JPA的persistence.xml的使用及常见问题

在J2SE项目中:

<?xml version="1.0" encoding="UTF-8" ?>  
<persistence 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_1_0.xsd" version="1.0">   
  <persistence-unit name="demo" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.ejb.HibernatePersistence</provider>
    <class>com.ncs.entity.UserInfo</class>
        <properties>  
            <property name="hibernate.hbm2ddl.auto" value="update"/>  
            <property name="hibernate.connection.driver_class" value="oracle.jdbc.driver.OracleDriver"/>  
            <property name="hibernate.connection.url" value="jdbc:oracle:thin:@127.0.0.1:1522:xe"/>  
            <property name="hibernate.connection.username" value="system"/>  
            <property name="hibernate.connection.password" value="151923"/>  
            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>  
            <property name="hibernate.show_sql" value="true"/>  
            <property name="hibernate.format_sql" value="true"/>  
        </properties> 
  </persistence-unit>
</persistence>

注意:配置参数hibernate.hbm2ddl.auto 各个取值的含义

validate  update  create  create-drop

  这几个参数的作用主要用于：自动创建  更新  验证数据库表结构

  如果不是此方面的需求取none

  validate：加载hibernate时，验证创建数据库表结构

  create：每次加载hibernate，重新创建数据库表结构，这就是导致数据库表结构丢失的原因

  create-drop：加载hibernate时创建，退出是删除表结构

  update：加载hibernate自动更新数据库结构

常见问题排错:

1.

Exception in thread "main" javax.persistence.PersistenceException: No Persistence provider for EntityManager named HelloTitan
 at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:56)
 at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:34)
 at Main.main(Main.java:10)

原因：没有Persistence provider

解决办法：错误提示很明显:No Persistence provider.

  (1)检查lib:jar丢失,则将需要的lib添加到classpath中，Jboss使用的是hibernate实现，这些lib在下面的目录中能找到

jboss-X\common\lib\

  (2)可能provider不对.检查xml文件里面的provider

2.

Exception in thread "main" javax.persistence.PersistenceException: [PersistenceUnit: HelloTitan] Unable to build EntityManagerFactory   
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:677)   
    at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:126)   
    at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:52)   
    at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:34)   
    at Main.main(Main.java:10)   
Caused by: org.hibernate.HibernateException: Could not find datasource   
    at org.hibernate.connection.DatasourceConnectionProvider.configure(DatasourceConnectionProvider.java:79)   
    at org.hibernate.connection.ConnectionProviderFactory.newConnectionProvider(ConnectionProviderFactory.java:137)   
    at org.hibernate.ejb.InjectionSettingsFactory.createConnectionProvider(InjectionSettingsFactory.java:29)   
    at org.hibernate.cfg.SettingsFactory.buildSettings(SettingsFactory.java:89)   
    at org.hibernate.cfg.Configuration.buildSettings(Configuration.java:2101)   
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1325)   
    at org.hibernate.cfg.AnnotationConfiguration.buildSessionFactory(AnnotationConfiguration.java:867)   
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:669)   
    ... 4 more   
Caused by: javax.naming.NoInitialContextException: Need to specify class name in environment or system property, or as an applet parameter, or in an application resource file:  java.naming.factory.initial   
    at javax.naming.spi.NamingManager.getInitialContext(NamingManager.java:645)   
    at javax.naming.InitialContext.getDefaultInitCtx(InitialContext.java:288)   
    at javax.naming.InitialContext.getURLOrDefaultInitCtx(InitialContext.java:325)   
    at javax.naming.InitialContext.lookup(InitialContext.java:392)   
    at org.hibernate.connection.DatasourceConnectionProvider.configure(DatasourceConnectionProvider.java:75)   
    ... 11 more  

原因：JNDI没有初始化成功

解决办法： 在classpath添加 jnid.properties 文件 （以JBoss为例，配置如下）

java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=127.0.0.1:1099

3.

程序成功运行，但是数据没有写入数据库

原因：没有提交数据

解决办法：em.getTransaction().commit();

附录

Hibernate SQL方言 (hibernate.dialect)

RDBMS	方言
DB2	org.hibernate.dialect.DB2Dialect
DB2 AS/400	org.hibernate.dialect.DB2400Dialect
DB2 OS390	org.hibernate.dialect.DB2390Dialect
PostgreSQL	org.hibernate.dialect.PostgreSQLDialect
MySQL	org.hibernate.dialect.MySQLDialect
MySQL with InnoDB	org.hibernate.dialect.MySQLInnoDBDialect
MySQL with MyISAM	org.hibernate.dialect.MySQLMyISAMDialect
Oracle (any version)	org.hibernate.dialect.OracleDialect
Oracle 9i/10g	org.hibernate.dialect.Oracle9Dialect
Sybase	org.hibernate.dialect.SybaseDialect
Sybase Anywhere	org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server	org.hibernate.dialect.SQLServerDialect
SAP DB	org.hibernate.dialect.SAPDBDialect
Informix	org.hibernate.dialect.InformixDialect
HypersonicSQL	org.hibernate.dialect.HSQLDialect
Ingres	org.hibernate.dialect.IngresDialect
Progress	org.hibernate.dialect.ProgressDialect
Mckoi SQL	org.hibernate.dialect.MckoiDialect
Interbase	org.hibernate.dialect.InterbaseDialect
Pointbase	org.hibernate.dialect.PointbaseDialect
FrontBase	org.hibernate.dialect.FrontbaseDialect
Firebird	org.hibernate.dialect.FirebirdDialect