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Spring中使用@within与@target 详解Spring中使用@within与@target的区别

It''''s my code life.   2021-09-13 我要评论
想了解详解Spring中使用@within与@target的区别的相关内容吗It''''s my code life.在本文为您仔细讲解Spring中使用@within与@target的相关知识和一些Code实例欢迎阅读和指正我们先划重点:Spring,@within与@target,Spring,@within,@target区别下面大家一起来学习吧

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sonhello2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@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@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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