Defining Capabilities and the Flow of Events

  • This section introduces AdvancedExampleServlet2, an example data source implementation that defines capabilities and the flow of events, and provides instructions on how to run and test it.

  • AdvancedExampleServlet2 defines the flow of events by overriding HttpServlet.doGet() and using helper functions from DataSourceHelper.

  • Error handling in AdvancedExampleServlet2 involves catching exceptions and using DataSourceHelper to format and set the servlet error response.

  • The generateMyDataTable method in AdvancedExampleServlet2 demonstrates using a URL parameter (tableId) to determine which data table is returned.

  • Running and testing AdvancedExampleServlet2 involves updating a web application on Apache Tomcat and using a visualization to view the data.

This section introduces AdvancedExampleServlet2. AdvancedExampleServlet2 is an example data source implementation that defines capabilities and the flow of events. This section also provides step-by-step instructions on how to run and test AdvancedExampleServlet2.

Note: You must complete the Getting Started section before you begin this section.

Introducing AdvancedExampleServlet2

The AdvancedExampleServlet2 class is located in the examples package. This class provides an example implementation that defines capabilities and flow of events.

The most important parts of AdvancedExampleServlet2 are described in the following sections:

Defining the flow of events

AdvancedExampleServlet2 defines the flow of events by overriding the HttpServlet.doGet() method and calling various helper functions provided by DataSourceHelper.

The following snippet overrides doGet(). The HttpServletRequest parameter encapsulates the request made by a visualization to the servlet. The HttpServletResponse parameter encapsulates the response from the servlet to the querying visualization. This snippet also sets dsRequest to null. dsRequest is used at various points throughout the rest of the code.

  @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException {
    DataSourceRequest dsRequest = null;

The following snippet extracts the request parameters from HttpServletRequest to create the context in which the request runs. 

    try {
      // Extract the request parameters.
      dsRequest = new DataSourceRequest(req);

The following snippet takes the query from the dsRequest object and splits it into two separate queries. One query is called the data source query, the other the completion query. Because the data source's declared capability is SELECT, the data source query consists of a SELECT operation if the dsRequest object includes a SELECT operation. The completion query consists of all other operations required by the request, which might also include a SELECT operation. For example, if the query requested is SELECT a ORDER BY b, then the data source query will be SELECT a, b and the completion query will be the same as the original query SELECT a ORDER BY b.

      // Split the query.
      QueryPair query = DataSourceHelper.splitQuery(dsRequest.getQuery(), Capabilities.SELECT);

The following snippet takes the data source query created by the previous snippet and the HttpServletRequest, and creates a data table. See the Using capabilities section for more details. 

      // Generate the data table.
      DataTable data = generateMyDataTable(query.getDataSourceQuery(), req);

The following code takes the completion query produced when the query was split, the datatable produced by the previous snippet, and the user locale from the querying visualization. The code then creates a new data table.

      // Apply the completion query to the data table.
      DataTable newData = DataSourceHelper.applyQuery(query.getCompletionQuery(), data,
          dsRequest.getUserLocale());

The following code takes the data table produced by the previous snippet, and the request parameters from HttpServletRequest. The code then sets the servlet response. The servlet container returns this response to the querying visualization.

      DataSourceHelper.setServletResponse(newData, dsRequest, resp);

Handling errors

The following snippet catches an exception, gets the appropriate message, formats the response, and sets the servlet response. If dsRequest is null, the DataSourceRequest is not available, possibly due to failure of the constructor. In this case, the HttpRequest is used instead of the DataSourceRequest.

    catch (RuntimeException rte) {
      log.error("A runtime exception has occured", rte);
      ResponseStatus status = new ResponseStatus(StatusType.ERROR, ReasonType.INTERNAL_ERROR,
          rte.getMessage());
      if (dsRequest == null) {
        dsRequest = DataSourceRequest.getDefaultDataSourceRequest(req);
      }
      DataSourceHelper.setServletErrorResponse(status, dsRequest, resp);
    } catch (DataSourceException e) {
      if (dsRequest != null) {
        DataSourceHelper.setServletErrorResponse(e, dsRequest, resp);
      } else {
        DataSourceHelper.setServletErrorResponse(e, req, resp);
      }
    }

Using URL parameters

The following snippet takes the data source query that was created when the query was split and the HttpServletRequest. The HttpServletRequest can optionally include a tableId parameter specified as a URL. This tableId parameter determines which data table is returned as follows:

  • If the tableId parameter is omitted, or is anything other than planets, the data source returns the animals data table.
  • If the tableId parameter is specified as planets, the data source returns the planets data table.

When you write your own code to return a data table, you decide which parameters to take. 

  private DataTable generateMyDataTable(Query query, HttpServletRequest req)
      throws TypeMismatchException {
    String tableID = req.getParameter("tableId");
    if ((tableID != null) && (tableID.equalsIgnoreCase("planets"))) {
      return generatePlanetsTable(query);
    }
    return generateAnimalsTable(query);
  }

Using capabilities

The following snippet takes the query and generates the animals data table.

private DataTable generateAnimalsTable(Query query) throws TypeMismatchException {
  DataTable data = new DataTable();
  List requiredColumns = getRequiredColumns(query,
      ANIMAL_TABLE_COLUMNS);
  data.addColumns(requiredColumns);

  // Populate the data table
  for (String key : animalLinksByName.keySet()) {
    TableRow row = new TableRow();
    for (ColumnDescription selectionColumn : requiredColumns) {
      String columnName = selectionColumn.getId();
      if (columnName.equals(ANIMAL_COLUMN)) {
        row.addCell(key);
      } else if (columnName.equals(ARTICLE_COLUMN)) {
        row.addCell(animalLinksByName.get(key));
      }
    }
    data.addRow(row);
  }
  return data;
}

The following snippet takes the query and generates the planets data table.

private DataTable generatePlanetsTable(Query query) throws TypeMismatchException {
  DataTable data = new DataTable();
  List requiredColumns = getRequiredColumns(
      query, planetTableColumns);
  data.addColumns(requiredColumns);

  // Populate data table
  for (Planet planet : Planet.values()) {
    TableRow row = new TableRow();
    for (ColumnDescription selectionColumn : requiredColumns) {
      String columnName = selectionColumn.getId();
      if (columnName.equals(PLANET_COLUMN)) {
        row.addCell(planet.name());
      } else if (columnName.equals(MASS_COLUMN)) {
        row.addCell(planet.getMass());
      } else if (columnName.equals(GRAVITY_COLUMN)) {
        row.addCell(planet.getSurfaceGravity());
      } else if (columnName.equals(MOONS_COLUMN)) {
        row.addCell(planet.getNumberOfMoons());
      }
    }
    data.addRow(row);
  }
  return data;
}

Running and Testing AdvancedExampleServlet2

This section provides instructions on how to run and test AdvancedExampleServlet2.

To run and test AdvancedExampleServlet2, update your web application, and set up a visualization that queries the data source, as described in the following sections:

Updating a Web Application on Apache Tomcat

Follow or adapt the instructions below to update your web application on Apache Tomcat. These instructions are specific to Apache Tomcat on a Windows system:

  1. The web.xml file you previously copied to the WEB-INF directory already contains the definition and mapping required for this example. The lines that define this are:

    <servlet>
  <servlet-name>AdvancedExampleServlet2</servlet-name>
  <description>
  AdvancedExampleServlet2
  </description>
  <servlet-class>AdvancedExampleServlet2</servlet-class>
</servlet>
  
<servlet-mapping>
  <servlet-name>AdvancedExampleServlet2</servlet-name>
  <url-pattern>/advanced</url-pattern>
</servlet-mapping> 
  2. Start Tomcat, or restart Tomcat if it is already running.
     
  3. Click the following link:http://localhost:8080/myWebApp/advanced
    The screen displays 6-7 lines of text, depending on your screen width. The text begins with google.visualization.Query.setResponse and ends with {v:'http://en.wikipedia.org/wiki/Tiger'}]}]}});
    This is the response that the example CSV data source sends to a visualization.

Using a Visualization to View the Data

The all_examples.html file in the <data_source_library_install>/examples/src/html directory can be used to view a visualization of the data.

The following snippet from all_examples specifies the advanced servlet, the planets table, a select query, and a bar chart visualization.

query = new google.visualization.Query('advanced?tableId=planets&tq=select planet,mass');
...
var chart = new google.visualization.BarChart(document.getElementById('advanced_div'));

For an explanation of the other visualizations included in all_examples.html, see the Using an External Data Store section.

For more information on how to specify a visualization and use the query language, see Using Charts and the Query Language Reference.

Follow, or adapt, the instructions below to view a visualization of the data provided by the advanced data source:

  1. If you haven't already done so, copy the all_examples.html file from the <data_source_library_install>/examples/src/html directory
    to the <tomcat_home>/webapps/myWebApp/ directory.
     
  2. Click the following link: http://localhost:8080/myWebApp/all_examples.html in a browser. You should see the following:

Next Steps

To further explore the examples provided with the library, see Examples Quick Reference. To learn more about implementing a complex data source, see Implementation Tips.