In addition to supporting traditional JavaTM applications, the JavaTM 2 Platform, Mobile Edition (J2METM) Personal Basis Profile (PBP) also supports the Xlet application model. Eric Giguere's tech tip "The Xlet Application Model" teaches you how to use the Xlet application model.

Derived from the Java TVTM APIs, Xlets are applications whose lifecycle is closely controlled and monitored by the system. They are very similar to the MIDlets defined by the Mobile Information Device Profile (MIDP). Xlets are also supported by the J2METM Personal Profile, which is a superset of the PBP.

An Xlet application's entry point is always a class that has a public no-argument constructor, and that implements the Xlet interface.

Along with the Xlet interface, the new javax.microedition.xlet package also defines an XletContext interface and two classes, XletStateChangeException and UnavailableContainerException.

The timing of an Xlet's creation is up to the application management software on the device.

The single argument passed to initXlet) is an object that implements the XletContext interface. The Xlet customarily stores a reference to this object because the methods it exposes allow the Xlet to communicate with the system, in order to effect state changes and to access its environment.

Once the Xlet is in the active state, the system can pause it at any time. When the system needs to pause the Xlet, it invokes the Xlet's pauseXlet() method. The Xlet is then no longer active, so it should release as many system resources as it can, to make them available to other applications. On return from pauseXlet() the Xlet enters the paused state. The Xlet is not suspended, however, and any background threads it started continue to run.

The system terminates the Xlet by invoking its destroyXlet() method. If the boolean argument to destroyXlet() is true, the termination is unconditional: the Xlet is destroyed without any choice in the matter. If the argument is false, the Xlet can refuse termination by throwing an XletStateChangeException.

Usually, the system handles Xlet state transitions. As with MIDlets, however, Xlets have ways to request state transitions explicitly. They can invoke methods of the XletContext interface.

An Xlet can pause or terminate itself at any point by calling the notifyPaused() or notifyDestroyed() method, respectively. The Xlet immediately transitions to the desired state. Note that the Xlet is the one initiating the state transition, and neither pauseXlet() nor destroyXlet() is called, so the Xlet should perform the appropriate cleanup before calling notifyPaused() or notifyDestroyed().

An Xlet in the paused state can request activation by calling the resumeRequest() method. Note that immediate activation is not guaranteed: the system controls activation and may deny the request. When it approves activation, it invokes the Xlet's startXlet() method.

The remaining methods of the XletContext interface are used to retrieve properties, typically during Xlet initialization.

Giguere provides a simple Xlet that displays a message, then waits for the user to press a key before terminating itself to be used as a template for your own Xlets. For code and additional technical details, please see:

http://wireless.java.sun.com/personal/ttips/xlet [...read more...]

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