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);
                 }

         } catch (ConfigurationException e) {
     }
 }