Java Spring AOP之PointCut案例详解

时间：2021-12-02 15:12:19

一、PointCut接口

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/*
 * 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

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/*
 * 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 &mdash; 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

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/*
 * 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 &mdash; 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:

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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。

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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。

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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。

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原文链接：https://blog.csdn.net/qq1309664161/article/details/120061132

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