struts2 实现原理解析

struts2 实现原理解析

struts2 操作 action servlet,是通过在web.xml中配置一个filter实现的

<filter>         <filter-name>struts2</filter-name>         <filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class>      </filter>        <filter-mapping>         <filter-name>struts2</filter-name>         <url-pattern>/*</url-pattern>     </filter-mapping>

 
   注:现在新版本的struts2已经改成StrutsPrepareAndExecuteFilter,但原理大同小异,我们还按FilterDispatcher讲解
<filter-class>
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
</filter-class>

  

而这个Filter FilterDispatcher 实际做了下列工作  

 public class FilterDispatcher implements StrutsStatics, Filter
1.filter的init()方法建立一个Dispatcher对象

以后struts2 都是围绕这个Dispatcher对象进行操作
init()方法将参数FilterConfig对象传给Dispatcher对象,实际就是将ServletContext对象传递给Dispatcher对象

而ServletContext包含了各种基本Servlet的运行环境,比如ServletContext提供了getAttribute(), setAttribute() 等方法

所以,实际是Dispatcher对象接管了基本Servlet的一切功能

public void init(FilterConfig filterConfig) throws ServletException {         dispatcher = createDispatcher(filterConfig);         ...         }

protected Dispatcher createDispatcher(FilterConfig filterConfig) {         ...         return new Dispatcher(filterConfig.getServletContext(), params);     }

2.filter的doFilter()方法继续操作前面init()方法建立的作Dispatcher对象,将HttpServletRequest对象和HttpServletResponse对象传入Dispatcher中

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {         HttpServletRequest request = (HttpServletRequest) req;         HttpServletResponse response = (HttpServletResponse) res;         ServletContext servletContext = getServletContext();                 ....         dispatcher.serviceAction(request, response, servletContext, mapping);     }

 

 Dispatcher对象是Struts2的核心操作对象,它主要实现实现了下面的工作
 1.它是线程安全的,即Dispatcher对象支持多线程,且每线程一副本

public class Dispatcher {  private static ThreadLocal<Dispatcher> instance = new ThreadLocal<Dispatcher>();  //Store the dispatcher instance for this thread.   public static void setInstance(Dispatcher instance) {      Dispatcher.instance.set(instance);      // Tie the ObjectFactory threadlocal instance to this Dispatcher instance               if (instance != null) {          Container cont = instance.getContainer();                   if (cont != null) {              ObjectFactory.setObjectFactory(cont.getInstance(ObjectFactory.class));          } else {              LOG.warn("This dispatcher instance doesn't have a container, so the object factory won't be set.");          }      } else {          ObjectFactory.setObjectFactory(null);      }  }  //Provide the dispatcher instance for the current thread.    public static Dispatcher getInstance() {      return instance.get();  }

2.上边说了,Dispatcher对象的建立/构造,是接受了参数FilterDispatcher过滤器的FilterConfig传来的ServletContext,这样才接管了基本Servlet的一切功能

private ServletContext servletContext; private Map<String, String> initParams;  public  Dispatcher(ServletContext servletContext, Map<String, String> initParams) {      this.servletContext = servletContext;      this.initParams = initParams;  }

3. 上边说了,FilterDispatcher过滤器的doFilter()方法,调用了Dispatcher对象的serviceAction()方法,并把HttpServletRequest对象和HttpServletResponse对象传入

 这个serviceAction()方法,就是整个Strtus2的主引擎.

 serviceAction内部完成了多项功能:

配置文件加载,配置初始化

调用ActionProxy对象实现对Action类的执行

ActionProxy对象又会按照Struts2的Stack结构依次执行Inteceptor,action,method


 

public void serviceAction(HttpServletRequest request, HttpServletResponse response, ServletContext context,ActionMapping mapping) throws ServletException {        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);         if (stack != null) {          extraContext.put(ActionContext.VALUE_STACK, ValueStackFactory.getFactory().createValueStack(stack));      }         try {          String namespace = mapping.getNamespace();          String name = mapping.getName();          String method = mapping.getMethod();          Configuration config = configurationManager.getConfiguration();          ActionProxy proxy = config.getContainer().getInstance(ActionProxyFactory.class).createActionProxy(                  namespace, name, extraContext, true, false);          proxy.setMethod(method);          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 {              proxy.execute();                  }          // If there was a previous value stack then set it back onto the request             if (stack != null) {              request.setAttribute(ServletActionContext.STRUTS_VALUESTACK_KEY, stack);