详解Spring中使用@within与@target的区别

  • Z时代
  • 2024-01-10
  • 分类：IT

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

注解定义：
@Target({ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface MyAnnotation {
    String value() default "me";
}
切面定义：
@Order(-1)
@Aspect
@Component
public 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：

father.hello()
father.hello2()
before, myAnnotation.value : son
son.hello()
father.hello2()

父类有注解，子类无注解：

before, myAnnotation.value : father
father.hello()
before, myAnnotation.value : father
father.hello2()
before, myAnnotation.value : father
son.hello()
before, myAnnotation.value : father
father.hello2()

父类有注解，子类有注解(其实就是上面那个例子的结果)：

before, myAnnotation.value : father
father.hello()
before, myAnnotation.value : father
father.hello2()
before, myAnnotation.value : son
son.hello()
before, myAnnotation.value : father
father.hello2()

@target

把切面代码改成如下：

@Order(-1)
@Aspect
@Component
public 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：

father.hello()
father.hello2()
before, myAnnotation.value : son
son.hello()
before, myAnnotation.value : son
father.hello2()

父类有注解，子类无注解：

before, myAnnotation.value : father
father.hello()
before, myAnnotation.value : father
father.hello2()
son.hello()
father.hello2()

父类有注解，子类有注解

before, myAnnotation.value : father
father.hello()
before, myAnnotation.value : father
father.hello2()
before, myAnnotation.value : son
son.hello()
before, myAnnotation.value : son
father.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

@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：

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：

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：

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：

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：

case AT_WITHIN_VAR:
    if (annotationFinder != null) { 
        return annotationFinder.getAnnotationFromClass(getType(), withinType);
    } else return null;

withinType追踪到如下：

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：

case AT_TARGET_VAR:
    if (annotationFinder != null) {
        return annotationFinder.getAnnotation(getType(), targetObject);
    } else return null;

targetObject 追踪到如下：

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

@Order(-1)
@Aspect
@Component
public 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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