Java 带有JPA和Hibernate的ORM:EntityManager为空

我正在尝试在JavaSE应用程序中使用Hibernate开始使用ORM。我已经读到，现代的方法是使用JPA，然后使用Hibernate作为持久性提供者。但是，我的EntityManager为空：

依赖项：

    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>4.3.10.Final</version>
    </dependency>

    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-entitymanager</artifactId>
        <version>4.3.10.Final</version>
    </dependency>

要使用数据库，请执行以下操作：

public class PersistenceManager {

    private static EntityManagerFactory emFactory;

    private PersistenceManager() {
        emFactory = Persistence.createEntityManagerFactory("pers-unit"); // defined in a persistence.xml
    }

    public static EntityManager getEntityManager() {
        return emFactory.createEntityManager();
    }

    public static void close() {
        emFactory.close();
    }
}

然后：

    String sql = "SELECT m FROM table m WHERE m.id = :id";
    EntityManager em = PersistenceManager.getEntityManager();
    em.getTransaction().begin();
    Query q = em.createQuery(sql);
    q.setParameter("id", key);
    Content result = (Content) q.getSingleResult();
    em.getTransaction().commit();
    em.close();
    PersistenceManager.close();
    return result;

persistence.xml：

<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence

http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"

    version="2.1">

    <persistence-unit name="pers-unit"
        transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

        <properties>

            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost/dbdb" />
            <property name="javax.persistence.jdbc.user" value="root" />
            <property name="javax.persistence.jdbc.password" value="" />
            <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />

            <property name="hibernate.show_sql" value="true" />
            <property name="hibernate.format_sql" value="true" />
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect" />
            <property name="hibernate.hbm2ddl.auto" value="validate" />
        </properties>
    </persistence-unit>
</persistence>

已经有一些不推荐使用的代码

---

怎么做

我认为您的PersistenceManager类中有一些错误。EntityManagerFactory成员是静态的，获取EntityManager的方法也是静态的，但是当您以静态方式调用该方法时，您没有PersistenceManager的实例。这意味着根本不调用构造函数，因此无法对null引用调用createEntityManager（）

你可以试试这个：

public class PersistenceManager {

    private static EntityManagerFactory emFactory;

    static {
        emFactory = Persistence.createEntityManagerFactory("pers-unit"); // defined in a persistence.xml
    }

    public static EntityManager getEntityManager() {
       return emFactory.createEntityManager();
    }

    public static void close() {
        emFactory.close();
    }
}

---

关于你的问题，如果这是一条路。。。没有明确的答案，因为这取决于个人的选择。使用SessionFactory等的教程指的是Hibernate，而命名EntityManagerFactory和EntityManager的教程指的是JPA规范（Hibernate实现了该规范，但它也有非JPA API）。

Yepp，就是这样。没有想到静态访问的后果。谢谢

Configuration configuration = new Configuration();
configuration.configure();
ServiceRegistryBuilder srBuilder = new ServiceRegistryBuilder();
srBuilder.applySettings(configuration.getProperties());
ServiceRegistry serviceRegistry = srBuilder.buildServiceRegistry();
factory = configuration.buildSessionFactory(serviceRegistry);

public class PersistenceManager {

    private static EntityManagerFactory emFactory;

    static {
        emFactory = Persistence.createEntityManagerFactory("pers-unit"); // defined in a persistence.xml
    }

    public static EntityManager getEntityManager() {
       return emFactory.createEntityManager();
    }

    public static void close() {
        emFactory.close();
    }
}