EJB Concepts
Internal
EJB and JNDI
Session beans are bound to JNDI according to a portable naming scheme. The most generic format is:
ejb:[/<application-name>]/<module-name>/<distinct-name>/<bean-name>[!<view-class-name>]?stateful
Simplified formats are possible within the context of specific namespaces, as described in the Namespaces section.
JNDI Path Components
application-name
application-name is the name of the EAR application the session bean is deployed as part of. It is an optional component. If the session bean is not deployed as part of an EAR, that part is left blank, and the JNDI name starts with ejb:/<module-name>.... If the session bean deployed as part of an EAR, the part is by default the name of the EAR file, without the .ear suffix. The name can be overridden in the application.xml deployment descriptor by specifying the <application-name> element.
module-name
module-name is the name of the JAR (or WAR) file the session bean is deployed in. It is a mandatory component. By default, is the file name of the JAR without the ".jar" suffix. The module name can be override in ejb-jar.xml by specifying the <module-name> element.
distinct-name
Each deployment may specify an optional distinct name. It is an optional component. If the deployment does not have a distinct name, the corresponding JNDI path element should be left blank.
The JNDI bindings for session bean named 'SimpleStatelessBean' in deployment unit 'deployment "stateless-ejb-example.jar"' are as follows:
bean-name
The simple name of the class implementing the session bean. It is a mandatory component.
view-class-name
The fully qualified class name of the remote interface. It is an optional component.
?stateful
This suffix is required when the JNDI name refers to a stateful session bean.
Namespaces
Upon deployment, the component will be registered in JNDI, in the following contexts: java:global, java:app and java:module, which are accessible only to the current JVM, and also in java:jboss/exported, which is accessible remotely.
For more details on JEE JNDI Namespaces, see
JNDI Name Examples
EJB deployed in JAR
The following bean has been deployed as part of stateless-ejb-example.jar, it is implemented by io.novaordis.playground.jee.ejb.stateless.SimpleStatelessBean and exposes the io.novaordis.playground.jee.ejb.stateless.SimpleStateless remote interface.
java:global/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:app/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:module/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:jboss/exported/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:global/stateless-ejb-example/SimpleStatelessBean java:app/stateless-ejb-example/SimpleStatelessBean java:module/SimpleStatelessBean
The last three names are short forms, and they are registered if the EJB component only exposed one interface.
EJB deployed in EAR
The following bean has been deployed as part of a stateless-ejb-example.jar, which was embedded in stateless-ejb-and-servlet.ear.
java:global/stateless-ejb-and-servlet/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:app/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:module/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:jboss/exported/stateless-ejb-and-servlet/stateless-ejb-example/SimpleStatelessBean!io.novaordis.playground.jee.ejb.stateless.SimpleStateless java:global/stateless-ejb-and-servlet/stateless-ejb-example/SimpleStatelessBean java:app/stateless-ejb-example/SimpleStatelessBean java:module/SimpleStatelessBean
The last three names are short forms, and they are registered if the EJB component only exposed one interface.
Remote EJB Applications
An EJB remote application can be one of the following:
- a standalone Java application
- another component that runs within a application server instance, and makes invocations into remote EJBs. In this case, each deployed application will have its own EJB client context, and the application's components will share the EJB client context.
Invoking into EJBs
The invocation mechanics is different depending on where the caller and the target EJBs reside relatively to each other. There are several cases:
Collocated Client Component and Target EJB
- The caller is a component collocated with the target EJB in the same deployment unit (JAR, WAR or EAR).
- The caller is a component deployed as part of a different deployment unit than the target EJB, but both components are deployed on the same Application Server (JBoss) instance. This is an example that shows how the client component looks up the EJB via JNDI and also how can it get it via @EJB injection:
Remote EJB
- The caller is a component deployed on an Application Server (JBoss) instance, while the target EJB is deployed on a different Application Server (JBoss) instance. This case qualifies as a remote EJB application. Remote invocations are necessary. This is an example that shows how the client component looks up the EJB deployed on a different JBoss instance and invokes remotely:
- The caller is a standalone application. Remote invocations are necessary.
Clustered Remote EJB
- The caller invokes into a cluster of remote EJBs.
Invoking into EJBs TODO
Desktop/Learning/NOKB TODO/Invoking into EJBs/Invoking into EJBs TODO.docx
EJB Client API
JBoss EAP 6 introduced the EJB client API for managing remote EJB invocations. The EJB client API uses the EJB Client Context.
EJB Client Context
The EJB client context is a JBoss-specific mechanism that allows a client application to look up and invoke into a remote EJB.
The EJB client context may be associated with, and used by more than one thread concurrently.
There can potentially be more than one EJB client contexts into a JVM. For deployed applications, each application will have its own EJB client context. Whenever that application invokes another EJB, the corresponding EJB client context is used to find the correct EJB receiver, which then handles the invocation.
For standalone applications, a remote standalone client typically has just one EJB client context backed by any number of EJB receivers
An EJB client context may contain any number of EJB receivers.
The EJB client context is configured with jboss-ejb-client.properties for standalone applications and with jboss-ejb-client.xml for applications deployed within the EAP.
EJB Receiver
An EJB receiver is a component that knows how to communicate with a server that is capable of handling EJB invocations.
Exceptions
Application Exception
An application exception is an exception defined by the EJB as part of the business logic of the application, to inform the client of abnormal application-level conditions. Clients can typically recover from application exceptions.
In order to designate a checked exception as an application exception, the developer may declared it in the throws clause of the bean's business interface or web service endpoint.
An unchecked exception may be designated an application exception by annotating it with @ApplicationException or declaring it as application-exception in the descriptor.
Application exceptions should be handled differently than a system exception by the container.
Application Exceptions and Transactions
An application exception does not automatically result in marking the transaction for rollback unless the @ApplicationException annotation is applied to the exception class and is specified with the rollback element value true (or the corresponding deployment descriptor element).
System Exception
A system exception is an exception that is a java.mi.RemoteException or one of its subclasses, or a RuntimeException that is not an application exception.
Concurrency
If a concurrent access timeout occurs on a stateful session or a singleton bean, the container will throw javax.ejb.ConcurrentAccessTimeoutException.
If concurrent access is not allowed, and the container detects concurrent access attempt to a stateful session or singleton bean, it will throw javax.ejb.ConcurrentAccessException.