JPA - Persistence与EntityManagerFactory

【1】Persistence类

Persistence 类是用于获取 EntityManagerFactory 实例。

该类包含一个名为 createEntityManagerFactory 的 静态方法 。

createEntityManagerFactory 方法有如下两个重载版本。

① 带有一个参数的方法：以 JPA 配置文件 persistence.xml 中的持久化单元名为参数；

② 带有两个参数的方法：前一个参数含义相同，后一个参数 Map类型，用于设置 JPA 的相关属性，这时将忽略其它地方设置的属性。Map 对象的属性名必须是 JPA 实现库提供商的名字空间约定的属性名。

//1. 创建 EntitymanagerFactory
String persistenceUnitName = "jpa-1";

Map<String, Object> properites = new HashMap<String, Object>();
properites.put("hibernate.show_sql", false);

EntityManagerFactory entityManagerFactory = 
//              Persistence.createEntityManagerFactory(persistenceUnitName);
        Persistence.createEntityManagerFactory(persistenceUnitName, properites);

---

其源码如下：

/*
 * Copyright (c) 2008, 2009 Sun Microsystems. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Linda DeMichiel - Java Persistence 2.0 - Version 2.0 (October 1, 2009)
 *     Specification available from http://jcp.org/en/jsr/detail?id=317
 */

// $Id: Persistence.java 20957 2011-06-13 09:58:51Z stliu $

package javax.persistence;

import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import javax.persistence.spi.LoadState;
import javax.persistence.spi.PersistenceProvider;
import javax.persistence.spi.PersistenceProviderResolverHolder;

/**
 * Bootstrap class that provides access to an EntityManagerFactory.
 */
public class Persistence {

    @Deprecated
    public static final String PERSISTENCE_PROVIDER = "javax.persistence.spi.PeristenceProvider";

    @Deprecated
    protected static final Set providers = new HashSet();

    /**
     * Create and return an EntityManagerFactory for the named persistence unit.
     *
     * @param persistenceUnitName The name of the persistence unit
     *
     * @return The factory that creates EntityManagers configured according to the specified persistence unit
     */
    public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName) {
        return createEntityManagerFactory( persistenceUnitName, null );
    }

    /**
     * Create and return an EntityManagerFactory for the named persistence unit using the given properties.
     *
     * @param persistenceUnitName The name of the persistence unit
     * @param properties Additional properties to use when creating the factory. The values of these properties override
     * any values that may have been configured elsewhere
     *
     * @return The factory that creates EntityManagers configured according to the specified persistence unit
     */
    public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName, Map properties) {
        EntityManagerFactory emf = null;
        List providers = getProviders();
        for ( PersistenceProvider provider : providers ) {
            emf = provider.createEntityManagerFactory( persistenceUnitName, properties );
            if ( emf != null ) {
                break;
            }
        }
        if ( emf == null ) {
            throw new PersistenceException( "No Persistence provider for EntityManager named " + persistenceUnitName );
        }
        return emf;
    }

    private static List getProviders() {
        return PersistenceProviderResolverHolder
                .getPersistenceProviderResolver()
                .getPersistenceProviders();
    }

    /**
     * @return Returns a PersistenceUtil instance.
     */
    public static PersistenceUtil getPersistenceUtil() {
        return util;
    }

    private static PersistenceUtil util =
            //TODO add an Hibernate specific optimization
        new PersistenceUtil() {
            public boolean isLoaded(Object entity, String attributeName) {
                List providers = Persistence.getProviders();
                for ( PersistenceProvider provider : providers ) {
                    final LoadState state = provider.getProviderUtil().isLoadedWithoutReference( entity, attributeName );
                    if ( state == LoadState.UNKNOWN ) continue;
                    return state == LoadState.LOADED;
                }
                for ( PersistenceProvider provider : providers ) {
                    final LoadState state = provider.getProviderUtil().isLoadedWithReference( entity, attributeName );
                    if ( state == LoadState.UNKNOWN ) continue;
                    return state == LoadState.LOADED;
                }
                return true;
            }

            public boolean isLoaded(Object object) {
                List providers = Persistence.getProviders();
                for ( PersistenceProvider provider : providers ) {
                    final LoadState state = provider.getProviderUtil().isLoaded( object );
                    if ( state == LoadState.UNKNOWN ) continue;
                    return state == LoadState.LOADED;
                }
                return true;
            }
        };
}

---

【2】EntityManagerFactory 接口

EntityManagerFactory 接口主要用来创建 EntityManager 实例。

EntityManager 的作用参考博客：博客地址

该接口主要约定了如下4个方法：

① createEntityManager()：用于创建实体管理器对象实例。

② createEntityManager(Map map)：用于创建实体管理器对象实例的重载方法，Map 参数用于提供 EntityManager 的属性。

③ isOpen()：检查 EntityManagerFactory 是否处于打开状态。实体管理器工厂创建后一直处于打开状态，除非调用close()方法将其关闭。

④ close()：关闭 EntityManagerFactory 。

EntityManagerFactory 关闭后将释放所有资源，isOpen()方法测试将返回 false，其它方法将不能调用，否则将导致IllegalStateException异常。

---

其接口源码如下：

/*
 * Copyright (c) 2008, 2009 Sun Microsystems. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Linda DeMichiel - Java Persistence 2.0 - Version 2.0 (October 1, 2009)
 *     Specification available from http://jcp.org/en/jsr/detail?id=317
 */

// $Id: EntityManagerFactory.java 20957 2011-06-13 09:58:51Z stliu $

package javax.persistence;

import java.util.Map;
import javax.persistence.metamodel.Metamodel;
import javax.persistence.criteria.CriteriaBuilder;

/**
 * Interface used to interact with the entity manager factory
 * for the persistence unit.
 *
 * 
When the application has finished using the entity manager
 * factory, and/or at application shutdown, the application should
 * close the entity manager factory.  Once an
 * EntityManagerFactory has been closed, all its entity managers
 * are considered to be in the closed state.
 *
 * @since Java Persistence 1.0
 */
public interface EntityManagerFactory {

    /**
     * Create a new application-managed EntityManager.
     * This method returns a new EntityManager instance each time
     * it is invoked.
     * The isOpen method will return true on the returned instance.
     * @return entity manager instance
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     */
    public EntityManager createEntityManager();

    /**
     * Create a new application-managed EntityManager with the
     * specified Map of properties.
     * This method returns a new EntityManager instance each time
     * it is invoked.
     * The isOpen method will return true on the returned instance.
     * @param map properties for entity manager
     * @return entity manager instance
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     */
    public EntityManager createEntityManager(Map map);

    /**
     * Return an instance of CriteriaBuilder for the creation of
     * CriteriaQuery objects.
     * @return CriteriaBuilder instance
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     *
     * @since Java Persistence 2.0
     */
    public CriteriaBuilder getCriteriaBuilder();

    /**
     * Return an instance of Metamodel interface for access to the
     * metamodel of the persistence unit.
     * @return Metamodel instance
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     *
     * @since Java Persistence 2.0
     */
    public Metamodel getMetamodel();

    /**
     * Indicates whether the factory is open. Returns true
     * until the factory has been closed.
     * @return boolean indicating whether the factory is open
     */
    public boolean isOpen();

    /**
     * Close the factory, releasing any resources that it holds.
     * After a factory instance has been closed, all methods invoked
     * on it will throw the IllegalStateException, except
     * for isOpen, which will return false. Once an
     * EntityManagerFactory has been closed, all its
     * entity managers are considered to be in the closed state.
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     */
    public void close();

    /**
     * Get the properties and associated values that are in effect
     * for the entity manager factory. Changing the contents of the
     * map does not change the configuration in effect.
     * @return properties
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     *
     * @since Java Persistence 2.0
     */
    public Map getProperties();

    /**
     * Access the cache that is associated with the entity manager
     * factory (the "second level cache").
     * @return instance of the Cache interface
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     *
     * @since Java Persistence 2.0
     */
    public Cache getCache();

    /**
     * Return interface providing access to utility methods
     * for the persistence unit.
     * @return PersistenceUnitUtil interface
     * @throws IllegalStateException if the entity manager factory
     * has been closed
     *
     * @since Java Persistence 2.0
     */
    public PersistenceUnitUtil getPersistenceUnitUtil();
}

EntityManagerFactory (对应 Hibernate 中的 SessionFactory)。

EntityManager (对应 Hibernate 中的Session)。