Struts2中使用execAndWait后，在 Action中调用getXXX()方法报告java.lang.NullPointerException异常的原因和解决方法

使用 Struts2 编写页面，遇到一个要长时间运行的接口，因此增加了一个execAndWait ，结果在 Action 中调用 getContext()的时候报告异常

 ActionContext context = ActionContext.getContext();
 ServletContext servletContext = (ServletContext) context.get(ServletActionContext.SERVLET_CONTEXT);  //抛空指针异常
 String rootPath = servletContext.getRealPath("/");

查询了很多评论，最终找到原因跟解决方案，具体解释在 http://stackoverflow.com/questions/16692658/execandwait-interceptor-not-redirecting-to-success-page-after-waiting。大致意思为：execAndWait 会导致执行的Action 在另外一个线程中被执行，而getText 依赖 ActionContext ，他从 ActionContext 中获得当前的Locale 从而根据语言的不同加载不同的文字，可是，由于ActionContext 是ThreadLocal 的，而execAndWait 新开线程的时候并没有把父线程的ActionContext 传递给子线程 结果导致在新开的子线程中的ActionContext中的数据都是null ,因此出现异常信息就不足为怪了。

解决方法如下：需要重载两个类，来解决这个问题
ActionInvocationEx.java

 package byrs.rms.interceptors;

 import com.opensymphony.xwork2.ActionContext;
 import com.opensymphony.xwork2.ActionEventListener;
 import com.opensymphony.xwork2.ActionInvocation;
 import com.opensymphony.xwork2.ActionProxy;
 import com.opensymphony.xwork2.Result;
 import com.opensymphony.xwork2.interceptor.PreResultListener;
 import com.opensymphony.xwork2.util.ValueStack;

 public class ActionInvocationEx implements ActionInvocation {

     /**
      *
      */
     private static final long serialVersionUID = 2434502343414625665L;

     private final ActionInvocation mActionInvocation;

     private final ActionContext context;

     public ActionInvocationEx(ActionInvocation aActionInvocation,ActionContext aContext)
     {
         mActionInvocation = aActionInvocation;
         context = aContext;
     }

     public Object getAction() {
         return mActionInvocation.getAction();
     }

     public boolean isExecuted() {
         return mActionInvocation.isExecuted();
     }

     public ActionContext getInvocationContext() {
         return mActionInvocation.getInvocationContext();
     }

     public ActionProxy getProxy() {
         return mActionInvocation.getProxy();
     }

     public Result getResult() throws Exception {
         return mActionInvocation.getResult();
     }

     public String getResultCode() {
         return mActionInvocation.getResultCode();
     }

     public void setResultCode(String resultCode) {
         mActionInvocation.setResultCode(resultCode);
     }

     public ValueStack getStack() {
         return mActionInvocation.getStack();
     }

     public void addPreResultListener(PreResultListener listener) {
         mActionInvocation.addPreResultListener(listener);
     }

     public String invoke() throws Exception {
         return mActionInvocation.invoke();
     }

     public String invokeActionOnly() throws Exception {
         return mActionInvocation.invokeActionOnly();
     }

     public void setActionEventListener(ActionEventListener listener) {
         mActionInvocation.setActionEventListener(listener);
     }

     public void init(ActionProxy proxy) {
         mActionInvocation.init(proxy);
     }

     public ActionInvocation serialize() {
         return mActionInvocation.serialize();
     }

     public ActionInvocation deserialize(ActionContext actionContext) {
         return mActionInvocation.deserialize(actionContext);
     }

     /**
      * @return the context
      */
     public ActionContext getContext() {
         return context;
     }

 }

ExecAndWaitInterceptorEx.java

 package byrs.rms.interceptors;

 import org.apache.struts2.interceptor.BackgroundProcess;
 import org.apache.struts2.interceptor.ExecuteAndWaitInterceptor;

 import com.opensymphony.xwork2.ActionContext;
 import com.opensymphony.xwork2.ActionInvocation;

 public class ExecAndWaitInterceptorEx extends ExecuteAndWaitInterceptor {

     /**
      *
      */
     private static final long serialVersionUID = 8829373762598564300L;

     /**
      * {@inheritDoc}
      */
     @Override
     protected BackgroundProcess getNewBackgroundProcess(String arg0, ActionInvocation arg1, int arg2) {
         ActionInvocationEx aActionInvocationEx = new ActionInvocationEx(arg1,ActionContext.getContext());
         return new BackgroundProcessEx(arg0, aActionInvocationEx, arg2);
     }

     private class BackgroundProcessEx extends BackgroundProcess {
         public BackgroundProcessEx(String threadName,
                 ActionInvocation invocation, int threadPriority) {
             super(threadName, invocation, threadPriority);
         }

         private static final long serialVersionUID = -9069896828432838638L;
         /**
          * {@inheritDoc}
          * @throws InterruptedException
          */
         @Override
         protected void beforeInvocation() throws InterruptedException {
             ActionInvocationEx aActionInvocationEx = (ActionInvocationEx)this.invocation;
             ActionContext context = aActionInvocationEx.getContext();
             ActionContext.setContext(context);
         }

         /**
          * {@inheritDoc}
          */
        @Override
         protected void afterInvocation() {
             ActionContext.setContext(null);
         }

     }

 }

然后在struts.xml中覆盖默认拦截器即可

 <interceptors >
             <interceptor name="execAndWait" class="byrs.rms.interceptors.ExecAndWaitInterceptorEx"/>
         </interceptors >

参考自：http://www.mobibrw.com/?p=1046