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Java Spring AOP之PointCut案例詳解

 更新時間:2021年09月03日 08:26:56   作者:敲代碼的小小酥  
這篇文章主要介紹了Java Spring AOP之PointCut案例詳解,本篇文章通過簡要的案例,講解了該項技術的了解與使用,以下就是詳細內容,需要的朋友可以參考下

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;

}

由源碼可知,PointCut接口就是定義了兩個元素,ClassFilter和MethodMatcher。PointCut接口就是為了獲得這兩個元素。換句話說,PointCut的功能,都包含在了這兩個元素里。下面看這 兩個元素源碼。

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 &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;

}

這個接口用來過濾要生成代理的類和給定的類是否匹配。
該接口的實現類應該提供正確的equals()方法和hashCode()方法,以便于能在緩存中使用。例如通過cglib生成的代理對象。(這句話什么意思,不理解)。
matches方法就是判斷參數中的class類是否和切點定義的類相匹配。如果匹配,則生成代理對象,如果不匹配,則過濾掉。

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 &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,用來判斷方法是否需要進行增強。ClassFilter過濾需要生成代理的類。而這個類里,不是所有的方法都需要增強的,所以要通過MethodMatcher接口匹配出要增強的方法來。
MethodMatcher分為靜態(tài)匹配和動態(tài)匹配。靜態(tài)匹配是根據方法名匹配。動態(tài)匹配是根據參數進行匹配(不知道這么理解對不對)
MethodMatcher的實現類中,如果isRuntime()方法返回false,則使用靜態(tài)匹配,無論參數是什么,只要方法名匹配,則都會進行增強。而且這個接口中帶有3個參數的matches方法,即matches(java.lang.reflect.Method, Class, Object[])就永遠不會執(zhí)行。
實現類中,如果兩個參數的matches方法返回true,且isRuntime()也返回true。那么在執(zhí)行增強之前,會執(zhí)行三個參數的matches方法,來判斷這個增強是否要執(zhí)行。

下面來看接口三個方法的注釋:

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

靜態(tài)核對給定的方法是否需要增強。如果返回false。則不再會執(zhí)行三個參數的matches方法。

boolean isRuntime();

當兩個參數的matches方法返回true時,是否要執(zhí)行三個參數的matches方法。該方法在AOP代理生成的時候執(zhí)行,而不是每次調用代理方法之前執(zhí)行。意思就是生成代理對象的時候,就已經判斷好要不要執(zhí)行三個參數的matches方法了。

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

該方法執(zhí)行的時機是增強方法要執(zhí)行之前,判斷參數是否滿足要求,如果滿足,則執(zhí)行增強。注意isRuntime方法是創(chuàng)建代理對象時就決定好的,而是否執(zhí)行三個參數的matches方法是調用增強之前判斷的。

總結

從上面的源碼可以看出,PointCut就是起到過濾的作用,首先是過濾類,然后再過濾方法,篩選出需要加強的方法來。由接口可知,我們要將參數中的class或method進行比較,然后過濾,那么,和誰進行比較呢?要過濾的類和方法的規(guī)則存在哪里了呢?請看下回分解。

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