StrutsPrepareAndExecuteFilter源码剖析

一、概述
     Struts2的核心是一个Filter，Action可以脱离web容器，那么是什么让http请求和action关联在一起的，下面深入源码来分析下Struts2是如何工作的。
FilterDispatcher API写道  Deprecated. Since Struts 2.1.3, use StrutsPrepareAndExecuteFilter instead or StrutsPrepareFilter and StrutsExecuteFilter if needing using the ActionContextCleanUp filter in addition to this one
     鉴于官方推荐使用StrutsPrepareAndExecuteFilter替代FilterDispatcher，这里剖析StrutsPrepareAndExecuteFilter，其在工程中作为一个Filter配置在web.xml中，配置如下：
<filter>
   <filter-name>struts2</filter-name>
   <filter-class>org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter</filter-class>
</filter>
<filter-mapping>
   <filter-name>struts2</filter-name>
   <url-pattern>/*</url-pattern>
</filter-mapping>
二、源码属性方法简介
下面研究下StrutsPrepareAndExecuteFilter源码，类的主要信息如下：
protected  List<Pattern> excludedPatterns
protected  ExecuteOperations execute
protected  PrepareOperations prepare
StrutsPrepareAndExecuteFilter与普通的Filter并无区别，方法除继承自Filter外，仅有一个回调方法，第三部分我们将按照Filter方法调用顺序，由init—>doFilter—>destroy顺序地分析源码。
方法摘要
 void destroy() 继承自Filter，用于资源释放
 void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) 继承自Filter，执行方法
 void init(FilterConfig filterConfig) 继承自Filter，初始化参数
protected  void postInit(Dispatcher dispatcher, FilterConfig filterConfig) 是Callback for post initialization（一个空的方法，用于方法回调初始化）
三、源码剖析   
    1、init方法是Filter第一个运行的方法，看下struts2的核心Filter在调用init方法初始化时做哪些工作：
public void init(FilterConfig filterConfig) throws ServletException {
  InitOperations init = new InitOperations();
//封装filterConfig，其中有个主要方法getInitParameterNames将参数名字以String格式存储在List中
   FilterHostConfig config = new FilterHostConfig(filterConfig);
// 初始化struts内部日志
   init.initLogging(config);
//创建dispatcher ，并初始化，这部分下面我们重点分析，初始化时加载那些资源
   Dispatcher dispatcher = init.initDispatcher(config);
   init.initStaticContentLoader(config, dispatcher);
//初始化类属性：prepare 、execute
   prepare = new PrepareOperations(filterConfig.getServletContext(), dispatcher);
   execute = new ExecuteOperations(filterConfig.getServletContext(), dispatcher);
   this.excludedPatterns = init.buildExcludedPatternsList(dispatcher);
//回调空的postInit方法
   postInit(dispatcher, filterConfig);
   init.cleanup();
}
   首先看下FilterHostConfig ，源码如下：
public class FilterHostConfig implements HostConfig {
   private FilterConfig config; 
public FilterHostConfig(FilterConfig config) { this.config = config;}
public String getInitParameter(String key) {//根据init-param配置的param-name获取param-value的值
  return config.getInitParameter(key);}
public Iterator<String> getInitParameterNames() {//返回初始化参数名的List
   return MakeIterator.convert(config.getInitParameterNames());}
public ServletContext getServletContext() { return config.getServletContext();}
   只有短短的几行代码，getInitParameterNames是这个类的核心，将Filter初始化参数名称有枚举类型转为Iterator。此类的主要作为是对filterConfig 封装。
 
    重点来了，创建并初始化Dispatcher    
public Dispatcher initDispatcher( HostConfig filterConfig ) {
   Dispatcher dispatcher = createDispatcher(filterConfig);
   dispatcher.init();
   return dispatcher;
}
     创建Dispatcher，会读取 filterConfig 中的配置信息，将配置信息解析出来，封装成为一个Map，然后根绝servlet上下文和参数Map构造Dispatcher ：
private Dispatcher createDispatcher( HostConfig filterConfig ) {
   Map<String, String> params = new HashMap<String, String>();
   for ( Iterator e = filterConfig.getInitParameterNames(); e.hasNext(); ) {
      String name = (String) e.next();
      String value = filterConfig.getInitParameter(name);
      params.put(name, value);
   }
   return new Dispatcher(filterConfig.getServletContext(), params);
}
  Dispatcher初始化，加载struts2的相关配置文件，将按照顺序逐一加载：default.properties，struts-default.xml,struts-plugin.xml,struts.xml，……
 
public void init() {
  if (configurationManager == null) configurationManager = new ConfigurationManager(BeanSelectionProvider.DEFAULT_BEAN_NAME);
//加载org/apache/struts2/default.properties
    init_DefaultProperties(); 
//加载struts-default.xml,struts-plugin.xml,struts.xml
   init_TraditionalXmlConfigurations();
   init_LegacyStrutsProperties(); 
//用户自己实现的ConfigurationProviders类
   init_CustomConfigurationProviders(); 
//Filter的初始化参数
   init_FilterInitParameters() ; 
   init_AliasStandardObjects() ; 
   Container container = init_PreloadConfiguration();
   container.inject(this);
   init_CheckConfigurationReloading(container);
   init_CheckWebLogicWorkaround(container);
   if (!dispatcherListeners.isEmpty()) 
      for (DispatcherListener l : dispatcherListeners)  l.dispatcherInitialized(this);
}
 
   初始化default.properties，具体的初始化操作在DefaultPropertiesProvider类中
private void init_DefaultProperties() {
   configurationManager.addConfigurationProvider(new DefaultPropertiesProvider());  }
   
   下面看下DefaultPropertiesProvider类源码：
public void register(ContainerBuilder builder, LocatableProperties props)throws ConfigurationException {
   Settings defaultSettings = null;
   defaultSettings = new PropertiesSettings("org/apache/struts2/default");
   loadSettings(props, defaultSettings);
}
其他的再次省略，可以浏览下各个初始化操作都加载了那些文件
 
3、doFilter方法
     doFilter是过滤器的执行方法，它拦截提交的HttpServletRequest请求，HttpServletResponse响应，作为strtus2的核心拦截器，在doFilter里面到底做了哪些工作，将逐行解读其源码，源码如下：
public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
//父类向子类转：强转为http请求、响应
   HttpServletRequest request = (HttpServletRequest) req;
   HttpServletResponse response = (HttpServletResponse) res;
//设置编码和国际化
   prepare.setEncodingAndLocale(request, response);
//创建Action上下文（重点）
   prepare.createActionContext(request, response);
   prepare.assignDispatcherToThread();
   if ( excludedPatterns != null && prepare.isUrlExcluded(request, excludedPatterns)) {
      chain.doFilter(request, response);
   } else {
      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 execute.executeAction(request, response, mapping);
  }
  prepare.cleanupRequest(request);
}
    setEncodingAndLocale调用了dispatcher方法的prepare方法：
public void setEncodingAndLocale(HttpServletRequest request, HttpServletResponse response) {
   dispatcher.prepare(request, response); }
 
   下面看下prepare方法，这个方法很简单只是设置了encoding 、locale ，做的只是一些辅助的工作：
public void prepare(HttpServletRequest request, HttpServletResponse response) {
   String encoding = null;
   if (defaultEncoding != null)encoding = defaultEncoding;
   Locale locale = null;
   if (defaultLocale != null)locale = LocalizedTextUtil.localeFromString(defaultLocale, request.getLocale());
   if (encoding != null)request.setCharacterEncoding(encoding);
   if (locale != null)response.setLocale(locale);
   if (paramsWorkaroundEnabled) request.getParameter("foo");
}
 
   Action上下文ActionContext是一个容器，这个容易主要存储request、session、application、parameters等相关信息.ActionContext是一个线程的本地变量，这意味着不同的action之间不会共享ActionContext，所以也不用考虑线程安全问题。其实质是一个Map，key是标示request、session、……的字符串，值是其对应的对象：
static ThreadLocal actionContext = new ThreadLocal();
Map<String, Object> context;
   下面看下如何创建action上下文的，代码如下：
public ActionContext createActionContext(HttpServletRequest request, HttpServletResponse response) {
   ActionContext ctx;
   Integer counter = 1;
   Integer oldCounter = (Integer) request.getAttribute(CLEANUP_RECURSION_COUNTER);
   if (oldCounter != null)counter = oldCounter + 1;
//注意此处是从ThreadLocal中获取此ActionContext变量
   ActionContext oldContext = ActionContext.getContext();
   if (oldContext != null) ctx = new ActionContext(new HashMap<String, Object>(oldContext.getContextMap()));
   else {
      ValueStack stack = dispatcher.getContainer().getInstance(ValueStackFactory.class).createValueStack();
      stack.getContext().putAll(dispatcher.createContextMap(request, response, null, servletContext));
//stack.getContext()返回的是一个Map<String，Object>，根据此Map构造一个ActionContext
      ctx = new ActionContext(stack.getContext());
   }
   request.setAttribute(CLEANUP_RECURSION_COUNTER, counter);
//将ActionContext存如ThreadLocal
   ActionContext.setContext(ctx);
   return ctx;
}
   上面代码中dispatcher.createContextMap，如何封装相关参数：
public Map<String,Object> createContextMap(HttpServletRequest request, HttpServletResponse response,ActionMapping mapping, ServletContext context) {
// request map wrapping the http request objects
   Map requestMap = new RequestMap(request);
// parameters map wrapping the http parameters.  ActionMapping parameters are now handled and applied separately
   Map params = new HashMap(request.getParameterMap());
// session map wrapping the http session
   Map session = new SessionMap(request);
// application map wrapping the ServletContext
   Map application = new ApplicationMap(context);
//requestMap、params、session等Map封装成为一个上下文Map，逐个调用了map.put(Map p).
   Map<String,Object> extraContext = createContextMap(requestMap, params, session, application, request, response, context);
   if (mapping != null)extraContext.put(ServletActionContext.ACTION_MAPPING, mapping);
   return extraContext;
}
  RequestMap类实现了抽象Map，故其本身是一个Map，主要方法实现：
//map的get实现
public Object get(Object key) {return request.getAttribute(key.toString());}
//map的put实现
public Object put(Object key, Object value) {
   Object oldValue = get(key);
   entries = null;
   request.setAttribute(key.toString(), value);
   return oldValue;
}
 
   下面是源码展示了如何执行Action控制器：
public void executeAction(HttpServletRequest request, HttpServletResponse response, ActionMapping mapping) throws ServletException {
   dispatcher.serviceAction(request, response, servletContext, mapping);
}
public void serviceAction(HttpServletRequest request, HttpServletResponse response, ServletContext context,ActionMapping mapping) throws ServletException {
//封装执行的上下文环境，主要讲相关信息存储入map
   Map<String, Object> extraContext = createContextMap(request, response, mapping, context);
// 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";
   UtilTimerStack.push(timerKey);
   String namespace = mapping.getNamespace();//获取命名空间
   String name = mapping.getName();//获取action配置的name属性
   String method = mapping.getMethod();//获取action配置的method属性
   Configuration config = configurationManager.getConfiguration();
//根据执行上下文参数，命名空间，名称等创建用户自定义Action的代理对象
   ActionProxy proxy = config.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!
//执行execute方法，并转向结果
    if (mapping.getResult() != null) {
         Result result = mapping.getResult();
         result.execute(proxy.getInvocation());
    } else 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);
  sendError(request, response, context, HttpServletResponse.SC_NOT_FOUND, e);
  UtilTimerStack.pop(timerKey); }
}
 
   因为StrutsXmlConfigurationProvider继承XmlConfigurationProvider，并在register方法回调父类的register，有兴趣的可以深入阅读下下XmlConfigurationProvider源码：
public void register(ContainerBuilder containerBuilder, LocatableProperties props) throws ConfigurationException {
if (servletContext != null && !containerBuilder.contains(ServletContext.class)) {
   containerBuilder.factory(ServletContext.class, new Factory<ServletContext>() {
      public ServletContext create(Context context) throws Exception {
         return servletContext;
      }
   });
}
//调用父类的register，关键点所在
   super.register(containerBuilder, props);
}
struts2-core-2.2.1.jar包中struts-2.1.7.dtd对于Action的定义如下：
<!ELEMENT action (param|result|interceptor-ref|exception-mapping)*>
<!ATTLIST action
name CDATA #REQUIRED
class CDATA #IMPLIED
method CDATA #IMPLIED
converter CDATA #IMPLIED
>
    从上述DTD中可见Action元素可以含有name 、class 、method 、converter 属性。
XmlConfigurationProvider解析struts.xml配置的Action元素：
protected void addAction(Element actionElement, PackageConfig.Builder packageContext) throws ConfigurationException {
   String name = actionElement.getAttribute("name");
   String className = actionElement.getAttribute("class");
   String methodName = actionElement.getAttribute("method");
   Location location = DomHelper.getLocationObject(actionElement);
   methodName = (methodName.trim().length() > 0) ? methodName.trim() : null;
   Map<String, ResultConfig> results;
   results = buildResults(actionElement, packageContext);
   List<InterceptorMapping> interceptorList = buildInterceptorList(actionElement, packageContext);
   List<ExceptionMappingConfig> exceptionMappings = buildExceptionMappings(actionElement,packageContext);
   ActionConfig actionConfig = new ActionConfig.Builder(packageContext.getName(), name, className)
.methodName(methodName).addResultConfigs(results).addInterceptors(interceptorList).addExceptionMappings(exceptionMappings).addParams(XmlHelper.getParams(actionElement)).location(location).build();
   packageContext.addActionConfig(name, actionConfig);
}