java - 调用 Persistence.createEntityManagerFactory > 1 次

Question

我有一个 servlet，当我使用 PU 中指定的 H2 调用此方法时，每次调用它时它都会重新创建所有数据库结构。我只能调用这个方法1次吗？如果我可以调用它> 1次，我该怎么做？

entityManagerFactory = Persistence
                    .createEntityManagerFactory("MYPU");

用于持久性的 XML

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" 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">
  <persistence-unit name="MyJPAJAXRS" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
    <non-jta-data-source>jdbc/myds</non-jta-data-source>
    <properties>
      <property name="javax.persistence.schema-generation.database.action"
                value="drop-and-create"/>
      <property name="javax.persistence.sql-load-script-source" value="META-INF/seed.sql"/>
    </properties>
  </persistence-unit>
</persistence>

Answer 1

您需要一个单例方法来创建 EntityManagerFactory，然后使用该单例实例获取 EntityManager 的新实例。注意:EntityManager 不是线程安全的，您需要为每个线程获取一个新的 EntityManager 实例。下面是如何实现这一点的示例

public class JpaUtil {

private static HashMap<String, String> properties = new HashMap<String, String>();
private volatile static EntityManagerFactory factory;

static {
    properties.put("javax.persistence.jdbc.driver", System.getProperty("DRIVER"));
    properties.put("javax.persistence.jdbc.user", System.getProperty("USER"));
    properties.put("javax.persistence.jdbc.password", System.getProperty("PASSWORD"));
    properties.put("javax.persistence.jdbc.url", System.getProperty("DATABASEURL"));
}

private static EntityManagerFactory getInstance() {
    if (factory == null) {
        synchronized (EntityManagerFactory.class) {
            if (factory == null) {

                factory = Persistence.createEntityManagerFactory("PU", properties);
            }
        }
    }
    return factory;
}

public static EntityManager getEntityManager() throws Exception {
    return getInstance().createEntityManager();
}

}

然后要获取实体管理器，只需调用:JpaUtil.getEntityManager()