2.2. Execution

The Page class provide a number of empty handler methods which subclasses can override to provide functionality:

The ClickServlet relies on instantiating Pages using a public no arguments constructor, so when you create Page subclasses you must ensure you don't add an incompatible constructor. The GET request execution sequence for Pages is summarized below in the Figure 2.

GET Request Sequence Diagram

Figure 2.2. GET Request Sequence Diagram

Stepping through this GET request sequence, a new Page instance is created and the attributes for the Page are set (format, headers, path). Next, request parameter values are bound to any matching public Page variables.

Then the onSecurityCheck() handler is executed. This method can be used to ensure the user is authorized to access the page, and if necessary abort any further processing.

The next method invoked is onInit(), this is where you place any post constructor initialization code. onInit() is the ideal place to create controls such as Forms, Fields and Tables. As illustrated by the diagram, after a Page's onInit() is called, each Control, available at that stage, will have their onInit() method called.

The next step is the processing of the Page's controls. The ClickServlet gets the list of Controls from the page and then iterates through the list calling onProcess(). If any of the Control's onProcess() methods return false, processing of subsequent controls and the Page's onGet() method is aborted.

If everything is executing normally the Page's onGet() method is now called.

The next step is rendering the page template to generate the displayed HTML. The ClickServlet gets the model (Map) from the Page then adds the following objects to the model:

It then merges the template with the page model and writes out results to the HttpServletResponse. When the model is being merged with the template, any Controls in the model may be rendered using their toString() method.

The final step in this sequence is invoking each control's onDestroy() method and lastly invoke the Page's onDestroy() method. This method can be used to clean up resource associated with the Control or Page before it is garbage collected. The onDestroy() method is guaranteed to be called even if an exception occurs in the previous steps.

The execution sequence for POST requests is almost identical, except the onPost() method is invoked instead on onGet(). See the POST Request Sequence Diagram.

Another view on the execution flow of Pages is illustrated in the Activity diagram below.

Page Execution Activity Diagram

Figure 2.3. Page Execution Activity Diagram