struts2源码分析及拦截器实现原理
2016-07-08 05:47
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一:拦截器(interceptor)简介
拦截器(interceptor)是struts2中的最重要的核心之一,是Struts2最强大的特性之一。拦截器可以让你在Action和result被执行之前或之后进行一些处理。同时,拦截器也可以让你将通用的代码模块化并作为可重用的类。Struts2中的很多特性都是由拦截器来完成的。拦截是AOP的一种实现策略。在Webwork的中文文档的解释为:拦截器是动态拦截Action调用的对象。它提供了一种机制可以使开发者可以定义在一个action执行的前后执行的代码,也可以在一个action执行前阻止其执行。同时也是提供了一种可以提取action中可重用的部分的方式。谈到拦截器,还有一个词大家应该知道——拦截器链(Interceptor Chain,在Struts 2中称为拦截器栈Interceptor Stack)。拦截器链就是将拦截器按一定的顺序联结成一条链。在访问被拦截的方法或字段时,拦截器链中的拦截器就会按其之前定义的顺序被调用(这里借鉴引用他人总结)。二:struts2实现原理(源码剖析)
struts2整个执行过程大致如下图所示:下图是在debug跟踪的一个执行过程(由下往上执行顺序):
通过上图可以看出,struts2请求处理的一个过程:
1、客户端发起request请求action请求到达StrutsPrepareAndExecuteFilter的doFilter方法:
/* * $Id: DefaultActionSupport.java 651946 2008-04-27 13:41:38Z apetrelli $ * * Licensed to the Apache Software Foundation (ASF) under one * or more contributor license agreements. See the NOTICE file * distributed with this work for additional information * regarding copyright ownership. The ASF licenses this file * to you 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 * * http://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.apache.struts2.dispatcher.ng.filter; import org.apache.struts2.StrutsStatics; import org.apache.struts2.dispatcher.Dispatcher; import org.apache.struts2.dispatcher.mapper.ActionMapping; import org.apache.struts2.dispatcher.ng.ExecuteOperations; import org.apache.struts2.dispatcher.ng.InitOperations; import org.apache.struts2.dispatcher.ng.PrepareOperations; import javax.servlet.Filter; import javax.servlet.FilterChain; import javax.servlet.FilterConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; import java.util.List; import java.util.regex.Pattern; /** * Handles both the preparation and execution phases of the Struts dispatching process. This filter is better to use * when you don't have another filter that needs access to action context information, such as Sitemesh. */ public class StrutsPrepareAndExecuteFilter implements StrutsStatics, Filter { protected PrepareOperations prepare; protected ExecuteOperations execute; protected List<Pattern> excludedPatterns = null; public void init(FilterConfig filterConfig) throws ServletException { InitOperations init = new InitOperations(); Dispatcher dispatcher = null; try { FilterHostConfig config = new FilterHostConfig(filterConfig); init.initLogging(config); dispatcher = init.initDispatcher(config); init.initStaticContentLoader(config, dispatcher); prepare = new PrepareOperations(dispatcher); execute = new ExecuteOperations(dispatcher); this.excludedPatterns = init.buildExcludedPatternsList(dispatcher); postInit(dispatcher, filterConfig); } finally { if (dispatcher != null) { dispatcher.cleanUpAfterInit(); } init.cleanup(); } } /** * Callback for post initialization */ protected void postInit(Dispatcher dispatcher, FilterConfig filterConfig) { } public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException { HttpServletRequest request = (HttpServletRequest) req; HttpServletResponse response = (HttpServletResponse) res; try { if (excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) { chain.doFilter(request, response); } else { prepare.setEncodingAndLocale(request, response); prepare.createActionContext(request, response); prepare.assignDispatcherToThread(); request = prepare.wrapRequest(request); ActionMapping mapping = prepare.findActionMapping(request, response, true); if (mapping == null) { boolean handled = execute.executeStaticResourceRequest(request, response); if (!handled) { chain.doFilter(request, response); } } else {
<span style="white-space:pre"> </span><span style="color:#ff6600;">//这里实际调用了dispatcher.java的serviceAction方法</span> execute.executeAction(request, response, mapping); } } } finally { prepare.cleanupRequest(request); } } public void destroy() { prepare.cleanupDispatcher(); } }
2、通过execute.executeAction(request, response, mapping); 调用Dispatcher.java的serviceAction方法:
<pre name="code" class="java"> public void serviceAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping) throws ServletException { Map<String, Object> extraContext = createContextMap(request, response, mapping); // If there was a previous value stack, then create a new copy and pass it in to be used by the new Action ValueStack stack = (ValueStack) request.getAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY); boolean nullStack = stack == null; if (nullStack) { ActionContext ctx = ActionContext.getContext(); if (ctx != null) { stack = ctx.getValueStack(); } } if (stack != null) { extraContext.put(ActionContext.VALUE_STACK, valueStackFactory.createValueStack(stack)); } String timerKey = "Handling request from Dispatcher"; try { UtilTimerStack.push(timerKey); String namespace = mapping.getNamespace(); String name = mapping.getName(); String method = mapping.getMethod(); <span style="white-space:pre"> </span><span style="color:#ff6600;">//通过动态代理,创建出一个ActionProxy对象</span> ActionProxy proxy = getContainer().getInstance(ActionProxyFactory.class).createActionProxy( namespace, name, method, extraContext, true, false); request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, proxy.getInvocation().getStack()); // if the ActionMapping says to go straight to a result, do it! if (mapping.getResult() != null) { Result result = mapping.getResult(); result.execute(proxy.getInvocation()); } else { <span style="color:#ff6600;"> //调用execute方法</span> proxy.execute(); } // If there was a previous value stack then set it back onto the request if (!nullStack) { request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack); } } catch (ConfigurationException e) { logConfigurationException(request, e); sendError(request, response, HttpServletResponse.SC_NOT_FOUND, e); } catch (Exception e) { if (handleException || devMode) { sendError(request, response, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, e); } else { throw new ServletException(e); } } finally { UtilTimerStack.pop(timerKey); } }
3、通过动态代理,创建一个ActionProxy的对象,然后调用ActionProxy对象的execute方法:
<pre name="code" class="java" style="line-height: 26px;">ActionProxy对象是创建action的关键对象
<pre name="code" class="java">public String execute() throws Exception { ActionContext nestedContext = ActionContext.getContext();
ActionContext.setContext(invocation.getInvocationContext()); String retCode = null; String profileKey = "execute: "; try { UtilTimerStack.push(profileKey); <span style="white-space:pre"> </span><span style="color:#ff6600;">//调用了ActionInvocation的invoke方法</span> retCode = invocation.invoke(); } finally { if (cleanupContext) { ActionContext.setContext(nestedContext); } UtilTimerStack.pop(profileKey); } return retCode; }
4、通过ActionProxy的execute方法调用了ActionInvocation的invoke方法,这是实现拦截器的核心,ActionInvocation是Action调度者:
<pre name="code" class="java"> public String invoke() throws Exception { String profileKey = "invoke: "; try { UtilTimerStack.push(profileKey); if (executed) { throw new IllegalStateException("Action has already executed"); } if (interceptors.hasNext()) { final InterceptorMapping interceptor = interceptors.next(); String interceptorMsg = "interceptor: " + interceptor.getName(); UtilTimerStack.push(interceptorMsg); try {
<span style="white-space:pre"> </span>//调用interceptor的intercept方法 resultCode = interceptor.getInterceptor().intercept(DefaultActionInvocation.this); } finally { UtilTimerStack.pop(interceptorMsg); } } else {
<span style="white-space:pre"> </span>//执行完默认的intercept方法后调用我们的action resultCode = invokeActionOnly(); }
拦截器部分的执行调用:resultCode = interceptor.getInterceptor().intercept(DefaultActionInvocation.this);
在这个实现类中,实际上已经实现了最简单的拦截器的雏形。这里需要指出的是一个很重要的方法invocation.invoke()。这是ActionInvocation中的方法,而ActionInvocation是Action调度者,所以这个方法具备以下2层含义:
1. 如果拦截器堆栈中还有其他的Interceptor,那么invocation.invoke()将调用堆栈中下一个Interceptor的执行。
2. 如果拦截器堆栈中只有Action了,那么invocation.invoke()将调用Action执行。
所以,我们可以发现,invocation.invoke()这个方法其实是整个拦截器框架的实现核心。基于这样的实现机制,我们还可以得到下面2个非常重要的推论:
1. 如果在拦截器中,我们不使用invocation.invoke()来完成堆栈中下一个元素的调用,而是直接返回一个字符串作为执行结果,那么整个执行将被中止。
2. 我们可以以invocation.invoke()为界,将拦截器中的代码分成2个部分,在invocation.invoke()之前的代码,将会在Action之前被依次执行,而在invocation.invoke()之后的代码,将会在Action之后被逆序执行。
由此,我们就可以通过invocation.invoke()作为Action代码真正的拦截点,从而实现AOP。
5、通过遍历/struts-default.xml中一系列的intercept参数,调用interceptor的intercept方法:
<pre name="code" class="java"> public String intercept(ActionInvocation invocation) throws Exception { String result; try {
<span style="white-space:pre"> </span><span style="color:#ff6600;">//这里又调用ActionInvocation的invoke方法</span> result = invocation.invoke(); } catch (Exception e) { if (isLogEnabled()) { handleLogging(e); } List<ExceptionMappingConfig> exceptionMappings = invocation.getProxy().getConfig().getExceptionMappings(); ExceptionMappingConfig mappingConfig = this.findMappingFromExceptions(exceptionMappings, e); if (mappingConfig != null && mappingConfig.getResult()!=null) { Map parameterMap = mappingConfig.getParams(); // create a mutable HashMap since some interceptors will remove parameters, and parameterMap is immutable invocation.getInvocationContext().setParameters(new HashMap<String, Object>(parameterMap)); result = mappingConfig.getResult(); publishException(invocation, new ExceptionHolder(e)); } else { throw e; } } return result; }原来在intercept()方法又对ActionInvocation的invoke()方法进行递归调用,ActionInvocation循环嵌套在intercept()中,一直到语句result = invocation.invoke()执行结束。这样,Interceptor又会按照刚开始执行的逆向顺序依次执行结束。一个有序链表,通过递归调用,变成了一个堆栈执行过程,将一段有序执行的代码变成了2段执行顺序完全相反的代码过程,从而巧妙地实现了AOP。这也就成为了Struts2的Action层的AOP基础。
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