抛出 servlet 和堆栈跟踪的 Java 问题

Question

我有以下内容，我必须在其中编写一个 servlet，该 servlet 需要年龄、婚姻状况、家庭收入和 child 数量，输出到数据库，然后将更新后的平均值返回给用户。但是，我遇到了这个堆栈跟踪:

java.lang.NullPointerException
    at HouseSurvey$SurveyResults.access$200(HouseSurvey.java:86)
    at HouseSurvey.doPost(HouseSurvey.java:43)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
    at org.apache.catalina.servlets.InvokerServlet.serveRequest(InvokerServlet.java:419)
    at org.apache.catalina.servlets.InvokerServlet.doPost(InvokerServlet.java:169)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:709)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:868)
    at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:663)
    at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
    at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
    at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
    at java.lang.Thread.run(Unknown Source)

这到底是什么意思？我想我得到了一个 NullPointerException，但我看不到在哪里。这是我的程序:

import java.text.DecimalFormat;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HouseSurvey extends HttpServlet
{
    private final static String SURVEY_FILE = "HouseSurvey.dat";
    SurveyResults results;
    Household h;
    DecimalFormat form = new DecimalFormat("#,###,#00.00");

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
    {
        PrintWriter out = response.getWriter();  //make a printwriter to print the page
        File survey = new File(SURVEY_FILE);
        if(!survey.exists())    //check to see if the file exists
            results = new SurveyResults();
        else   
        {     //If the file exists, read it the latest survey tallies
            try
            {
                ObjectInputStream ips = new ObjectInputStream(new FileInputStream(survey));
                results = (SurveyResults)ips.readObject(); //read the file into 'results'
                ips.close();   //close the input stream
            }
            catch(ClassNotFoundException e) {e.printStackTrace();}
            catch(FileNotFoundException f)  {f.printStackTrace();}
            catch(IOException ioe)          {ioe.printStackTrace();}
        }
        int       ageValue = Integer.parseInt(request.getParameter("age")),
        childValue = Integer.parseInt(request.getParameter("children"));
        double incomeValue = Double.parseDouble(request.getParameter("income"));
        Boolean marriedValue = true;
        if (request.getParameter("status").equals("married"))
            marriedValue = true;
        else 
           if(request.getParameter("status").equals("single"))
           marriedValue = false;
        Household h = new Household(ageValue,childValue,incomeValue,marriedValue);
        results.totalResults(h);
        //Write results to disk.
        try
        {
            ObjectOutputStream ops = new ObjectOutputStream(new FileOutputStream(survey));
            ops.writeObject(results); ops.flush(); ops.close();
        }
        catch(IOException ioe)          {ioe.printStackTrace();}
        response.setContentType("text/html");       //contnent type for the responding webpage
            out.println("<html>\n"+
                "<head><title>Thanks!</title></head>"+
                "<body style='background:cyan;'>"+
                "   <h1 style='text-align:center'>RRC BIT Department - Household Survey</h1>"+
                "   <hr><br/>"+
                "   <h2 style='text-align:center'>Up to Date Survey Results</h2>"+
                "   <h4 style='margin-left:200px'>Average Age: "+form.format(results.getAvgAge())+"</h4>"+
                "   <h4 style='margin-left:200px'>Average Number of Children: "+form.format(results.getAvgKids())+"</h4>"+
                "   <h4 style='margin-left:200px'>Average Number of Married Respondents: "+form.format(results.getTotalMarried())+"</h4>"+
                "   <h4 style='margin-left:200px'>Average Number of Single Respondents: "+form.format(results.getTotalSingle())+"</h4>"+
                "   <h4 style='margin-left:200px'>Average Income: "+form.format(results.getAvgIncome())+"</h4></body>");
    }                           
    private class Household
    {
        private int age, children;
        private double income;
        private boolean married=false;
        /**Method: Household
         * Constructor
         * @ param age - age of person surveyed:int
         * @ param children - number of children person surveyed has:int
         * @ param married - true or false, used to determine married or single:boolean
         * @ param income - the family income of the person surveyed:double
         */
        private Household(int age, int children, double income, boolean married)
        {
            this.age=age; this.children=children; this.income=income; this.married=married;       
        } 
        //Getters
        private int getAge()         {return age;}
        private int getChildren()    {return children;}
        private double getIncome()   {return income;}
        private boolean getMarried() {return married;}
    }
    private class SurveyResults implements Serializable
    {
        private double totalAge, totalChildren, totalMarried, totalSingle, totalIncome;
        /**Method: SurveyResults
         * Used for totals
         * @ param h - Household object created above:Household
         */      
        private void totalResults(Household h)
        {
            totalAge += h.getAge(); totalChildren += h.getChildren(); totalIncome += h.getIncome();            
            if(h.getMarried()) totalMarried += 1;
               else
                  totalSingle += 1;
        }
        private double getTotalHouseholds() {return totalSingle + totalMarried;}
        private double getAvgAge()          {return totalAge/getTotalHouseholds();}
        private double getAvgKids()         {return totalChildren/getTotalHouseholds();}
        private double getTotalMarried()    {return totalMarried;}
        private double getTotalSingle()     {return totalSingle;}
        private double getAvgIncome()       {return totalIncome/getTotalHouseholds();}
    }
}

最初在我的 HTML 输出中，我不小心有一行

"Average Number of Married Respondents: "+form.format(results.getTotalMarried())+"</h4>" 

作为

"Average Number of Married Respondents: "+form.format(results.getAvgKids())+"</h4>"

而且效果很好。然后我将它切换到前者以获得 totalMarried，现在它给了我异常(exception)。问题出在哪里？

提前致谢。

Answer 1

我将其添加为单独的答案，因为这是完全不同的思路。如果编辑更好，请告诉我，我会删除重复的。

看起来您将在读取结果对象时抛出 IOException。原因是，您尝试序列化的类不可序列化。由于 SurveyResults 是一个内部类，要序列化它也必须序列化包含类(在本例中为您的 Servlet)。我建议将您的类更改为静态嵌套类，或者可能是默认作用域的顶级类。 (根据需要更改方法访问级别。)

您可能还需要删除您的调查结果数据文件并重新开始。在我的测试中，它引用了不可序列化的类，在删除它之前我无法让代码运行。