转：ServletContext,ActionContext,ServletActionContext

ServletContext

ServletContext从他的package信息可以看出,它是标准的JavaEE WebApplication类库
javax.servlet.ServletContext
ServletContext提供了标准的Servlet运行环境,其实就是一些servlet和web container进行通信的方法
public interface ServletContext {

 // Returns the URL prefix for the ServletContext.
 public String getServletContextName();

 //Returns the ServletContext for the uri.
 public ServletContext getContext(String uri);

 //Returns the context-path for the web-application.
 public String getContextPath();

 //Returns the real file path for the given uri.
 public String getRealPath(String uri);

 public RequestDispatcher getRequestDispatcher(String uri);
 public RequestDispatcher getNamedDispatcher(String servletName);
public Object getAttribute(String name);
public Enumeration getAttributeNames();
public void setAttribute(String name, Object value);
public void removeAttribute(String name);
注意:一个ServletContext对应一个命名空间的servlet( 比如/struts下的所有servlet),是被所有servlet共享的.
There is one context per "web application" per Java Virtual Machine.
(A "web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace such as /catalog and possibly installed via a .war file.)

ServletContext被包含在ServletConfig对象中,ServletConfig对象通常被servlet或filter的init()方法读取
ServletConfig.getServletContext()
filterconfig.getServletContext()

ActionContext来源于Struts2 与 Struts1的本质不同.
struts1时，由一个servlet (servlet org.apache.struts.action.ActionServlet)处理所有的*.do
struts2时，由一个filter(org.apache.struts2.dispatcher.FilterDispatcher)处理所有的请求
struts1 仍旧属于servlet范畴,struts1 action 其本质仍是servlet.
struts2 action 已经是普通的Java bean了,已经跳出了servlet 框架
ActionContext就是为了弥补strtus2 action跳出标准servlet框架而造成的和WEB环境失去联系的缺陷

ActionContext的主要作用:

提供Web环境Context
解决线程安全问题
解决一些和其他框架或容器(siteMesh,webLogic)的兼容问题

分析ActionContext源码

 public class ActionContext implements Serializable {
  //////////ThreadLocal模式下的ActionContext实例,实现多线程下的线程安全///////////////

     static ThreadLocal actionContext = new ThreadLocal();

     //Sets the action context for the current thread.
     public static void setContext(ActionContext context) {
         actionContext.set(context);
     }
     //Returns the ActionContext specific to the current thread.
     public static ActionContext getContext() {
         return (ActionContext) actionContext.get();
     }
 ///////////////定义放置"名/值对"的Map容器,这是ActionContext的主要功能/////////////// 

     Map<String, Object> context;

     // constractor
     // Creates a new ActionContext initialized with another context.
     public ActionContext(Map<String, Object> context) {
         this.context = context;
     }

     public void setContextMap(Map<String, Object> contextMap) {
         getContext().context = contextMap;
     }
     public Map<String, Object> getContextMap() {
         return context;
     }

//Returns a value that is stored in the current ActionContext by doing a lookup using the value's key.
     public Object get(String key) {
         return context.get(key);
     }
 //Stores a value in the current ActionContext. The value can be looked up using the key.
     public void put(String key, Object value) {
         context.put(key, value);
     }
///////////////////将各种功能属性放置入Map容器中/////////////////////

     //action name, Constant for the name of the action being executed.
     public static final String ACTION_NAME = "com.opensymphony.xwork2.ActionContext.name";

     // ognl value stack
     public static final String VALUE_STACK = ValueStack.VALUE_STACK;

     public static final String SESSION = "com.opensymphony.xwork2.ActionContext.session";
     public static final String APPLICATION = "com.opensymphony.xwork2.ActionContext.application";
     public static final String PARAMETERS = "com.opensymphony.xwork2.ActionContext.parameters";
     public static final String LOCALE = "com.opensymphony.xwork2.ActionContext.locale";
     public static final String TYPE_CONVERTER = "com.opensymphony.xwork2.ActionContext.typeConverter";
     public static final String ACTION_INVOCATION = "com.opensymphony.xwork2.ActionContext.actionInvocation";
     public static final String CONVERSION_ERRORS = "com.opensymphony.xwork2.ActionContext.conversionErrors";
     public static final String CONTAINER = "com.opensymphony.xwork2.ActionContext.container";

 ////// 各种Action主属性:ActionName, ActionInvocation(调用action的相关信息), ognl value stack///

     //Gets the name of the current Action.
     public String getName() {
         return (String) get(ACTION_NAME);
     }
     //Sets the name of the current Action in the ActionContext.
     public void setName(String name) {
         put(ACTION_NAME, name);
     }

     //Sets the action invocation (the execution state).
     public void setActionInvocation(ActionInvocation actionInvocation) {
         put(ACTION_INVOCATION, actionInvocation);
     }
     public ActionInvocation getActionInvocation() {
         return (ActionInvocation) get(ACTION_INVOCATION);
     }

     // Sets the OGNL value stack.
     public void setValueStack(ValueStack stack) {
         put(VALUE_STACK, stack);
     }
     //Gets the OGNL value stack.
     public ValueStack getValueStack() {
         return (ValueStack) get(VALUE_STACK);
     }

////////////////各种 request请求包含的内容////////////////////
     //Returns a Map of the HttpServletRequest parameters
     public Map<String, Object> getParameters() {
         return (Map<String, Object>) get(PARAMETERS);
     }
     public void setParameters(Map<String, Object> parameters) {
         put(PARAMETERS, parameters);
     }    

     public void setSession(Map<String, Object> session) {
         put(SESSION, session);
     }
     public Map<String, Object> getSession() {
                  return (Map<String, Object>) get(SESSION);
     }
     public void setApplication(Map<String, Object> application) {
         put(APPLICATION, application);
     }
     public Map<String, Object> getApplication() {
         return (Map<String, Object>) get(APPLICATION);
     }

     public void setConversionErrors(Map<String, Object> conversionErrors) {
         put(CONVERSION_ERRORS, conversionErrors);
     }
     public Map<String, Object> getConversionErrors() {
         Map<String, Object> errors = (Map) get(CONVERSION_ERRORS);

         if (errors == null) {
             errors = new HashMap<String, Object>();
             setConversionErrors(errors);
         }
         return errors;
     }

     //Sets the Locale for the current action.
     public void setLocale(Locale locale) {
         put(LOCALE, locale);
     }
     public Locale getLocale() {
         Locale locale = (Locale) get(LOCALE);

         if (locale == null) {
             locale = Locale.getDefault();
             setLocale(locale);
         }

         return locale;
     }
     public void setContainer(Container cont) {
         put(CONTAINER, cont);
     }
     public Container getContainer() {
         return (Container) get(CONTAINER);
     }

     public <T> T getInstance(Class<T> type) {
         Container cont = getContainer();
         if (cont != null) {
             return cont.getInstance(type);
         } else {
             throw new XWorkException("Cannot find an initialized container for this request.");
         }
     }
 }

ServletActionContext 其实是ActionContext的子类,其功能脱胎于ActionContext,对ActionContext的方法做了一定的包装,提供了更简便直观的方法

 public class ServletActionContext extends ActionContext implements StrutsStatics {
/////////////////Servlet Context 提供了多种操作ActionContext的静态方法,使获得Web对象更方便

        //HTTP servlet request
      public static void setRequest(HttpServletRequest request) {
          ActionContext.getContext().put(HTTP_REQUEST, request);
      }
      public static HttpServletRequest getRequest() {
          return (HttpServletRequest) ActionContext.getContext().get(HTTP_REQUEST);
      }

      //HTTP servlet response
      public static void setResponse(HttpServletResponse response) {
          ActionContext.getContext().put(HTTP_RESPONSE, response);
      }
      public static HttpServletResponse getResponse() {
          return (HttpServletResponse) ActionContext.getContext().get(HTTP_RESPONSE);
      }

      //servlet context.
      public static ServletContext getServletContext() {
          return (ServletContext) ActionContext.getContext().get(SERVLET_CONTEXT);
      }
      public static void setServletContext(ServletContext servletContext) {
          ActionContext.getContext().put(SERVLET_CONTEXT, servletContext);
      }

原文：http://blog.csdn.net/ocean1010/article/details/6160159