.config;import com.morris.spring.service.CatService;
import com.morris.spring.service.CatServiceImpl;
import org.aspectj.lang.annotation.Aspect;
import org.aspectj.lang.annotation.DeclareParents;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.EnableAspectJAutoProxy;@Configuration
@EnableAspectJAutoProxy
@Aspect
public class IntroduceConfig {@DeclareParents(value = "com.morris.spring.service.*+", defaultImpl = CatServiceImpl.class)private CatService catService;}

测试类

package com.morris.spring.demo.annotation;import com.morris.spring.config.IntroduceConfig;
import com.morris.spring.service.CatService;
import com.morris.spring.service.DogService;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;public class IntroduceDemo {public static void main(String[] args) {AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();applicationContext.register(DogService.class);applicationContext.register(IntroduceConfig.class);applicationContext.refresh();DogService dogService = applicationContext.getBean(DogService.class);dogService.hi();CatService catService = (CatService) dogService;catService.eat();}
}

运行结果如下：

cat eat
wangwang

从运行结果可以发现DogService拥有了CatService接口的能力。

源码实现

注解的扫描

解析带有@Aspect注解类下面的有@DeclareParents注解的属性：

org.springframework.aop.aspectj.annotation.ReflectiveAspectJAdvisorFactory#getAdvisors

public List<Advisor> getAdvisors(MetadataAwareAspectInstanceFactory aspectInstanceFactory) {... ...// Find introduction fields.// 引入for (Field field : aspectClass.getDeclaredFields()) {Advisor advisor = getDeclareParentsAdvisor(field);if (advisor != null) {advisors.add(advisor);}}return advisors;
}

遍历所有带有@DeclareParents注解的属性，封装成为DeclareParentsAdvisor对象。

org.springframework.aop.aspectj.annotation.ReflectiveAspectJAdvisorFactory#getDeclareParentsAdvisor

private Advisor getDeclareParentsAdvisor(Field introductionField) {DeclareParents declareParents = introductionField.getAnnotation(DeclareParents.class);if (declareParents == null) {// Not an introduction fieldreturn null;}if (DeclareParents.class == declareParents.defaultImpl()) {throw new IllegalStateException("'defaultImpl' attribute must be set on DeclareParents");}return new DeclareParentsAdvisor(introductionField.getType(), declareParents.value(), declareParents.defaultImpl());
}

DeclareParentsAdvisor包含两部分：

ClassFilter：TypePatternClassFilter，根据@DeclareParents中配置的表达式进行匹配
Advice：DelegatePerTargetObjectIntroductionInterceptor，增强逻辑

目标类的匹配

调用IntroductionAdvisor.getClassFilter().matches()进行类的匹配，上面注入的DeclareParentsAdvisor对象是IntroductionAdvisor的子类，也就是会根据@DeclareParents注解中指定的表达式与类的全限定名进行匹配，这里只匹配类，不会匹配方法。

org.springframework.aop.support.AopUtils#canApply

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {if (advisor instanceof IntroductionAdvisor) {return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);}else if (advisor instanceof PointcutAdvisor) {PointcutAdvisor pca = (PointcutAdvisor) advisor;return canApply(pca.getPointcut(), targetClass, hasIntroductions);}else {// It doesn't have a pointcut so we assume it applies.return true;}
}

与方法的增强一样，如果匹配到了目标类就会为目标类生成代理对象。

目标方法的调用

可以使用arthas工具导出内存中动态代理生成的代理类：

public class DogService$$EnhancerBySpringCGLIB$$13feac43 extends DogService implements CatService, SpringProxy, Advised, Factory {private boolean CGLIB$BOUND;public static Object CGLIB$FACTORY_DATA;private static final ThreadLocal CGLIB$THREAD_CALLBACKS;private static final Callback[] CGLIB$STATIC_CALLBACKS;private MethodInterceptor CGLIB$CALLBACK_0;private MethodInterceptor CGLIB$CALLBACK_1;private NoOp CGLIB$CALLBACK_2;private Dispatcher CGLIB$CALLBACK_3;private Dispatcher CGLIB$CALLBACK_4;private MethodInterceptor CGLIB$CALLBACK_5;private MethodInterceptor CGLIB$CALLBACK_6;private static Object CGLIB$CALLBACK_FILTER;private static final Method CGLIB$hi$0$Method;private static final MethodProxy CGLIB$hi$0$Proxy;private static final Object[] CGLIB$emptyArgs;private static final Method CGLIB$equals$1$Method;private static final MethodProxy CGLIB$equals$1$Proxy;private static final Method CGLIB$toString$2$Method;private static final MethodProxy CGLIB$toString$2$Proxy;private static final Method CGLIB$hashCode$3$Method;private static final MethodProxy CGLIB$hashCode$3$Proxy;private static final Method CGLIB$clone$4$Method;private static final MethodProxy CGLIB$clone$4$Proxy;private static final Method CGLIB$eat$5$Method;private static final MethodProxy CGLIB$eat$5$Proxy;static void CGLIB$STATICHOOK3() {CGLIB$THREAD_CALLBACKS = new ThreadLocal();CGLIB$emptyArgs = new Object[0];Class var0 = Class.forName("com.morris.spring.service.DogService$$EnhancerBySpringCGLIB$$13feac43");Class var1;Method[] var10000 = ReflectUtils.findMethods(new String[]{"equals", "(Ljava/lang/Object;)Z", "toString", "()Ljava/lang/String;", "hashCode", "()I", "clone", "()Ljava/lang/Object;"}, (var1 = Class.forName("java.lang.Object")).getDeclaredMethods());CGLIB$equals$1$Method = var10000[0];CGLIB$equals$1$Proxy = MethodProxy.create(var1, var0, "(Ljava/lang/Object;)Z", "equals", "CGLIB$equals$1");CGLIB$toString$2$Method = var10000[1];CGLIB$toString$2$Proxy = MethodProxy.create(var1, var0, "()Ljava/lang/String;", "toString", "CGLIB$toString$2");CGLIB$hashCode$3$Method = var10000[2];CGLIB$hashCode$3$Proxy = MethodProxy.create(var1, var0, "()I", "hashCode", "CGLIB$hashCode$3");CGLIB$clone$4$Method = var10000[3];CGLIB$clone$4$Proxy = MethodProxy.create(var1, var0, "()Ljava/lang/Object;", "clone", "CGLIB$clone$4");CGLIB$eat$5$Method = ReflectUtils.findMethods(new String[]{"eat", "()V"}, (var1 = Class.forName("com.morris.spring.service.CatService")).getDeclaredMethods())[0];CGLIB$eat$5$Proxy = MethodProxy.create(var1, var0, "()V", "eat", "CGLIB$eat$5");CGLIB$hi$0$Method = ReflectUtils.findMethods(new String[]{"hi", "()V"}, (var1 = Class.forName("com.morris.spring.service.DogService")).getDeclaredMethods())[0];CGLIB$hi$0$Proxy = MethodProxy.create(var1, var0, "()V", "hi", "CGLIB$hi$0");}public final void hi() {MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;if (var10000 == null) {CGLIB$BIND_CALLBACKS(this);var10000 = this.CGLIB$CALLBACK_0;}if (var10000 != null) {var10000.intercept(this, CGLIB$hi$0$Method, CGLIB$emptyArgs, CGLIB$hi$0$Proxy);} else {super.hi();}}public final void eat() {MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;if (var10000 == null) {CGLIB$BIND_CALLBACKS(this);var10000 = this.CGLIB$CALLBACK_0;}if (var10000 != null) {var10000.intercept(this, CGLIB$eat$5$Method, CGLIB$emptyArgs, CGLIB$eat$5$Proxy);} else {super.eat();}}
... ....
}

代理方法都会调用MethodInterceptor，类型为DynamicAdvisedInterceptor。

org.springframework.aop.framework.CglibAopProxy.DynamicAdvisedInterceptor

public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {Object oldProxy = null;boolean setProxyContext = false;Object target = null;TargetSource targetSource = this.advised.getTargetSource();try {if (this.advised.exposeProxy) {// Make invocation available if necessary.oldProxy = AopContext.setCurrentProxy(proxy);setProxyContext = true;}// Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...target = targetSource.getTarget();Class<?> targetClass = (target != null ? target.getClass() : null);// 获得所有合格的advice中的MethodInterceptorList<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);Object retVal;// Check whether we only have one InvokerInterceptor: that is,// no real advice, but just reflective invocation of the target.if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) {// We can skip creating a MethodInvocation: just invoke the target directly.// Note that the final invoker must be an InvokerInterceptor, so we know// it does nothing but a reflective operation on the target, and no hot// swapping or fancy proxying.// 直接调用目标方法Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);retVal = methodProxy.invoke(target, argsToUse);}else {// We need to create a method invocation...// 创建CglibMethodInvocation链式调用，与jdk动态代理的逻辑一样retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();}retVal = processReturnType(proxy, target, method, retVal);return retVal;}finally {if (target != null && !targetSource.isStatic()) {targetSource.releaseTarget(target);}if (setProxyContext) {// Restore old proxy.AopContext.setCurrentProxy(oldProxy);}}
}

此方法的逻辑与org.springframework.aop.framework.JdkDynamicAopProxy#invoke类似。

CglibMethodInvocation会调用advice中的MethodInterceptor。

先看上面DeclareParentsAdvisor的构造方法：

public DeclareParentsAdvisor(Class<?> interfaceType, String typePattern, Class<?> defaultImpl) {this(interfaceType, typePattern,new DelegatePerTargetObjectIntroductionInterceptor(defaultImpl, interfaceType));
}

将目标类封装到了DelegatePerTargetObjectIntroductionInterceptor，而DelegatePerTargetObjectIntroductionInterceptor是一个MethodInterceptor，最终目标方法的执行都会调到MethodInterceptor.invoke()：

public Object invoke(MethodInvocation mi) throws Throwable {if (isMethodOnIntroducedInterface(mi)) {Object delegate = getIntroductionDelegateFor(mi.getThis());// Using the following method rather than direct reflection,// we get correct handling of InvocationTargetException// if the introduced method throws an exception.// 调用目标方法Object retVal = AopUtils.invokeJoinpointUsingReflection(delegate, mi.getMethod(), mi.getArguments());// Massage return value if possible: if the delegate returned itself,// we really want to return the proxy.if (retVal == delegate && mi instanceof ProxyMethodInvocation) {retVal = ((ProxyMethodInvocation) mi).getProxy();}return retVal;}return doProceed(mi);
}

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