为什么我访问路径报出空指针异常（javaweb）

这是异常提示：

java.lang.NullPointerException
at java.io.FileOutputStream.<init>(FileOutputStream.java:206)
at java.io.FileOutputStream.<init>(FileOutputStream.java:104)

**at com.mifan.web.UpLoadServlet.doGet(UpLoadServlet.java:41)**
at com.mifan.web.UpLoadServlet.doPost(UpLoadServlet.java:57)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:644)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:291)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:239)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:219)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:106)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:501)

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:142)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:610)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:88)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:516)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1086)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:659)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:285)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2439)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2428)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:722)

这是源代码：
public class UpLoadServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    try{
        //创建工厂类  这是阿帕奇提供的那个第三包jar包
        DiskFileItemFactory factory = new DiskFileItemFactory();
        //生产文件上传核心类
        ServletFileUpload fileUpload = new ServletFileUpload(factory);
        List<FileItem> list = fileUpload.parseRequest(request);
        //遍历所有的FileItem
        for(FileItem item : list){
            if(item.isFormField()){
                //当前是一个普通的字段项
                String name = item.getFieldName();//普通字段的名字
                String value = item.getString();  //普通字段名字所对应的值
                System.out.println(name+":"+value);
            }else {//当前是一个文件上传项
                //获取文件上传项的文件名
                String filename = item.getName();
                InputStream in = item.getInputStream();
                OutputStream out = new FileOutputStream(
                        this.getServletContext().getRealPath("upload/"+filename));
                IOUtils.InAndOut(in, out);
                IOUtils.close(in, out);

            }
        }
        //利用文件上传核心类解析request
    }catch (Exception e) {
        e.printStackTrace();
    }
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    doGet(request, response);
}

}

Debug 跟一下你就知道哪个为空了。