After choosing the means for transmitting the data, the next step is to build the server side code. For Expresso, that means creating a new controller and a new state. A controller is similar to a Struts action; one controller may have many states. Please be aware that if you want to play with the new client code, you can use almost any dynamic web aware language, or even a static HTML page, to return the appropriate JSON, which will look similar to this: {"fivearm":"5.00","thirtyfixed":"6.50"}.
Adding a new state to handle a request for mortgage interest data in Expresso is not too complicated, though it could definitely be easier. I needed to:
- Add the values to the database. Expresso has a setup table for generic configuration information that I’ll use.
- Create a class that extends DBController, and have this class access the database and create the JSON.
- Update the configuration file to map a URL to a class. This version of Expresso is based on Struts.
- Add a JSP for output.
- Update the security tables so that this state can be accessed by everyone.
Adding the values to the database is just an insert statement. For the thirty year fixed interest rate, the SQL looks like this: insert into SETUP values ('com.cohomefinder.CoHomeFinderSchema', 'MTG_30_Year_Fixed_Rate','30 Year Fixed Mortgage Interest Rate','6.50');. One of the benefits of using the setup table is that Expresso ships with an Administrative Web Interface which lets non technical users change ‘setup values’ with a browser.
The next step is to create a class to respond to our request. That class is the MortgageCalculator controller. This is a fairly simple class, which uses the JSON.simple Java library to create a correctly formatted JSON object. Note that the rates are sent back as Strings even though JSON can handle converting to numbers. The reason for this is that I wanted some kind of formatting control; if the class sent back ‘6.5’ for the thirty year fixed interest rate, I might want it formatted as ‘6.50’. Formatting is simpler on the server, where I have the entire Java API to use, including NumberFormatter.
After that, I needed to create an entry in the configuration file to map some URL to this class.
<action path=”/MtgCalc” type=”com.cohomefinder.controller.MortgageCalculator” name=”default” scope=”request” validate=”false”>
<forward name=”getRatesAsJSON” path=”/expresso/components/registration/jsp/register/mtgcalc.jsp”/>
</action>
As you can see, this is very similar to configuring a struts-config.xml entry.
The JSP was extremely simple. It imports the expresso tag libraries in the standard manner, and then write the property that the MortgageCalculator class puts in the response (on the last line of the Java file).
<expresso:IfElementExists name=”JSON” type=”Output”><bean:write property=”JSON”/></expresso:IfElementExists>
The last thing to do is update the controller table to allow anyone to access this class. Again, it’s a simple SQL statement: insert into controllersecurity values ('com.cohomefinder.controller.MortgageCalculator','Everybody','*');. If I wanted to, I could restrict this information to certain classes of users, but in this case, everyone should have access to it.
After these steps, I can hit http://localhost:8080/MtgCalc.do?state=getRatesAsJSON and get back valid JSON containing values stored in the database.
The next step is to update the client to access the server data using HTTPRequest and to change the GUI accordingly.
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