* persistence.xml file
- Is a standard configuration file in JPA.
- Has to be included in the META-INF directory inside the JAR file that contains the entity beans.
- The <provider> specifies the underlying implementation of the JPA EntityManager.
- The <jta-data-source> points to the JNDI name of the database this persistence unit maps to.
* Since you might have multiple instances of persistence-unit defined in the same application, you typically need to explicitly tell the @PersistenceContext annotation which unit you want to inject. For instance, @PersistenceContext(name="myapp") injects the EntityManager from the persistence-unit named "myapp".
* JTA:
- One of the Java Enterprise Edition (Java EE) APIs allowing distributed transactions to be done across multiple XA resources in a Java environment.
- Provides for:
- demarcation of transaction boundaries
- X/Open XA API allowing resources to participate in transactions.
* <tx:annotation-driven>
- Enabled spring to interpret @Transactional annotation.
- Creates a BeanPostProcessor to be included in the BeanPostProcessor step in the initialization phase.
- When that BeanPostProcessor sees @Transactional in your Spring bean code, then it create a DynamicProxy to proxy that bean so that it will run transactionally.
* 一個 LocalContainerEntityManagerFactoryBean 只能管理一個持久化單元, 僅管其内部創建的DefaultPersistenceUnitManager 類可以得取多個持久化單元信息(PersistenceUnitInfo), 但最终只能有一個被返回给 LocalContainerEntityManagerFactoryBean 用於創建 JPA 實現提供者的 EntityManagerFactory
* Entity Manager 藉由 Persistence Context 來控制被管理的 Entity Instances，而每個 Persistence Context 必定由一個 Persistence Unit 來限制該 Context 可以管理的 Entity Classes 種類。
* Entity Manager 可以分為兩類
- Container-Managed Entity Manager
- 是由 JEE Container 所管理的 Entity Manager
- 主要是以 @PersistenceContext 標識在你的程式中，Container 會在執行期間自動注入對應的 instance。
- 可以再區分兩類：
- PersistenceContextType.TRANSACTION (Default):
- 主要是配合 Stateless Session Bean 與 JTA Transaction，在每次 method 被呼叫時都去檢查 JTA transaction 中是不是有 Persistence Context，如果有就續用否則就建一個新的。
- 當 JTA Transaction commit 時，在 persistence context 中的 entity instances 就會自動 persist 至 DB。
- PersistenceContextType.EXTENDED:
- 主要是配合 Stateful Session Bean，只有當執行到有 @Remove 標識的 method 時才會 persist 在 persistence context 中的 entity instances。
- Application-Managed Entity Manager
- 由 Application 自行管理，藉由呼叫 EntityManagerFactory.createEntityManager() 來取得，因此，一般的 J2SE程式也可以使用，當然在 JEE Container 也可以使用，特別是在某些特別情況或有特殊考慮時多一種方式可以應用。
- 如果要在 JEE Container 中使用的話，與 Container-Managed Entity Manager 不同之處是以 @PersistenceUnit 來標識要被注入的 EntityManagerFactory，而且也必需呼叫 EntityManager.close() 來指出要 persist 的時機。
- 產生 persistence context 的時機也有所不同，當呼叫 EntityManagerFactory.createEntityManager() 就會產生一個 persistence context。
* JPA allows us to work with entity classes, which are denoted as such using
- The annotation @Entity.
- configured in an XML file (we'll call this persistence meta information).
* To work with a database using JPA, we need an Entity Manager. Before we can do that, we need to create an Entity Manager Factory.
=> Entity Manager Factory -> Entity Manager -> using JPA...
* To acquire an Entity Manager Factory, we use the class javax.persistence.Persistence.
- It reads a file called persistence.xml in the META-INF directory.
- Then creates the named Entity Manager Factory, which processes persistence meta information stored in XML files or annotations (we only use annotations).
* Once we have an Entity Manager, we can ask it to perform several operations such as persisting or removing an entity from the database or creating a query.
* Reference
- 1.2.1. The persistence.xml file
- Java Transaction API
- Understanding
- 非J2EE 容器环境下Spring +JPA 多持久化单元/多个JAR归档注解实体 的实体扫描问题及解决办法
- JPA2--Transaction Management
- Comments/Suggestions, please email: schuchert -at- yahoo -dot- com
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