Basics - Getting Things
Getting Stuff from the Container
Generally speaking the only way to get a Container-Managed Resource is via dependency injection or lookup from within a [Container-Managed Component] .
The unbreakable rules. Read these over and over again when things don’t work.
java:comp/env is the spec defined namespace for lookup of any Container-Managed Resource
java:comp/env is empty by default
java:comp/env is read-only at runtime
java:comp/env is populated by Declaring References to [Container-Managed Resource] via xml or annotation
only Container-Managed Component s, not their libraries, can [Declare References|Declaring References] via xml or annotation
only Container-Managed Component s, not their libraries, can get dependency injection of [Container-Managed Resource] s
only Container-Managed Component s, and their libraries, may lookup from java:comp/env
you must use the no-arg 'new InitialContext()' constructor to lookup something from java:comp/env
the annotations and xml for Declaring References are identical in functionality, both always configure lookup with optional dependency injection
Common mistakes, misunderstandings, and myths
"I tried it via annotation and it didn’t work, so I used xml and then it did work"
See rule 9. If one form worked and the other didn’t, it means you simply made a mistake in using one versus the other. Use what works for you, but understand both annotations or xml will work for either lookup or injection if used correctly.
"I need to use lookups, so I can’t use the annotation"
See rule 9. Annotations are not just for injection, that is just how they are typically used. Know that when you use an annotation for injection, it will always create an entry in java:comp/env. As well you can use the annotation at the class level and it will cause no dependency injection and only the entry creation in java:comp/env.
"I don’t want injection, so I can’t use the annotation"
See rule 9 and the above. You can use the annotation at the class level and it will cause no dependency injection and only the entry creation in java:comp/env.
"I tried to list java:comp/env but it’s empty?!"
See rule 2 and rule 4. There will be nothing in java:comp/env unless you Declare a Reference to it. It does not matter if is a DataSource configured at the server level, etc. Nothing is bound into java:comp/env unless you explicitly declare a reference to it. The Java EE 5 TCK (Technology Compatibility Kit) tests for this extensively and is a rule we cannot break. Java EE 6 does finally offer some new namesaces (java:global, java:app, and java:module) which will offer some great new options for more global-style lookups.
"I deployed the EJB but can’t look it up, it’s name is Foo"
See rule 2 and the above. Just creating an EJB doesn’t cause it to be
added to java:comp/env. If a
Container-Managed Component wants
to lookup the EJB they must [Declare a Reference|Declaring References]
to it via the
@EJB annotionation or <ejb-local-ref> or <ejb-ref> in xml.
In Java EE 6, however, EJBs will be automatically bound to
and can be looked up without declaring a reference first.
"Which InitialContextFactory do I use for java:comp/env?"
See rule 8. You are not allowed to use an InitialContextFactory for java:comp/env lookups. Setting an InitialContextFactory via 'java.naming.factory.initial' in either System properties, InitialContext properties, or a jndi.properties file is illegal and will cause java:comp/env lookups to fail.
"My Client can’t lookup the EJB from java:comp/env"
See rule 7. A plain, standalone, Java application cannot use java:comp/env. There is the official concept of a Java EE Application Client which can be packaged in an ear and deployed into the Container. In practice, most people find them restrictive, cumbersome, and hard to use and are therefore rarely employed in "real world" projects. Most people opt to use the non-standard, vendor-specific, approach to looking up EJBs from their plain java clients. In OpenEJB this can be done via either the RemoteInitialContextFactory (for remote clients) or the LocalInitialContextFactory (for local clients of an embedded container). The JNDI names can be configured as shown here .
"I declared the reference, but still can’t look it up"
See all of the above and reread the rules a few times. Always check the log output as well.