触发 `RequestDispatcher` 时出现 NullPointerException

Question

我正在使用 Retrofit 连接到我的 REST API。请检查以下代码

   import com.google.gson.Gson;
    import com.google.gson.GsonBuilder;
    import java.io.IOException;
    import java.io.PrintWriter;
    import java.util.Date;
    import java.util.List;
    import java.util.logging.Level;
    import java.util.logging.Logger;
    import javax.servlet.RequestDispatcher;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import retrofit2.Call;
    import retrofit2.Callback;
    import retrofit2.Response;
    import retrofit2.Retrofit;
    import retrofit2.converter.gson.GsonConverterFactory;

    /**
     *
     * @author The Ace
     */
    public class SignUpLoaderServlet extends HttpServlet {

        /**
         * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
         * methods.
         *
         * @param request servlet request
         * @param response servlet response
         * @throws ServletException if a servlet-specific error occurs
         * @throws IOException if an I/O error occurs
         */
        protected void processRequest(final HttpServletRequest request, final HttpServletResponse response)
                throws ServletException, IOException {
            response.setContentType("text/html;charset=UTF-8");
            PrintWriter out = response.getWriter();
            System.out.println("RUNNING!!!!!!!!!!!");

            try {
                GsonBuilder gsonBuilder = new GsonBuilder();
                gsonBuilder.registerTypeAdapter(Date.class, new DateTypeDeserializer());
                Gson gson = gsonBuilder.create();

                Retrofit retrofit = new Retrofit.Builder()
                        .baseUrl(BaseURLs.MESSAGING_URL)
                        .addConverterFactory(GsonConverterFactory.create(gson))
                        .build();

                RestEndPointsInterface endPoint = retrofit.create(RestEndPointsInterface.class);
                Call<List<ProfesionalBodyList>> call = endPoint.getAllProfesionalBodyLists();
                call.enqueue(new Callback<List<ProfesionalBodyList>>() {

                    @Override
                    public void onResponse(Call<List<ProfesionalBodyList>> call, Response<List<ProfesionalBodyList>> rspn) 
                    {
                        try {
                            List<ProfesionalBodyList> body = rspn.body();


                            for(int i=0;i<body.size();i++)
                            {
                                System.out.println(body.get(i).getProfessionalBody());
                            }

RequestDispatcher requestDispatcher = request.getRequestDispatcher("/create-account.jsp");
                            requestDispatcher.forward(request, response);


                        } catch (Exception ex) {
                            ex.printStackTrace();
                        } 
                    }

                    @Override
                    public void onFailure(Call<List<ProfesionalBodyList>> call, Throwable ex) {
                        ex.printStackTrace();
                    }
                });



            } finally {


            }
        }

        // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
        /**
         * Handles the HTTP <code>GET</code> method.
         *
         * @param request servlet request
         * @param response servlet response
         * @throws ServletException if a servlet-specific error occurs
         * @throws IOException if an I/O error occurs
         */
        @Override
        protected void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            processRequest(request, response);
        }

        /**
         * Handles the HTTP <code>POST</code> method.
         *
         * @param request servlet request
         * @param response servlet response
         * @throws ServletException if a servlet-specific error occurs
         * @throws IOException if an I/O error occurs
         */
        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
            processRequest(request, response);
        }

    }

此代码触发以下异常

java.lang.NullPointerException
    at com.tekhinno.xxx.signup.SignUpLoaderServlet$1.onResponse(SignUpLoaderServlet.java:80)
    at retrofit2.OkHttpCall$1.callSuccess(OkHttpCall.java:132)
    at retrofit2.OkHttpCall$1.onResponse(OkHttpCall.java:111)
    at okhttp3.RealCall$AsyncCall.execute(RealCall.java:135)
    at okhttp3.internal.NamedRunnable.run(NamedRunnable.java:32)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

但是，如果我将下面的代码行替换为 finally() 块，问题就消失了。

RequestDispatcher requestDispatcher = request.getRequestDispatcher("/create-account.jsp");
requestDispatcher.forward(request, response);

我不确定为什么它在 onResponse() 中不起作用。在那里运行很重要，因为我在前向操作发生之前从 REST API 加载项目。

有什么想法吗？

Answer 1

这个问题主要是因为调用是Asynchronous并且requestDispatcher对象已经被执行了。那么答案是一直待到 REST 调用完成其工作负载。这意味着，去做Synchronous。

改造也可以Synchronous 方式完成。下面是代码。

protected void processRequest(final HttpServletRequest request, final HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        System.out.println("RUNNING!!!!!!!!!!!");

        try {
            GsonBuilder gsonBuilder = new GsonBuilder();
            gsonBuilder.registerTypeAdapter(Date.class, new DateTypeDeserializer());
            Gson gson = gsonBuilder.create();

            Retrofit retrofit = new Retrofit.Builder()
                    .baseUrl(BaseURLs.MESSAGING_URL)
                    .addConverterFactory(GsonConverterFactory.create(gson))
                    .build();

            RestEndPointsInterface endPoint = retrofit.create(RestEndPointsInterface.class);
            Call<List<ProfesionalBodyList>> call = endPoint.getAllProfesionalBodyLists();
            body = call.execute().body();

            for (int i = 0; i < body.size(); i++) {
                System.out.println(body.get(i).getProfessionalBody());
            }

        } finally {
            RequestDispatcher requestDispatcher = request.getRequestDispatcher("/create-account.jsp");
            request.setAttribute("ProfesionalBodyList", body);
            requestDispatcher.forward(request, response);
        }
    }

Simpy 注意上面写着call.execute().body(); 的地方。这是Synchronous 电话。