| = Basics - Getting Things |
| :index-group: Unrevised |
| :jbake-date: 2018-12-05 |
| :jbake-type: page |
| :jbake-status: published |
| |
| = Getting Stuff from the Container |
| |
| Generally speaking the only way to get a |
| link:container-managed-resource.html[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. |
| |
| [arabic] |
| . java:comp/env is the spec defined namespace for lookup of any |
| link:container-managed-resource.html[Container-Managed Resource] |
| . java:comp/env is _empty_ by default |
| . java:comp/env is _read-only_ at runtime |
| . java:comp/env is populated by link:declaring-references.html[Declaring |
| References] to [Container-Managed Resource] via xml or annotation |
| . only link:container-managed-component.html[Container-Managed |
| Component] s, _not_ their libraries, can [Declare References|Declaring |
| References] via xml or annotation |
| . only link:container-managed-component.html[Container-Managed |
| Component] s, _not_ their libraries, can get dependency injection of |
| [Container-Managed Resource] s |
| . only link:container-managed-component.html[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 link:declaring-references.html[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 |
| link:declaring-references.html[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 |
| link:container-managed-component.html[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 |
| "java:global[/<app-name>]/<module-name>/<bean-name>[!<fully-qualified-interface-name>]" |
| 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 link:jndi-names.html[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. |
