Nullpointerexception java

首先，我是一名经验丰富的程序员，但对Java知之甚少。八年前，我在这方面有两年的经验

我在以下代码中得到一个NullPointerException：

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

    Response gfexResponse = null;

    try {

        ActionFactory actionFactory = ActionFactory.getInstance();
        String requestURL = request.getRequestURI();
        String actionId = actionFactory.getActionId(requestURL);

        IAction action = actionFactory.createAction(actionId);

        ActionEvent event = new ActionEvent(request, 0, actionId);
        gfexResponse = action.execute(event);

    } catch (Exception ex) {        
        gfexResponse = new Response();
        gfexResponse.setError(ex.getMessage());
        gfexResponse.setOutcome(IViewConstants.ERROR);

    } finally {

        if(request.getParameter("loginId") != null){
            request.setAttribute("loginId", request.getParameter("loginId"));
        }

        if(gfexResponse.getMessage()!= null){
            request.setAttribute("message", gfexResponse.getMessage());
        }

        if(gfexResponse.getError()!= null){
            request.setAttribute("error", gfexResponse.getError());
        }

        if (gfexResponse.getContentType() != null) {
            response.setContentType(gfexResponse.getContentType());
            OutputStream outputStream = response.getOutputStream();
            outputStream.write(gfexResponse.getOutputData());
            outputStream.flush();
            outputStream.close();
        }

        if(gfexResponse.getOutcome() != null){
            RequestDispatcher dispatcher = request.getRequestDispatcher(gfexResponse.getOutcome());
            dispatcher.forward(request, response);
        }
    }       
}

这是StackTrace：

[6/18/13 17:10:04:518 GMT] 00000023 ServletWrappe E   SRVE0068E: Uncaught exception thrown in one of the service methods of the servlet: GfexServlet. Exception thrown : java.lang.NullPointerException
    at com.svl.gfex.handlers.RequestHandler.handle(RequestHandler.java:44)
    at com.svl.gfex.servlets.GfexServlet.processRequest(GfexServlet.java:43)
    at com.svl.gfex.servlets.GfexServlet.doPost(GfexServlet.java:39)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:763)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:966)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:907)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:118)
    at com.ibm.ws.webcontainer.filter.WebAppFilterChain._doFilter(WebAppFilterChain.java:87)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:701)
    at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:646)
    at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:475)
    at com.ibm.ws.wswebcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:463)
    at com.ibm.ws.webcontainer.webapp.WebApp.handleRequest(WebApp.java:3129)
    at com.ibm.ws.webcontainer.webapp.WebGroup.handleRequest(WebGroup.java:238)
    at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:811)
    at com.ibm.ws.wswebcontainer.WebContainer.handleRequest(WebContainer.java:1433)
    at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:93)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:465)
    at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewInformation(HttpInboundLink.java:394)
    at com.ibm.ws.http.channel.inbound.impl.HttpICLReadCallback.complete(HttpICLReadCallback.java:102)
    at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:152)
    at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:213)
    at com.ibm.io.async.AbstractAsyncFuture.fireCompletionActions(AbstractAsyncFuture.java:195)
    at com.ibm.io.async.AsyncFuture.completed(AsyncFuture.java:136)
    at com.ibm.io.async.ResultHandler.complete(ResultHandler.java:194)
    at com.ibm.io.async.ResultHandler.runEventProcessingLoop(ResultHandler.java:741)
    at com.ibm.io.async.ResultHandler$2.run(ResultHandler.java:863)
    at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1510)

stacktrace指向此行：

        if(gfexResponse.getMessage()!= null){  <-------- this line
            request.setAttribute("message", gfexResponse.getMessage());
        }

---

if（gfexResponse.getMessage（）！=null）{该错误表明
gfexResponse
对象本身是
null
（即
action.execute（event）
在上面的代码中返回
null
，并且没有引发异常）
实际上问题在于
gfexResponse=action.execute>行（事件）；

下面一行中获得NPE的唯一机会是
gfexResponse
为空

if(gfexResponse.getMessage()!= null){  <-------- this line

如果（gfexResponse.getMessage（）！=null）{您确定
gfexResponse
从
action.execute（event）；
（在
try{}
中）获取实际值吗？我猜
action.execute（event）；
返回null。
您的基本轮廓如下：

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

    Response gfexResponse = null;

    try {
        //Try to get your gfexResponse
    } catch (Exception ex) {        
        //Provide a default gfexResponse
    } finally {
        //Do some stuff with gfexResponse
    }       
}

这是一种不好的做法：您试图对流控制使用异常处理。此外，您假设用于获取gfexResponse的方法在失败时会抛出异常，但显然不会。（一些简单的调试/跟踪将直接揭示这一点。）

您应该做的是以下几点：

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

    Response gfexResponse = null;

    try {
        //Try to get your gfexResponse
        //Make sure you got your response object and throw SomeAppropriateException if not
        //Do some stuff with gfexResponse
    } catch (SomeAppropriateException e) {
        //properly handle this case
    } catch (Exception ex) {
        //properly handle the general case that something else failed (But you should try to be more specific)
    } finally {
        //remove any resources that might not be properly cleaned up if an exception is thrown.
    }       
}

为了解决眼前的问题-您的
操作.执行（事件）；
调用可能返回
null
。但是，可以通过以下几种方式缓解此问题：


空检查，或
将
try-catch
块转换为它自己的单独方法调用，以返回
响应

从这里开始，
finally
块成为方法的主要焦点，您可以检查null，而不必担心
finally
添加检查以查看
gfexResponse==null
。Java中的null指针异常通常很容易调试，因为您试图在null对象上调用方法ce捕获关于
action.execute（事件）
。你的答案是唯一能说明为什么
gfexResponse
为
null
的答案。谢谢。我试过了，但还是在同一点上得到了相同的异常。你们搞定了，谢谢！我错了，它在同一点爆炸了。我想我必须弄清楚如何使用调试模式。你说得对，这是错误的Racce，@Nathaniel Ford。不幸的是，这是它最初的编码方式，管理层没有人会同意花费来修复所有这些。这只是一个小网站，但最初的开发者在任何地方都以同样的方式处理流控制。
public static void handle(HttpServletRequest request,HttpServletResponse response)throws IOException,ServletException {

    Response gfexResponse = null;

    try {
        //Try to get your gfexResponse
        //Make sure you got your response object and throw SomeAppropriateException if not
        //Do some stuff with gfexResponse
    } catch (SomeAppropriateException e) {
        //properly handle this case
    } catch (Exception ex) {
        //properly handle the general case that something else failed (But you should try to be more specific)
    } finally {
        //remove any resources that might not be properly cleaned up if an exception is thrown.
    }       
}