servlet中的Java持久化API

Question

我正在从数据库创建一个实体类，我想从servlet中创建一个实体。我编写了以下代码：

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

        EntityManagerFactory emFactory = Persistence.createEntityManagerFactory("UniAppPU"); 
        EntityManager em = emFactory.createEntityManager(); 
        Query query = em.createNamedQuery("Uniuser.findByUsername"); 
        query.setParameter("Username","clayd"); 
        List<Uniuser> result = query.getResultList(); 

        out.println("<p>" + result.size() + "</p>"); 

        em.close(); 

    }
}

我有一个简单的html页面，其中表单操作被发送到这个servlet。

    <form method="post" action="processLogin">
        <input type="submit" value="Sign in">

    </form>

以下是帖子：

 protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    processRequest(request, response);
}

持久性xml文件如下所示：

<persistence-unit name="UniversityApplicationPU" transaction-type="JTA">
<jta-data-source>UniversityEntity</jta-data-source>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties/>

当没有显示错误时，此操作不起作用。你知道怎么回事吗？多谢各位

Answer 1

此代码缺少类似

首先，您使用了错误的持久性单元名称。用这个

EntityManagerFactory emFactory=持久性。CreateEntityManager工厂（“UniversityApplicationPU”）

其次，您需要在任何获取/更新之前启动事务。您的实际代码如下所示

    EntityManagerFactory emFactory = Persistence.createEntityManagerFactory("UniversityApplicationPU"); 
    EntityManager em = emFactory.createEntityManager(); 
    em.getTransaction.begin();
    Query query = em.createNamedQuery("Uniuser.findByUsername"); 
    query.setParameter("Username","clayd"); 
    List<Uniuser> result = query.getResultList(); 
    out.println("<p>" + result.size() + "</p>"); 
    em.close();
    emFactory.close();

希望这能奏效