在我的服务器逻辑中，我试图调用map reduce作业。

KMeansClustering.startClustering(centerForClustering);

我一直在犯这个错误，不知道为什么

[WARN] Server class 'org.kmeans.clustering.VectorWritable' could not be found in the web app, but was found on the system classpath
   [WARN] Adding classpath entry 'file:/Users/yeshwanthvenkatesh/Documents/Hadoop/Research/KMeansClustering/bin/' to the web app classpath for this session
   For additional info see: file:/Applications/eclipse/plugins/com.google.gwt.eclipse.sdkbundle_2.5.0/gwt-2.5.0/doc/helpInfo/webAppClassPath.html
Starting Jetty on port 8888
   [WARN] Exception while dispatching incoming RPC call
com.google.gwt.user.server.rpc.UnexpectedException: Service method 'public abstract edu.neu.scolopax.web.client.entities.PlotSetOutput edu.neu.scolopax.web.client.SummaryGenerationService.generateSummary(edu.neu.scolopax.web.client.entities.PlotSetInput)' threw an unexpected exception: java.lang.NullPointerException
    at com.google.gwt.user.server.rpc.RPC.encodeResponseForFailure(RPC.java:389)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:579)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processCall(RemoteServiceServlet.java:208)
    at com.google.gwt.user.server.rpc.RemoteServiceServlet.processPost(RemoteServiceServlet.java:248)
    at com.google.gwt.user.server.rpc.AbstractRemoteServiceServlet.doPost(AbstractRemoteServiceServlet.java:62)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:362)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:729)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.handler.RequestLogHandler.handle(RequestLogHandler.java:49)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:324)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
    at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:843)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:647)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)
Caused by: java.lang.NullPointerException
    at org.kmeans.clustering.KMeansClustering.startClustering(KMeansClustering.java:170)
    at edu.neu.scolopax.web.server.SummaryGenerationServiceImpl.generateSummary(SummaryGenerationServiceImpl.java:73)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.google.gwt.user.server.rpc.RPC.invokeAndEncodeResponse(RPC.java:561)
    ... 22 more