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我在當前的Web項目中使用JP2。我的主數據庫包含主要實體。JPA2運行時數據庫連接
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數據源在陽光resources.xml中定義的：要在此DB我定義了一個Persitence單位用JTA數據源連接
<?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>
而且這是我如何訪問我的DAO類的數據庫（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 { ... }
}
其中一個實體（客戶）具有連接到單獨（每個客戶）數據庫的屬性，女巫在運行時定義。這些特性包括：
- DATABSE名
- 主機和端口
- 用戶和密碼
我的問題是：
- 如何有效地創建在運行一個數據庫連接-時間？
- 如果沒有定義每個客戶的PersistanceUnit和Datasources（女巫是在部署時定義的），如何從容器管理的資源創建新的EntityManager？
- 如果我必須手動處理EntityManagerFactory（女巫，就像我在大學學到的那樣，是一個沉重而膨脹的對象），我該如何有效地做到這一點？有沒有好的做法或模式？
- DAO模式如何工作？我的DAO類如何獲得EntityManager？
非常感謝巴西。