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Introduction

Geronimo Beans or GBeans are entities in the Geronimo Application Server that are available for use within the Geronimo system (through the Geronimo Kernel). GBeans allow loosely coupled interactions with other entities in the system and dependency injection between modules and subsystems. GBeans are interoperable with JMX tools but are implemented separately from JMX MBeans. GBeans are able to be accessed via JMX technology through adapter classes in the geronimo-system module.

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A GBean is a Java object that has a method called getGBeanInfo() that returns an instance of the GBeanInfo class. GBeanInfo has three main purposes. One is to define the methods that should be exposed to the other subsystems, the second is to define attributes and references so that they can injected at runtime and the third is to define information about the GBean so that it can be located later. Each of these will be discussed in detail below. When defining the instance of the GBeanInfo object for the GBean, GBeanInfo shouldn't be constructed directly. Instead there is a GBeanInfoBuilder class that will simplify the process (see //1 below). Below is an example of creating a GBeanInfo from the Log4jService class and then returning it in the getGBeanInfo() method:

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There are a few important things to note from above. By convention, the GBeanInfo object is constructed in a static initializer. Also by convention, the GBeanInfo object is named GBEAN_INFO. Below is the portion of a deployment plan that is relevant to the Log4jService (found in $GERONIMO_HOME/configs/j2ee-system/src/plan/plan.xml).

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The deployment plan allows the injection of dependencies, attributes etc and maintains a loose coupling between the systems. The Geronimo Kernel will (using the deployment plan) inject these dependencies automatically when the GBean is loaded. Below the code samples are explained in more detail.

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Although implementing any interfaces for a GBean is not required it is often useful to be able to execute code when a GBean starts up, or shuts down. Implementing the GBeanLifecycle interface will allow the GBean to be notified on startup, shutdown and failure. The required methods to implement in the GBeanLifecycle interface are: doStart(), doStop() and doFail(). Below is some example code from the same Log4jService GBean:

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Understanding all of the details around the above code is not necessary, it's sufficient to just understand that a Timer, or what could be thought of as a sleeping thread, is created in doStart(). The person writing the code wanted to ensure that the thread would be stopped when the GBean was stopped. When the GBean is stopped, the doStop() method will be called and then the Timer can be stopped safely. Note also that in the event of a failure, the doStop() method is also called, safely stopping the Timer.