Other Examples of Messengers

Messengers often appear in APIs as the types of parameters and return values. For example, collections such as arrays, Lists, and Sets are often used as messengers to pass multiple pieces of data to or from methods or constructors. Occasionally, messenger classes appear in APIs whose instances are intended to be used solely to pass specific data in parameters or return values.

Another place that messengers appear in APIs is to transfer information from one node of a distributed system to another. In the J2EE world such messengers are called transfer objects, which are serializable objects that contain business data. If a client needs several pieces of data from a business object, invoking a separate remote method to get each piece of data is generally less efficient than invoking a single method that returns all needed data. Transfer objects enable clients to receive a collection of needed business data from a business object as the return value of a single remote method call.

A specific example of a messenger used to transmit information across a network is class Locales from the ServiceUI API. As shown in Diagram 7-5, class Locales describes the locales supported by a service UI associated with a Jini service. In the ServiceUI architecture, services describe UIs with data such as Locales objects. Clients inspect the description information and select a best-fit service UI based on the client's capabilities and its user's preferences. Messengers are used to describe service UIs, because although information about UIs is known to UI providers, the behavior of selecting a best-fit UI exists at the client. Thus, UI providers use messengers such as Locales to send UI description data across the network to clients that know how to use that information to select a best-fit UI.

Diagram 7-5. The Locales class