项目里用到@within时，出现了一些问题，使用@target就可以解决，但又会出现一些新的问题，因此本文探讨了在spring中，使用@within和@target的一些区别。
背景
项目里有一个动态切换数据源的功能，我们是用切面来实现的，是基于注解来实现的，但是父类的方法是可以切换数据源的，如果有一个类直接继承这个类，调用这个子类时，这个子类是不能够切换数据源的，除非这个子类重写父类的方法。
模拟项目例子
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注解定义：@Target({ElementType.METHOD, ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Inherited@Documentedpublic @interface MyAnnotation { String value() default "me";}切面定义：@Order(-1)@Aspect@Componentpublic class MyAspect { @Before("@within(myAnnotation)") public void switchDataSource(JoinPoint point, MyAnnotation myAnnotation) { System.out.println("before, myAnnotation.value : " + myAnnotation.value()); }}父类Bean：@MyAnnotation("father")public class Father { public void hello() { System.out.println("father.hello()"); } public void hello2() { System.out.println("father.hello2()"); }}子类Bean：@MyAnnotation("son")public class Son extends Father { @Override public void hello() { System.out.println("son.hello()"); }}配置类：@Configuration@EnableAspectJAutoProxy(exposeProxy = true)public class Config { @Bean public Father father() { return new Father(); } @Bean public Son son() { return new Son(); }}测试类：public class Main { public static void main(String[] args) { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Config.class, MyAspect.class); Father father = context.getBean("father", Father.class); father.hello(); father.hello2(); Son son = context.getBean(Son.class); son.hello(); son.hello2(); }} |
我们定义了一个@Before通知，方法参数有point, myAnnotation，方法里输出了myAnnotation.value的值
下面是输出结果：
before, myAnnotation.value : father
father.hello()
before, myAnnotation.value : father
father.hello2()
before, myAnnotation.value : son
son.hello()
before, myAnnotation.value : father
father.hello2()
从上面的输出结果看出：Son类重写了hello方法，myAnnotation.value的输出的值是son，hello2方法没有重写，myAnnotation.value的输出的值是father
根据需求，我们肯定希望调用Son类的所有方法时，都希望myAnnotation.value的输出的值是son，因此就需要重写父类的所有public方法
那有没有办法不重写这些方法也能达到相同的效果呢，答案是可以的。
看看使用@within和@target的区别
我们分别在父类和子类上加上注解和去掉注解，一起来看看对应的结果
@within
父类无注解，子类有注解：
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father.hello()father.hello2()before, myAnnotation.value : sonson.hello()father.hello2() |
父类有注解，子类无注解：
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before, myAnnotation.value : fatherfather.hello()before, myAnnotation.value : fatherfather.hello2()before, myAnnotation.value : fatherson.hello()before, myAnnotation.value : fatherfather.hello2() |
父类有注解，子类有注解(其实就是上面那个例子的结果)：
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before, myAnnotation.value : fatherfather.hello()before, myAnnotation.value : fatherfather.hello2()before, myAnnotation.value : sonson.hello()before, myAnnotation.value : fatherfather.hello2() |
@target
把切面代码改成如下：
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@Order(-1)@Aspect@Componentpublic class MyAspect { @Before("@target(myAnnotation)") public void switchDataSource(JoinPoint point, MyAnnotation myAnnotation) { System.out.println("before, myAnnotation.value : " + myAnnotation.value()); }} |
我们再一起来看看测试结果：
父类无注解，子类有注解：
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father.hello()father.hello2()before, myAnnotation.value : sonson.hello()before, myAnnotation.value : sonfather.hello2() |
父类有注解，子类无注解：
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before, myAnnotation.value : fatherfather.hello()before, myAnnotation.value : fatherfather.hello2()son.hello()father.hello2() |
父类有注解，子类有注解
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before, myAnnotation.value : fatherfather.hello()before, myAnnotation.value : fatherfather.hello2()before, myAnnotation.value : sonson.hello()before, myAnnotation.value : sonfather.hello2() |
我们从上面总结出一套规律：
@within：@Before通知方法的myAnnotation参数指的是调用方法所在的类上面的注解，就是这个方法是在哪个类上定义的
@target：@Before通知方法的myAnnotation参数指的是调用方法运行时所属于的类上面的注解
我们最后总结一下，如果父类和子类上都标有注解，@within和@target的所得到实际注解的区别
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@within |
@target | |
|---|---|---|
| 父类方法 | 父类注解 | 父类注解 |
| 子类不重写方法 | 父类注解 | 子类注解 |
| 子类重写方法 | 子类注解 | 子类注解 |
@target 看起来跟合理一点
从上面的分析可以看出，其实用@target更符合我们想要的结果，在某个类上面加一个注解，拦截的时候就会获取这个类上面的注解，跟父类完全没有关系了
但这个时候会遇到一个问题，就是不相关的类都会生从代理类，
例子如下：
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public class NormalBean { public void hello() { }}@Configuration@EnableAspectJAutoProxy(exposeProxy = true)public class Config { @Bean public Father father() { return new Father(); } @Bean public Son son() { return new Son(); } @Bean public NormalBean normalBean() { return new NormalBean(); }}public class Main { public static void main(String[] args) { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Config.class, MyAspect.class); Father father = context.getBean("father", Father.class); father.hello(); father.hello2(); Son son = context.getBean(Son.class); son.hello(); son.hello2(); NormalBean normalBean = context.getBean(NormalBean.class); System.out.println(normalBean.getClass()); }} |
输出：
class cn.eagleli.spring.aop.demo.NormalBean$$EnhancerBySpringCGLIB$$eebc2a39
可以看出NormalBean自己什么都没做，但却被代理了
我们再把@target换成@within：
class cn.eagleli.spring.aop.demo.NormalBean
可以看出使用@within时，不相关的类没有被代理
我们一起来看看为什么
在AbstractAutoProxyCreator类中的wrapIfNecessary方法打断点，看看什么情况：
@within
@target
我们从上面的图片就可以理解为什么@target会生成代理类
我们再深入看一下：
@within会走到如下：
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public class ExactAnnotationTypePattern extends AnnotationTypePattern { @Override public FuzzyBoolean matches(AnnotatedElement annotated, ResolvedType[] parameterAnnotations) { // ...... }} |
我没深入研究，大致意思就是只要这个类或者这个类的祖先们带有这个注解，即匹配成功
@target会走到如下：
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public class ThisOrTargetAnnotationPointcut extends NameBindingPointcut { @Override protected FuzzyBoolean matchInternal(Shadow shadow) { if (!couldMatch(shadow)) { return FuzzyBoolean.NO; } ResolvedType toMatchAgainst = (isThis ? shadow.getThisType() : shadow.getTargetType()).resolve(shadow.getIWorld()); annotationTypePattern.resolve(shadow.getIWorld()); if (annotationTypePattern.matchesRuntimeType(toMatchAgainst).alwaysTrue()) { return FuzzyBoolean.YES; } else { // a subtype may match at runtime return FuzzyBoolean.MAYBE; } }}public class AspectJExpressionPointcut extends AbstractExpressionPointcut implements ClassFilter, IntroductionAwareMethodMatcher, BeanFactoryAware { @Override public boolean matches(Method method, Class<?> targetClass, boolean hasIntroductions) { obtainPointcutExpression(); ShadowMatch shadowMatch = getTargetShadowMatch(method, targetClass); // Special handling for this, target, @this, @target, @annotation // in Spring - we can optimize since we know we have exactly this class, // and there will never be matching subclass at runtime. if (shadowMatch.alwaysMatches()) { return true; } else if (shadowMatch.neverMatches()) { return false; } else { // the maybe case if (hasIntroductions) { return true; } // A match test returned maybe - if there are any subtype sensitive variables // involved in the test (this, target, at_this, at_target, at_annotation) then // we say this is not a match as in Spring there will never be a different // runtime subtype. RuntimeTestWalker walker = getRuntimeTestWalker(shadowMatch); return (!walker.testsSubtypeSensitiveVars() || walker.testTargetInstanceOfResidue(targetClass)); // 这里会返回true } }} |
我没深入研究，大致意思是匹配的话就返回YES，否则就返回MAYBE，匹配逻辑是和@within一样的
因此所有不相关的类都会是一个MAYBE的结果，这个结果会让不相关的类最后生成代理类
通知方法中注解参数的值为什么是不一样的
经过调试，最终是在这里获取的：
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public final class ReflectionVar extends Var { static final int THIS_VAR = 0; static final int TARGET_VAR = 1; static final int ARGS_VAR = 2; static final int AT_THIS_VAR = 3; static final int AT_TARGET_VAR = 4; static final int AT_ARGS_VAR = 5; static final int AT_WITHIN_VAR = 6; static final int AT_WITHINCODE_VAR = 7; static final int AT_ANNOTATION_VAR = 8; public Object getBindingAtJoinPoint( Object thisObject, Object targetObject, Object[] args, Member subject, Member withinCode, Class withinType) { switch( this.varType) { case THIS_VAR: return thisObject; case TARGET_VAR: return targetObject; case ARGS_VAR: if (this.argsIndex > (args.length - 1)) return null; return args[argsIndex]; case AT_THIS_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotation(getType(), thisObject); } else return null; case AT_TARGET_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotation(getType(), targetObject); } else return null; case AT_ARGS_VAR: if (this.argsIndex > (args.length - 1)) return null; if (annotationFinder != null) { return annotationFinder.getAnnotation(getType(), args[argsIndex]); } else return null; case AT_WITHIN_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotationFromClass(getType(), withinType); } else return null; case AT_WITHINCODE_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotationFromMember(getType(), withinCode); } else return null; case AT_ANNOTATION_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotationFromMember(getType(), subject); } else return null; } return null; }} |
@within：
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case AT_WITHIN_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotationFromClass(getType(), withinType); } else return null; |
withinType追踪到如下：
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public class PointcutExpressionImpl implements PointcutExpression { private ShadowMatch matchesExecution(Member aMember) { Shadow s = ReflectionShadow.makeExecutionShadow(world, aMember, this.matchContext); ShadowMatchImpl sm = getShadowMatch(s); sm.setSubject(aMember); sm.setWithinCode(null); sm.setWithinType(aMember.getDeclaringClass()); // 这里设置withinType return sm; }}public abstract class AopUtils { public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) { Assert.notNull(pc, "Pointcut must not be null"); if (!pc.getClassFilter().matches(targetClass)) { return false; } MethodMatcher methodMatcher = pc.getMethodMatcher(); if (methodMatcher == MethodMatcher.TRUE) { // No need to iterate the methods if we're matching any method anyway... return true; } IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null; if (methodMatcher instanceof IntroductionAwareMethodMatcher) { introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher; } Set<Class<?>> classes = new LinkedHashSet<>(); if (!Proxy.isProxyClass(targetClass)) { classes.add(ClassUtils.getUserClass(targetClass)); } classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass)); for (Class<?> clazz : classes) { Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz); for (Method method : methods) { // 这里获取所有method if (introductionAwareMethodMatcher != null ? introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) : methodMatcher.matches(method, targetClass)) { return true; } } } return false; }} |
@target：
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case AT_TARGET_VAR: if (annotationFinder != null) { return annotationFinder.getAnnotation(getType(), targetObject); } else return null; |
targetObject 追踪到如下：
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public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport implements SmartInstantiationAwareBeanPostProcessor, BeanFactoryAware { protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) { if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) { return bean; } if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) { return bean; } if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) { this.advisedBeans.put(cacheKey, Boolean.FALSE); return bean; } // Create proxy if we have advice. Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null); if (specificInterceptors != DO_NOT_PROXY) { this.advisedBeans.put(cacheKey, Boolean.TRUE); Object proxy = createProxy( bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean)); // 这里，targetObject就是生成的bean this.proxyTypes.put(cacheKey, proxy.getClass()); return proxy; } this.advisedBeans.put(cacheKey, Boolean.FALSE); return bean; } public SingletonTargetSource(Object target) { Assert.notNull(target, "Target object must not be null"); this.target = target; }} |
想用@within，但又想得到想要的注解
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@Order(-1)@Aspect@Componentpublic class MyAspect { @Before("@within(myAnnotation)") public void switchDataSource(JoinPoint point, MyAnnotation myAnnotation) { System.out.println(point.getTarget() + " " + point + " " + myAnnotation.value() + " " + point.getTarget().getClass().getAnnotation(MyAnnotation.class).value()); }} |
很简单，从JoinPoint中得到target，然后从这个类上得到对应的注解即可
此时，父类和子类都加有注解，一起来看看输出结果：
cn.eagleli.spring.aop.demo.Father@194fad1 execution(void cn.eagleli.spring.aop.demo.Father.hello()) father father
cn.eagleli.spring.aop.demo.Father@194fad1 execution(void cn.eagleli.spring.aop.demo.Father.hello2()) father father
cn.eagleli.spring.aop.demo.Son@14fc5f04 execution(void cn.eagleli.spring.aop.demo.Son.hello()) son son
cn.eagleli.spring.aop.demo.Son@14fc5f04 execution(void cn.eagleli.spring.aop.demo.Father.hello2()) father son
能力有限，只能先探讨这么多了，不懂的或者有其他见解的，欢迎一起讨论呀~
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原文链接：https://www.cnblogs.com/eaglelihh/p/15201208.html
