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我在当前的Web项目中使用JP2。我的主数据库包含主要实体。JPA2运行时数据库连接
Persistance.xml：
<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
<persistence-unit name="MyPU" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>MyDB</jta-data-source>
<exclude-unlisted-classes>false</exclude-unlisted-classes>
<properties>
<property name="eclipselink.logging.level" value="FINE"/>
<property name="eclipselink.logging.parameters" value="true"/>
<property name="eclipselink.logging.logger" value="ServerLogger"/>
<property name="eclipselink.ddl-generation" value="drop-and-create-tables"/>
</properties>
</persistence-unit>
</persistence>
和JTA数据源在阳光resources.xml中定义的：要在此DB我定义了一个Persitence单位用JTA数据源连接
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE resources PUBLIC "-//GlassFish.org//DTD GlassFish Application Server 3.1 Resource Definitions//EN" "http://glassfish.org/dtds/glassfish-resources_1_5.dtd">
<resources>
<jdbc-connection-pool allow-non-component-callers="false" associate-with-thread="false" connection-creation-retry-attempts="0" connection-creation-retry-interval-in-seconds="10" connection-leak-reclaim="false" connection-leak-timeout-in-seconds="0" connection-validation-method="auto-commit" datasource-classname="org.postgresql.ds.PGSimpleDataSource" fail-all-connections="false" idle-timeout-in-seconds="300" is-connection-validation-required="false" is-isolation-level-guaranteed="true" lazy-connection-association="false" lazy-connection-enlistment="false" match-connections="false" max-connection-usage-count="0" max-pool-size="32" max-wait-time-in-millis="60000" name="post-gre-sql_mydb_mypool" non-transactional-connections="false" pool-resize-quantity="2" res-type="javax.sql.DataSource" statement-timeout-in-seconds="-1" steady-pool-size="8" validate-atmost-once-period-in-seconds="0" wrap-jdbc-objects="false">
<property name="serverName" value="localhost"/>
<property name="portNumber" value="5432"/>
<property name="databaseName" value="mydb"/>
<property name="User" value="myuser"/>
<property name="Password" value="mypass"/>
<property name="URL" value="jdbc:postgresql://localhost:5432/mydb"/>
<property name="driverClass" value="org.postgresql.Driver"/>
<property name="characterEncoding" value="UTF-8" />
</jdbc-connection-pool>
<jdbc-resource enabled="true" jndi-name="MyDB" object-type="user" pool-name="post-gre-sql_mydb_mypoll"/>
</resources>
而且这是我如何访问我的DAO类的数据库（女巫是@ManagedBeans和@SessionScoped）：
@ManagedBean(name = "pageDao")
@SessionScoped
public class PageDao implements Serializable {
@Resource
private UserTransaction utx = null;
@PersistenceUnit(unitName = "MyPU")
private EntityManagerFactory emf = null;
public EntityManager getEntityManager() {
return emf.createEntityManager();
}
public List<PageEnt> getAll() { ... }
public PageEnt getOne(long pageId) { ... }
public void addPage(PageEnt newPage) throws RollbackFailureException, PreexistingEntityException, Exception { ... }
public PageEnt update(PageEnt page) throws RollbackFailureException, NonexistentEntityException, Exception { ... }
public void remove(PageEnt page) throws RollbackFailureException, Exception { ... }
}
其中一个实体（客户）具有连接到单独（每个客户）数据库的属性，女巫在运行时定义。这些特性包括：
- DATABSE名
- 主机和端口
- 用户和密码
我的问题是：
- 如何有效地创建在运行一个数据库连接-时间？
- 如果没有定义每个客户的PersistanceUnit和Datasources（女巫是在部署时定义的），如何从容器管理的资源创建新的EntityManager？
- 如果我必须手动处理EntityManagerFactory（女巫，就像我在大学学到的那样，是一个沉重而膨胀的对象），我该如何有效地做到这一点？有没有好的做法或模式？
- DAO模式如何工作？我的DAO类如何获得EntityManager？
非常感谢巴西。