Reviews : Trader Pattern for Invoking Services :

The best and most popular technique of specifying such configuration information is to define it in a XML file. The Trader object can then parse this XML file and load the appropriate configuration information to look for the appropriate component instance form the class libraries provided to its access. As the Trader object demands a particular set of information that should always be present in the configuration XML file, it is a good strategy to define an XML Schema for the same.

  We start with the XML Schema definition for the Trader configuration. Listing 1 presents the XML schema that we came up with.

View Listing 1

  Note that the Schema defines the following items.

1. The root element is <component-definition> .
2. The <component> element has a required attribute "name".
3. The <component> element has required child elements <inteface> , <instance> and <service-type>.

    A sample XML file following the trader.xsd Schema definition may look like the following in listing 2.

View Listing 2

   This configuration XML file defines a component named "Account". The component implementation follows an interface definition in sam.services.IAccount. Also, the concrete implementation of that interface is sam.ejb.AccountHome and the Trader object should create an instance of the AccountHome object and return it to the client. The <service-type> defined has a value "ejb" which indicates the Trader object that the component requested is an EJB component.

Hold on a minute. How does the Trader know where to look for this EJB component! Yes, there is a clue. We need to define it somewhere. We decide to define the service configuration in a separate configuration file. Listing 3 shows a sample service configuration file.

View Listing 3

  This XML file provides the required information for the EJB lookup for the specified object. Notice that, the component in question is identified with the unique name attached to them. As you can see, this configuration file can be used to define other configuration parameters that a particular service may need to know about. For example, for a normal Java implementation of the Account component, you may decide to supply the database connection information. Of course, the Trader object implementation grows with such requirements in place. However, it might be useful to employ some helper classes to achieve the configuration settings on the component instance.

  In the next section, we take a closer look at parsing and caching the information provided in the configuration files in order to develop a Trader object.

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