JAVAEE容器如何管理EntityManager和PersistenceContext

by linxh  小猪商队（QQ:124157155）

 

EJB3.0---JPA1.0

 

容器托管EntityManager

 

@PersistenceContext，存放unitName指向的DataBase对应的EntityBean实例集合，以及对这些实例进行生命周期管理

@PersistenceContext(
     name="entityManagerName",
     properties=@PersistenceProperty[],
     type=""//PersistenceContextType  TRANSACTION (default,transaction-scope)/EXTENDED
     unitName=""//the persistence unit name
)

 

EntityManager

必须关联一个PersistenceContext对象，提供一系列方法对PersistenceContext中的EntityBean实例进行操作。

 

可以使用依赖注入，将某个PersistenceContext对象注入给EntityManager对象

如：

@PersistenceContext(unitName="lxhPU")

private EntityManager entityManager;

JAVAEE容器会智能选取容器中的PersistenceContext实例跟这个entityManager实例来关联

 

 

a、一般情况下，一个persistence unit只需对应一个PersistenceContext实例，如

class UserDao{

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager;

}



class OrderDao{

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager;

}

 

UserDao和OrderDao中的entityManager对象关联的PersistenceContext是同一个实例

 

 

当然，我们也可以手动为PersistenceUnit增加PersistenceContext实例

class UserDao{

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager;

    @PersistenceContext(name="pc2" ,unitName="lxhPU")

    private EntityManager entityManager2;

}

 UserDao 中的entityManager和entityManager2关联的2个PersistenceContext实例是不同的，尽管他们指向的是同一个PersistenceUnit

 

 

b、JAVAEE容器会维护PersistenceContext实例及其关联的EntityManger实例

一个PersistenceContext实例可以被多个EntityManager实例关联

class UserDao{

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager;

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager2;

}



class OrderDao{

    @PersistenceContext(unitName="lxhPU")

    private EntityManager entityManager;

}

 

还是上面的代码：

UserDao和OrderDao中的entityManager对象关联的PersistenceContext是同一个实例，其实这2个entityManager也是指向容器中的同一个EntityManager实例

但是UserDao中的entityManager和entityManager2却是不同的EntityManager实例，在同一个类中多次定义，JAVAEE容器会为他们分配不同的EntityManager实例，无则新建，而这2个实例所关联的PersistenceContext对象却是同一个。

 

 

做个简单的测试就一目了然了

@Stateless
@Remote
public class UserDao1 implements IUserDiffEmDao{
	@PersistenceContext(unitName="lxhPU")
	private EntityManager em;
	
	@PersistenceContext(unitName="lxhPU")
	private EntityManager em2;
	
	@PersistenceContext(name="pc3", unitName="lxhPU")
	private EntityManager em3;
	
	@PersistenceContext(unitName="lxh2PU")
	private EntityManager em4;

	public void test() {
		System.out.println("-----------------------");
		System.out.println("em="+em);//----------------EntityManager@001
		System.out.println("em2="+em2);//----------------EntityManager@002
		System.out.println("em3="+em3);//----------------EntityManager@003
		System.out.println("em4="+em4);//----------------EntityManager@004
		//4个结果是不一样的
		return null;
	}

}



@Stateless
@Remote
public class UserDao2 implements IUserDiffEmDao{
	@PersistenceContext(unitName="lxhPU")
	private EntityManager em;
	
	@PersistenceContext(unitName="lxhPU")
	private EntityManager em2;
	
	@PersistenceContext(name="pc3", unitName="lxhPU")
	private EntityManager em3;
	
	@PersistenceContext(unitName="lxh2PU")
	private EntityManager em4;

	public void test() {
		System.out.println("-----------------------");
		System.out.println("em="+em);//----------------EntityManager@001
		System.out.println("em2="+em2);//----------------EntityManager@002
		System.out.println("em3="+em3);//----------------EntityManager@003
		System.out.println("em4="+em4);//----------------EntityManager@004
		//4个结果是不一样的，但是其值分别对应UserDao1打印的4个值

		
		try {
			PersistenceContext context1 = this.getClass().getDeclaredField("em").getAnnotation(PersistenceContext.class);//-----PersistenceContext@001
			PersistenceContext context2 = this.getClass().getDeclaredField("em2").getAnnotation(PersistenceContext.class);//-----PersistenceContext@001
			PersistenceContext context3 = this.getClass().getDeclaredField("em3").getAnnotation(PersistenceContext.class);//-----PersistenceContext@002
			PersistenceContext context4 = this.getClass().getDeclaredField("em4").getAnnotation(PersistenceContext.class);//-----PersistenceContext@003
			System.out.println("UserDao2.context1="+context1.hashCode());
			System.out.println("UserDao2.context2="+context2.hashCode());
			System.out.println("UserDao2.context3="+context3.hashCode());
			System.out.println("UserDao2.context4="+context4.hashCode());
			//context1和context2指向的是同一个PersistenceContext实例
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NoSuchFieldException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		
		return null;
	}

}

 

 

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