com.jobyab.controllers.registerController.doPost 处的 java.lang.NullPointerException

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

persist 似乎出了问题方法EntityManager类(class)。我使用 apache tomcat 并收到 HTTP Status 500-每次我尝试时都会页面。

这是DataQuery类:

public class DataQuery {
    EntityManagerFactory emf;
    EntityManager em;

    public DataQuery() {
        emf = Persistence.createEntityManagerFactory("com.sbu.ac.jobyab");
        em = emf.createEntityManager();
        em.getTransaction().begin();
    }

    public void registerUser(Employee e){
        em.persist(e);
    }
}

这是接收用户输入并执行注册任务的 registerController 类:

public class registerController extends HttpServlet {
    private DataQuery query;
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String email = request.getParameter("register_email");
        String pwd = request.getParameter("register_password");
        Employee e = new Employee(10, email, pwd);
        query.registerUser(e);
    }
}

这是完整的堆栈错误:

type Exception report

message

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerException com.jobyab.controllers.registerController.doPost(registerController.java:39) javax.servlet.http.HttpServlet.service(HttpServlet.java:648) javax.servlet.http.HttpServlet.service(HttpServlet.java:729) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) note The full stack trace of the root cause is available in the Apache Tomcat/8.0.33 logs.

Answer 1

尝试在 doPost() 方法中添加 query = new DataQuery();