谷歌应用引擎jpa数据检索异常

Question

我是Google应用引擎的新手。我用JPA编写了一个简单的webapps来存储和检索数据。我的程序成功地将数据插入数据库（JPA）（数据显示在谷歌应用程序引擎的数据存储页面中），但在检索时我收到“java.lang.NullPointerException”错误。

我的节目

package com.alokpanda;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;
import javax.persistence.Query;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@SuppressWarnings("serial")
public class SaveAndView extends HttpServlet {


    @SuppressWarnings("unchecked")
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String name = request.getParameter("name");
        String address = request.getParameter("address");

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("transactions-optional");
        EntityManager em = emf.createEntityManager();

        EntityTransaction tx = em.getTransaction();
        tx.begin();

        User user = new User();
        user.setName(name);
        user.setAddress(address);

        em.persist(user);


        em.getTransaction().commit();
        em.close();
        emf.close();
        response.setContentType("plain/text");
        PrintWriter out = response.getWriter();

        EntityManagerFactory emf2 = null;
        EntityManager em2 = null;
        Query query = null;
        List<User> list = null;
        try {
        emf2 = Persistence.createEntityManagerFactory("transactions-optional");
        } catch (Exception e) {
            out.print(e);
        }
        try {
        em2 = emf2.createEntityManager();
        } catch (Exception e) {
            out.print("em problem :"+e);
        }
        try {


        query = em2.createQuery("SELECT u FROM User u", User.class);
        list = query.getResultList();


        if(list != null) {
            out.print(list);
        } else {
            out.print("null");
        }
        } catch (Exception e) {
            out.print(e+" : ");
            out.print(query+" : ");
            out.print(list+" : ");
            out.print(em);
        }


    }

}

节目输出

java.lang.NullPointerException : SELECT u FROM User u : null : org.datanucleus.api.jpa.JPAEntityManager@161b59f3