关于TOMCAT报错。求助解决~~~~~~~~~
java.lang.NullPointerException
at   com.publish.jxc.charpter9.FlowControlServlet.doPost(FlowControlServlet.java:57)
at   javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
at   javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at   com.publish.jxc.charpter9.FlowControlServlet.service(FlowControlServlet.java:78)
at   org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:237)
at   org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:157)
at   org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:214)
at   org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at   org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at   org.apache.catalina.core.StandardContextValve.invokeInternal(StandardContextValve.java:198)
at   org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:152)
at   org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at   org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at   org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:137)
at   org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at   org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:118)
at   org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:102)
at   org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at   org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at   org.apache.catalina.core.StandardValveContext.invokeNext(StandardValveContext.java:104)
at   org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:520)
at   org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:929)
at   org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:160)
at   org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
at   org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
at   org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
at   org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
at   java.lang.Thread.run(Unknown   Source)
求助帮忙解决。。不知道那里出了问题

------解决方案--------------------
FlowControlServlet.java第57行，空指针异常，引用了一个空对象