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Note that @DependsOn is only required (and should only be used) if a Singleton uses another Singleton in its @PostConstruct method or @PreDestroy method. Simply having a reference to another Singleton and using it in other business methods does not require an @DependsOn declaration. The @DependsOn allows the Container to calculate the correct startup order and shutdown order so that it can guarantee the Singletons you need are available in your @PostConstruct or @PreDestroy methods. All Singletons will automatically be available to your business methods regardless if @DependsOn is used. Because of the greater chance of creating circular dependencies, it is better not to use the @DependsOn annotation "just in case" and should only be used when truly needed.

XML and Annotation Overriding

Singletons can be declared in the ejb-jar.xml as follows:

    <session>
      <ejb-name>MySingletonBean</ejb-name>
      <ejb-class>org.superbiz.MySingletonBean</ejb-class>
      <session-type>Singleton</session-type>
      <load-on-startup>true</load-on-startup>
      <depends-on>
        <ejb-name>SingletonFoo</ejb-name>
        <ejb-name>SingletonBar</ejb-name>
      </depends-on>
    </session>

Example Code

{snippet:url=openejb3/examples/simple-singleton/src/main/java/org/superbiz/registry/ComponentRegistryBean.java|lang=java}

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