org.apache.catalina.connector.Request.parseParameters(Request.java:2446) NullPointerException异常处理

1、环境

tomcat6.0.8 + jdk6 + struts1 

2、问题的产生

页面有两个异常的请求

http://127.0.0.1/project/a.action?do=testA&m=10
http://127.0.0.1/project/a.action?do=testB&m=10

两个请求几乎同步到达server。结果出现异常:

java.lang.NullPointerException
	at org.apache.catalina.connector.Request.parseParameters(Request.java:2446)
	at org.apache.catalina.connector.Request.getParameter(Request.java:1040)
	at org.apache.catalina.connector.RequestFacade.getParameter(RequestFacade.java:355)
	at com.tepper.common.util.JsonUtil.jsonResponse(JsonUtil.java:101)
	at com.tepper.flow.action.TechnologyFlowAction.loadLinkers(TechnologyFlowAction.java:126)
	at com.tepper.flow.action.TechnologyFlowAction.judgeRequest(TechnologyFlowAction.java:86)
	at com.tepper.flow.action.TechnologyFlowAction.execute(TechnologyFlowAction.java:59)
	at org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
	at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
	at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	at org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:198)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at com.tepper.common.SessionFilter.doFilter(SessionFilter.java:76)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at com.tepper.common.ResFilter.doFilter(ResFilter.java:49)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:433)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
	at java.lang.Thread.run(Thread.java:619)

或者后台没异常，前台第一个请求的response返回的是第二个请求的结果。而第二次请求则没有response。

经调试，发现两次请求的request对象是同一对象。然后想起Struts1是非线程安全的，而我在代码中，为了让一个Action接收多个页面的请求，把request对象写成了全局对象。

3、解决

struts1是非线程安全的。两个同一时间的异步请求，会得到同一个request对象，两个请求会造成冲突。

在execution方法中使用局部变量是最简单的保证线程安全的方法了。也就是说在使用struts1的action时，不要在类中定义属性。

把全局的request对象，放回execution方法中，变成局部变量，问题得到解决。