The first step in creating a Composer service or component is usually to create the appropriate kind of Connection Resource. It's the Connection Resource, after all, that lets you connect to your back-end data source, whatever it happens to be. To obtain the connection, you generally have to supply some basic information regarding ports and/or driver names and/or user authentication. Usually, you know this information in advance and can just enter it once into the Connection Resource. But sometimes you may not know the information until runtime. The question arises: How can you create the Connection Resource (and store its information as a static file on disk) if you don't know the necessary parameter values until runtime?

The solution to this problem is to use expression-based parameters. In Composer, the "Create a New Connection Resource" wizard will let you specify connection parameters in two ways: as Constants or as Expressions. By default, the wizard's parameter-entry fields are static or constant-based, which means that the literal value you enter for any given parameter is utilized unchanged, every time the connection is used. An expression-based parameter, by contrast, gets its value programmatically, at runtime, via an ECMAScript expression which you supply in the wizard at design time. The value of an expression-driven parameter, therefore, can be different each time a connection is used, depending on the conditions prevailing at runtime.

 For instance, one very simple use of an expression driven parameter in a JDBC Connection would be to define the User ID and Password as PROJECT Variables: for example, PROJECT.XPath("USERCONFIG/MyDeployUser". (From Designer's main menubar, choose Tools, then Configuration, then select the Project Variables tab.) When you go to deploy the project, you can update your  PROJECT Variables in the Deployment Wizard to values appropriate for the final deployment environment.

As another example, suppose Queue1 in your MOM environment is scheduled for maintenance on the 15th day of every month, in which case Queue2 should be used instead. You could assign an expression to the Queue Name parameter of the connection: 

(new Date).getDate() == 15 ? "Queue1" : "Queue2" 

You can also use an ECMAScript expression to read information from a file on disk, call a Java object in the Application Server, etc. 

As you can see, the use of expressions to provide parameter information in Connection Resources affords great flexibility and power. Keep this in mind the next time you design a Composer service. 

1. Click the right mouse button in the parameter field you are interested in changing.

2. Select Expression from the menu and the Expression Builder icon will become visible.

3. Click on the button and then create an expression that evaluates to a valid parameter value at runtime.

On a Connection resource dialog you can RMB in a parameter field and enter an ECMAScript expression that is evaluated at runtime allowing the settings to be dynamically chosen based on certain conditions you specify.