Question

我不确定 @EJB 注释如何分配字段等。

我们假设我们有3个班级：

A - 无状态（有一个带 @EJB 注释的B字段）
B - Singleton （有一个带 @EJB 注释的C字段）
C - 无国籍

在这种情况下，当用户尝试访问bean时会发生什么？

用户 1 访问bean（第一次）
1. 已创建，需要 B 才能正常工作
2. B 已创建，需要 C 才能正常工作
3. C 已创建
用户2连接和
1. 创建，因为用户1仍在使用 A
2. B 是必需的，但它是一个单身，因此只返回了唯一的实例
3. C 未创建为Singleton B 已为其分配了引用

这是否意味着如果 @Singleton 需要任何其他bean，那么它将用作 @Singleton 的bean？（ @Stateless 将充当单身人士）

Answer 1

不一定。如果单例允许多个并发调用@Lock(READ)或@ConcurrencyManagement(BEAN)，那么将创建多个C bean，每个调用者线程一个与A bean相同。但是，如果单例bean使用@ConcurrencyManagement(CONTAINER)与@Lock(WRITE)的默认并发，那么是的，bean C实际上也将是一个单例（假设它不在其他地方访问）。

Answer 2

我为您的问题创建了一个示例实现。你可以在GitHub上找到它： https://github.com/StefanHeimberg/stackoverflow-28194928

使用NetBeans 8.0.2打开此项目，并在Glassfish 4.1上部署（运行）此项目。

部署后会创建以下日志：

Information:   visiting unvisited references
Information:   visiting unvisited references
Information:   Portable JNDI names for EJB C: [java:global/mavenproject1/C!com.mycompany.mavenproject1.C, java:global/mavenproject1/C]
Schwerwiegend:   C[uuid=e1c73e4b-998c-4666-bac9-4825556aa20a] => created
Information:   Portable JNDI names for EJB Bootstrap: [java:global/mavenproject1/Bootstrap!com.mycompany.mavenproject1.Bootstrap, java:global/mavenproject1/Bootstrap]
Information:   Portable JNDI names for EJB A: [java:global/mavenproject1/A, java:global/mavenproject1/A!com.mycompany.mavenproject1.ARemote]
Information:   Glassfish-specific (Non-portable) JNDI names for EJB A: [com.mycompany.mavenproject1.ARemote, com.mycompany.mavenproject1.ARemote#com.mycompany.mavenproject1.ARemote]
Information:   Portable JNDI names for EJB B: [java:global/mavenproject1/B!com.mycompany.mavenproject1.B, java:global/mavenproject1/B]
Schwerwiegend:   B[uuid=868f15c0-728d-4079-b90a-2b55ae413c33] => created
WARN:   WELD-000411: Observer method [BackedAnnotatedMethod] private org.glassfish.jersey.gf.cdi.internal.CdiComponentProvider.processAnnotatedType(@Observes ProcessAnnotatedType<Object>) receives events for all annotated types. Consider restricting events using @WithAnnotations or a generic type with bounds.
WARN:   WELD-000411: Observer method [BackedAnnotatedMethod] public org.glassfish.jms.injection.JMSCDIExtension.processAnnotatedType(@Observes ProcessAnnotatedType<Object>) receives events for all annotated types. Consider restricting events using @WithAnnotations or a generic type with bounds.
WARN:   WELD-000411: Observer method [BackedAnnotatedMethod] org.glassfish.sse.impl.ServerSentEventCdiExtension.processAnnotatedType(@Observes ProcessAnnotatedType<Object>, BeanManager) receives events for all annotated types. Consider restricting events using @WithAnnotations or a generic type with bounds.
Schwerwiegend:   ========================
Schwerwiegend:   Bootstrap.init()
Schwerwiegend:   ========================
Information:   mavenproject1 was successfully deployed in 419 milliseconds.

现在只运行焦点测试方法ARemoteIT.client1（）。生成以下输出：

Schwerwiegend:   A[uuid=9be077f8-945a-4db2-ba3e-ea0003692c0f] => created ()
Schwerwiegend:   A[uuid=9be077f8-945a-4db2-ba3e-ea0003692c0f] => B of type com.mycompany.mavenproject1.__EJB31_Generated__B__Intf____Bean__ injected
Schwerwiegend:   A[uuid=9be077f8-945a-4db2-ba3e-ea0003692c0f] => ready to use 
Schwerwiegend:   A[uuid=9be077f8-945a-4db2-ba3e-ea0003692c0f] [client1] => doSomething() called
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] => created
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] => C of type com.mycompany.mavenproject1.__EJB31_Generated__C__Intf____Bean__ injected
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] => ready to use
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] [client1]   => doSomething() called
Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] => created
Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] => ready to use
Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] [client1]     => doSomething() called

在C.doeSomething（）中，现在超时1分钟......

在此超时时间内，无法同时访问单例bean B，因为默认锁定类型为Write（@Lock（WRITE））。

如果你现在在1分钟的超时时间内开始使用已故的测试方法ARemoteIT.client2（），则会生成以下输出：

Schwerwiegend:   A[uuid=c358e9f3-8687-4d76-a9ca-38b31357649b] => created ()
Schwerwiegend:   A[uuid=c358e9f3-8687-4d76-a9ca-38b31357649b] => B of type com.mycompany.mavenproject1.__EJB31_Generated__B__Intf____Bean__ injected
Schwerwiegend:   A[uuid=c358e9f3-8687-4d76-a9ca-38b31357649b] => ready to use 
Schwerwiegend:   A[uuid=c358e9f3-8687-4d76-a9ca-38b31357649b] [client2] => doSomething() called

在这里你可以看到client2是Waitung for B来调用B.doSomething。在1分钟超时完成后。该可以访问单例bean，client2可以跟随B.doSomething。

这是client1 C.doSomething超时后的日志：

Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] [client1]     => doSomething() finished
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] [client1]   => doSomething() finished
Schwerwiegend:   A[uuid=9be077f8-945a-4db2-ba3e-ea0003692c0f] [client1] => doSomething() finished
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] [client2]   => doSomething() called
Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] [client2]     => doSomething() called
Schwerwiegend:   C[uuid=5f1bed5c-80e8-41c8-b524-51cac3e3de0f] [client2]     => doSomething() finished
Schwerwiegend:   B[uuid=ce1be5f1-0b82-4990-a9b5-153a2540a891] [client2]   => doSomething() finished
Schwerwiegend:   A[uuid=c358e9f3-8687-4d76-a9ca-38b31357649b] [client2] => doSomething() finished

<强> Additionals：

你可以看到

它不是真正注入B或C的实例！相反，这些只是代理。但您也可以看到glassfish在部署后自动创建B和C的新实例。

使用@Stateless，您无法保证您收到所需EJB的相同实例。它也可能是那样的每次在代理上调用方法时，都会收到所需EJB的另一个实例。容器正在管理这个。如果所有EJB都在使用中，那么容器使用一个EJB池或创建一个新实例。

在日志中，您会看到B仅在调用A.doSomething（）之后创建。那个男士在调用代理方法时。

如果@Singleton bean中有@Stateless字段会发生什么？

2 个答案: