Java Spring AOP之PointCut案例详解

一、PointCut接口

/*
* Copyright 2002-2012 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.aop;

/**
* Core Spring pointcut abstraction.
*
* <p>A pointcut is composed of a {@link ClassFilter} and a {@link MethodMatcher}.
* Both these basic terms and a Pointcut itself can be combined to build up combinations
* (e.g. through {@link org.springframework.aop.support.ComposablePointcut}).
*
* @author Rod Johnson
* @see ClassFilter
* @see MethodMatcher
* @see org.springframework.aop.support.Pointcuts
* @see org.springframework.aop.support.ClassFilters
* @see org.springframework.aop.support.MethodMatchers
*/
public interface Pointcut {

/**
* Return the ClassFilter for this pointcut.
* @return the ClassFilter (never {@code null})
*/
ClassFilter getClassFilter();

/**
* Return the MethodMatcher for this pointcut.
* @return the MethodMatcher (never {@code null})
*/
MethodMatcher getMethodMatcher();

/**
* Canonical Pointcut instance that always matches.
*/
Pointcut TRUE = TruePointcut.INSTANCE;

}

由源码可知，PointCut接口就是定义了两个元素，ClassFilter和MethodMatcher。PointCut接口就是为了获得这两个元素。换句话说,PointCut的功能，都包含在了这两个元素里。下面看这 两个元素源码。

二、ClassFilter接口

/*
* Copyright 2002-2019 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.aop;

/**
* Filter that restricts matching of a pointcut or introduction to
* a given set of target classes.
*
* <p>Can be used as part of a {@link Pointcut} or for the entire
* targeting of an {@link IntroductionAdvisor}.
*
* <p>Concrete implementations of this interface typically should provide proper
* implementations of {@link Object#equals(Object)} and {@link Object#hashCode()}
* in order to allow the filter to be used in caching scenarios — for
* example, in proxies generated by CGLIB.
*
* @author Rod Johnson
* @see Pointcut
* @see MethodMatcher
*/
@FunctionalInterface
public interface ClassFilter {

/**
* Should the pointcut apply to the given interface or target class?
* @param clazz the candidate target class
* @return whether the advice should apply to the given target class
*/
boolean matches(Class<?> clazz);

/**
* Canonical instance of a ClassFilter that matches all classes.
*/
ClassFilter TRUE = TrueClassFilter.INSTANCE;

}

这个接口用来过滤要生成代理的类和给定的类是否匹配。
该接口的实现类应该提供正确的equals()方法和hashCode()方法，以便于能在缓存中使用。例如通过cglib生成的代理对象。(这句话什么意思，不理解)。
matches方法就是判断参数中的class类是否和切点定义的类相匹配。如果匹配，则生成代理对象，如果不匹配，则过滤掉。

三、MethodMatcher接口

/*
* Copyright 2002-2019 the original author or authors.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/

package org.springframework.aop;

import java.lang.reflect.Method;

/**
* Part of a {@link Pointcut}: Checks whether the target method is eligible for advice.
*
* <p>A MethodMatcher may be evaluated <b>statically</b> or at <b>runtime</b> (dynamically).
* Static matching involves method and (possibly) method attributes. Dynamic matching
* also makes arguments for a particular call available, and any effects of running
* previous advice applying to the joinpoint.
*
* <p>If an implementation returns {@code false} from its {@link #isRuntime()}
* method, evaluation can be performed statically, and the result will be the same
* for all invocations of this method, whatever their arguments. This means that
* if the {@link #isRuntime()} method returns {@code false}, the 3-arg
* {@link #matches(java.lang.reflect.Method, Class, Object[])} method will never be invoked.
*
* <p>If an implementation returns {@code true} from its 2-arg
* {@link #matches(java.lang.reflect.Method, Class)} method and its {@link #isRuntime()} method
* returns {@code true}, the 3-arg {@link #matches(java.lang.reflect.Method, Class, Object[])}
* method will be invoked <i>immediately before each potential execution of the related advice</i>,
* to decide whether the advice should run. All previous advice, such as earlier interceptors
* in an interceptor chain, will have run, so any state changes they have produced in
* parameters or ThreadLocal state will be available at the time of evaluation.
*
* <p>Concrete implementations of this interface typically should provide proper
* implementations of {@link Object#equals(Object)} and {@link Object#hashCode()}
* in order to allow the matcher to be used in caching scenarios — for
* example, in proxies generated by CGLIB.
*
* @author Rod Johnson
* @since 11.11.2003
* @see Pointcut
* @see ClassFilter
*/
public interface MethodMatcher {

/**
* Perform static checking whether the given method matches.
* <p>If this returns {@code false} or if the {@link #isRuntime()}
* method returns {@code false}, no runtime check (i.e. no
* {@link #matches(java.lang.reflect.Method, Class, Object[])} call)
* will be made.
* @param method the candidate method
* @param targetClass the target class
* @return whether or not this method matches statically
*/
boolean matches(Method method, Class<?> targetClass);

/**
* Is this MethodMatcher dynamic, that is, must a final call be made on the
* {@link #matches(java.lang.reflect.Method, Class, Object[])} method at
* runtime even if the 2-arg matches method returns {@code true}?
* <p>Can be invoked when an AOP proxy is created, and need not be invoked
* again before each method invocation,
* @return whether or not a runtime match via the 3-arg
* {@link #matches(java.lang.reflect.Method, Class, Object[])} method
* is required if static matching passed
*/
boolean isRuntime();

/**
* Check whether there a runtime (dynamic) match for this method,
* which must have matched statically.
* <p>This method is invoked only if the 2-arg matches method returns
* {@code true} for the given method and target class, and if the
* {@link #isRuntime()} method returns {@code true}. Invoked
* immediately before potential running of the advice, after any
* advice earlier in the advice chain has run.
* @param method the candidate method
* @param targetClass the target class
* @param args arguments to the method
* @return whether there's a runtime match
* @see MethodMatcher#matches(Method, Class)
*/
boolean matches(Method method, Class<?> targetClass, Object... args);

/**
* Canonical instance that matches all methods.
*/
MethodMatcher TRUE = TrueMethodMatcher.INSTANCE;

}

PointCut的一部分，用来判断方法是否需要进行增强。ClassFilter过滤需要生成代理的类。而这个类里，不是所有的方法都需要增强的，所以要通过MethodMatcher接口匹配出要增强的方法来。
MethodMatcher分为静态匹配和动态匹配。静态匹配是根据方法名匹配。动态匹配是根据参数进行匹配(不知道这么理解对不对)
MethodMatcher的实现类中，如果isRuntime()方法返回false，则使用静态匹配，无论参数是什么，只要方法名匹配，则都会进行增强。而且这个接口中带有3个参数的matches方法，即matches(java.lang.reflect.Method, Class, Object[])就永远不会执行。
实现类中，如果两个参数的matches方法返回true，且isRuntime()也返回true。那么在执行增强之前，会执行三个参数的matches方法，来判断这个增强是否要执行。

下面来看接口三个方法的注释:

boolean matches(Method method, Class<?> targetClass);

静态核对给定的方法是否需要增强。如果返回false。则不再会执行三个参数的matches方法。

boolean isRuntime();

当两个参数的matches方法返回true时，是否要执行三个参数的matches方法。该方法在AOP代理生成的时候执行，而不是每次调用代理方法之前执行。意思就是生成代理对象的时候，就已经判断好要不要执行三个参数的matches方法了。

boolean matches(Method method, Class<?> targetClass, Object... args);

该方法执行的时机是增强方法要执行之前，判断参数是否满足要求，如果满足，则执行增强。注意isRuntime方法是创建代理对象时就决定好的，而是否执行三个参数的matches方法是调用增强之前判断的。

总结

从上面的源码可以看出，PointCut就是起到过滤的作用，首先是过滤类，然后再过滤方法，筛选出需要加强的方法来。由接口可知，我们要将参数中的class或method进行比较，然后过滤，那么，和谁进行比较呢？要过滤的类和方法的规则存在哪里了呢？请看下回分解。

来源：https://blog.csdn.net/qq1309664161/article/details/120061132