关于struts2中getSession()为空的问题，求助。。

Question

今天刚刚开始学struts2，学到在action中去的request，session的时候，总是得到控制，不知道是什么原因，请有经验的指点一下，代码如下：
public class TransAttr extends ActionSupport {
private Map request;
   private Map session;
   private Map application;
   
public TransAttr() {
ActionContext ac = ActionContext.getContext();
request =(Map) ac.get("request");
session =ac.getSession();
 
}
     
public String execute() throws Exception {
 request.put("r1", "r1");
 session.put("s1", "s1");
 return SUCCESS;
}
}但是总报下面的错误：
Messages:  
File: com/struts2/test_method/TransAttr.java 
Line number: 40 
--------------------------------------------------------------------------------Stacktraces
java.lang.NullPointerException 
    com.struts2.test_method.TransAttr.execute(TransAttr.java:40)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    java.lang.reflect.Method.invoke(Unknown Source)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:440)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:279)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:163)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:249)
    org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:148)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:93)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:128)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:104)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:148)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:128)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
    org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:468)
    org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:395)
    org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:215)
    org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
    org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
    org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
    org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
    org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
    org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:873)
    org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
    org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
    org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
    org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
    java.lang.Thread.run(Unknown Source)

Answer 1

获取Session：  Map map = ActionContext.getContext().getSession();
获取Response： HttpServletResponse response = ServletActionContext.getResponse();

Answer 2

1.非IoC方式 
  要获得上述对象，关键Struts 2.0中com.opensymphony.xwork2.ActionContext类。我们可以通过它的静态方法getContext()获取当前 Action的上下文对象。 另外，org.apache.struts2.ServletActionContext作为辅助类（Helper Class），可以帮助您快捷地获得这几个对象。 
HttpServletRequest request = ServletActionContext.getRequest(); 
HttpServletResponse response = ServletActionContext.getResponse(); 
HttpSession session = request.getSession(); 2.IoC方式 
　　要使用IoC方式，我们首先要告诉IoC容器（Container）想取得某个对象的意愿，通过实现相应的接口做到这点。 publicclass IoCServlet extends ActionSupport implements ServletRequestAware, ServletResponseAware { 
    private String message; 
    private HttpServletRequest request; 
    private HttpServletResponse response;     public String execute() {       
        HttpSession session = request.getSession(); 
        StringBuffer sb =new StringBuffer("Message from request: "); 
        sb.append(request.getParameter("msg")); 
        sb.append("<br>Response Buffer Size: "); 
        sb.append(response.getBufferSize()); 
        sb.append("<br>Session ID: "); 
        sb.append(session.getId()); 
        
        message = sb.toString(); 
        return SUCCESS; 
    }   
} 

Answer 3

参考 这里的
http://www.blogjava.net/lyyb2001/archive/2008/03/07/184593.html

Answer 4

request、与session这些对象是在得到action对象后设置进去的。如果你放在构造方法城去获取，当然是错的。

Answer 5

谢谢各位了，小弟终于找到了哪里错了，是配置文件的问题
<filter-class>
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter
</filter-class> 
   <!--  
    <filter-class>org.apache.struts2.dispatcher.FilterDispatcher
    </filter-class>
    -->
之前为了解决乱码问题把web.xml中的filter配成了较早的版本的，改成2.1.6的filter配制就好了。这里可能就是两个版本中filter不同配置的影响吧，希望以后有遇到相同问题的可以参考一下。最后还是谢谢各位了！

Answer 6

别放构造函数里，获取session可以implements SessionWare接口，也能获取

Answer 7

private Map request; 
  private Map session; 
  private Map application;有提供getter setter???