JAVAEE容器如何管理EntityManager和PersistenceContext

article/2025/9/14 8:39:30

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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;
 }
}

转载于:https://my.oschina.net/u/869416/blog/112454