Creating the servlet

A servlet performs the main tasks for getting and managing data for BIRT Studio users, and it is the key piece of any custom query builder. The example servlet, SampleServlet.java, extends the HTTPServlet class, and performs the following tasks:

Manages design sessions for concurrent BIRT Studio users

Uses ODA API to define the basic methods for constructing a query, result set, and column objects

Processes the information sent by ClassicModelQueryBuilder.jsp, and creates a query

Returns dynamic content that appears in Available Data, as shown in Figure 14-16

The servlet contains declarations of the ODA data driver’s data source and data set extensions:

private final static String Datasource_Extension_Id = "org.eclipse.birt.report.data.oda.jdbc";

private final static String Dataset_Extension_Id = "org.eclipse.birt.report.data.oda.jdbc.JdbcSelectDataSet";

These extension IDs are used to construct the definitions of the data source connection and data set query in the report design.

The servlet supports the GET method only. Its doGet method retrieves request parameters, creates a unique ODA design session, and stores a session ID in the ODA session, so that other requests in the same session can access the same ODA session object. In any given session, the servlet can receive multiple requests with different parameter values. Table 14-2 describes the supported parameters.

Table 14-2

Session request parameters

Parameter	Description
inedit	Indicates whether the user has started editing the values on the query builder page. Values are null or true.
state	Shows the editing status. Values are null, ok, or cancel.
selection	Contains information about the selected data fields.
sessionId	Contains the session ID.

The servlet executes a different action, depending on the parameter values. Table 14-3 describes the actions taken with the different inedit and state values.

Table 14-3

Actions corresponding to inedit and state values

inedit value	state value	Action
null	any	Generates a new session ID. Stores the ID and the callback URL in the session map. Changes the inedit parameter to true and sends a response to the calling page.
not null	null	Redirects the response to ClassicModelQueryBuilder.jsp.
not null	ok	The user has finished selecting data fields. The servlet deletes the session ID, and parses the value in the selection parameter to build the query. The response is redirected to a BIRT Studio page.
not null	cancel	The user cancelled out of the query builder page. The servlet deletes the session ID.

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