调用Persistence.createEntityManagerFactory> 1次

问题描述

我有一个servlet，当我用PU中指定的H2调用此方法时，每次调用它都会重新创建所有数据库结构.我只能将此方法调用1次吗，如果可以调用1次以上，该怎么办?

I have a servlet, and when I call this method with H2 specified in the PU, it re-creates all the database structure each time I call it. Can I only call this method 1 time, and if I can call it > 1 time, how do I do it?

entityManagerFactory = Persistence
                    .createEntityManagerFactory("MYPU");

XML持久化

<?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>

推荐答案

您将需要一个singleton方法来创建EntityManagerFactory，然后使用该Singleton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.下面是一个如何实现此目的的示例

You'll need a singleton method to create EntityManagerFactory and then get a new instance of EntityManager using that Singleton Instance. Note: EntityManager is not thread safe and you'll need to get a new EntityManager instance per thread. Below is an example of how to implement this

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()

And then to get the Entity manager simply call: JpaUtil.getEntityManager()

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