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尝试在数据存储中添加对象 (JPA) 时遇到问题。我正在将其部署到 App Engine。我尝试执行一些非常基本的调试，因为我是 Web 开发的新手，通过尝试在我认为可能出现问题的每个步骤之前和之后执行打印。显然，问题似乎是没有创建实体管理器，因此（显然）没有执行数据库中的添加。这是我的代码：

         // all the parameters are previously initialized.
         public void doGet(HttpServletRequest req, HttpServletResponse resp)
        throws IOException {
         Symptoms aSymptoms = new Symptoms(thePatientId,theHeadacke,theMorningHeadacke,theDiziness,theFatigue,theMuscularFatigue,
            theTinitus,theVisualImpairment,theWeightLoss,theHeartPalpitation,thePallor,theWeightProblems,theUpperBodyObesity,
            theBrutalIncreaseDiastolic,theLumbarPain,theFever,thePolyuria,theSweating,thePositionChangeDiziness);
    // using a SymptomsDAOJPA object I can perform add in the datastore.
    SymptomsDAO dao = new SymptomsDAOJPA();

    try {
        // Save the symptom.
        out.println("Entered try in doPost.");
        dao.add(aSymptoms);
        resp.setStatus(HttpServletResponse.SC_OK);
        out.println("Saved symptom.");
        out.println(aSymptoms);
    } catch (Exception e) {
        resp.setStatus(HttpServletResponse.SC_EXPECTATION_FAILED);
        out.println("Problem saving symptom.");
    }
 }

add(sSymptom) 方法如下：

        @Override
public void add(Symptoms setSymptoms) {
    EntityManager em = null;
    System.out.println("Entity Manager is null so far.");
    try {
        em = EMF.get().createEntityManager();
        System.out.println("Created Entity Manager.");
        SymptomsJPA setSymtomsJPA = new SymptomsJPA(setSymptoms);
        System.out.println("Created SymptomsJPA object to be added in the database.");
        em.persist(setSymtomsJPA);
    } finally {
        em.close();
    }

}

打印的最后一条消息是：Entity Manager 到目前为止为空。因此，我的假设是这就是问题所在：

          em = EMF.get().createEntityManager();

这里有我的 EMF 课程：

     public final class EMF {
private static final EntityManagerFactory emfInstance = Persistence
    .createEntityManagerFactory("transactions-optional");

private EMF() {
// intentionally empty
}
public static EntityManagerFactory get() {
return emfInstance;
}

}

我的persistance.xml（部分）是：

<persistence-unit name="transactions-optional">
    <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
    <properties>
        <property name="datanucleus.NontransactionalRead" value="true"/>
        <property name="datanucleus.NontransactionalWrite" value="true"/>
        <property name="datanucleus.ConnectionURL" value="appengine"/>
    </properties>
</persistence-unit>

我不知道为什么没有创建我的实体管理器。我一直在使用这段代码并且它有效，但现在它没有，我不知道我错过了什么。我真的很感激对此的任何想法。

谢谢你。

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