GWT JSON

JSON(JavaScript Object Notation) is a language independent format for data. It is similar to XML as XML uses tags JSON uses Object-Literal notation of JavaScript.

Implementation

Creating a source of JSON data of Stock Exchange example:

We have implemented stock price class and refresh watch list method as:

/**
   * Generate random stock prices.
   */
  private void refreshWatchList() {
    final double MAX_PRICE = 100.0; // $100.00
    final double MAX_PRICE_CHANGE = 0.02; // +/- 2%

    StockPrice[] prices = new StockPrice[stocks.size()];
    for (int i = 0; i < stocks.size(); i++) {
      double price = Random.nextDouble() * MAX_PRICE;
      double change = price * MAX_PRICE_CHANGE
          * (Random.nextDouble() * 2.0 - 1.0);

      prices[i] = new StockPrice(stocks.get(i), price, change);
    }

    updateTable(prices);
  }

Creating Servlet

In this we create a servlet file for stock quotes in JSON format. Following are the steps for creating servlet are:

1) Create a Servlet

In the Package Explorer, select the client package:com.google.gwt.sample.stockwatcher.client
In Eclipse, open the New Java Class wizard (File > New > Class).

2) Under Package, change the name from .client to .server

Under name, enter JsonStockData.
Eclipse will create a package for the server-side code and a stub for the JsonStockData class.

3) Replace the stub with following code:

package com.google.gwt.sample.stockexchange.server;

   import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

    import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

    public class JsonStockData extends HttpServlet {

      private static final double MAX_PRICE = 100.0; // $100.00
  private static final double MAX_PRICE_CHANGE = 0.02; // +/- 2%

      @Override
  protected void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {

        Random rnd = new Random();

        PrintWriter out = resp.getWriter();
    out.println('[');
    String[] stockSymbols = req.getParameter("q").split(" ");
    boolean firstSymbol = true;
    for (String stockSymbol : stockSymbols) {

          double price = rnd.nextDouble() * MAX_PRICE;
      double change = price * MAX_PRICE_CHANGE * (rnd.nextDouble() * 2f - 1f);

          if (firstSymbol) {
        firstSymbol = false;
      } else {
        out.println("  ,");
      }
      out.println("  {");
      out.print("    \"symbol\": \"");
      out.print(stockSymbol);
      out.println("\",");
      out.print("    \"price\": ");
      out.print(price);
      out.println(',');
      out.print("    \"change\": ");
      out.println(change);
      out.println("  }");
    }
    out.println(']');
    out.flush();
  }

    }

Including Server Side Code

GWT consists of in build servlet container Jetty which hosts the servlet that generates stock data in JSON format. Steps for including server side code in GWT are:

Edit the following code under StockWatcher/war/WEB-INF/web.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

    <web-app>

      <!-- Default page to serve -->
  <welcome-file-list>
    <welcome-file>StockExchange.html</welcome-file>
  </welcome-file-list>

      <!-- Servlets -->
 <servlet>
    <servlet-name>jsonStockData</servlet-name>
    <servlet-class>com.google.gwt.sample.stockexchange.server.JsonStockData</servlet-class>
  </servlet>

      <servlet-mapping>
    <servlet-name>jsonStockData</servlet-name>
    <url-pattern>/stockexchange/stockPrices</url-pattern>
  </servlet-mapping>

    </web-app>

Retrieving JSON Data

Debug StockExchange in development mode.

At this point, the stock data is still coming from the client-side code.

Test the stock quote server.

Ensure that the development mode code server is running and pass stock codes to the servlet URL http://localhost:8888/stockexchange/stockPrices?q=ABC+DEF