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Tuesday, April 03, 2007

Using JSON from Java

JSON (JavaScript Object Notation) is a lightweight data-interchange format based on a subset of the JavaScript Programming Language. JSON object structure is built on two structures:
  • A collection of name/value pairs: An associative array in Javascript which is analogous to the Java Map.
  • An ordered list of values: Analogous to a Java Array.
The rest of the datatypes (string, number, true/false etc.) have straight forward mappings to the Java datatypes.

Producing JSON from Java


Since JSON objects can be declared literally in JavaScript, it is possible to create these literals from Java classes. There are a couple of ways to achieve this
  • Using JSONObject: The source code for JSONObject and the supporting classes is available at http://www.json.org/java/index.html. Using this you can create JSONObject from a Java class as shown below
      JSONObject json = new JSONObject();
    json.put("name1",value1);
    json.put("name2",value2);
    The JSONObject class will appear as a Javascript Associate array which looks like this
     {"name1": "value1",
    "name2": "value2"}
  • Using JSON-lib: This is a Java library (available on sourceforge) that provides utility methods to convert different Java objects into JSONObject. This method is used in the example shown below.
The following Java class and JSP demonstrate how to use JSON with Java
  1. The Java class:
    package beans;

    import java.util.Map;
    import java.util.TreeMap;

    import net.sf.json.JSONObject;

    public class MapHolder {
    private Map map;

    public Map getMap() {
    return map;
    }

    public void setMap(Map map) {
    this.map = map;
    }

    public JSONObject getJsonObject() {
    JSONObject obj = JSONObject.fromMap(map);
    return obj;
    }

    public MapHolder() {
    map = new TreeMap();
    map.put("name1", "value1");
    map.put("name2", "value2");

    }
    }
    MapHolder.java
    • The getJsonObject() method uses JSON-lib to convert the Map to a JSONObject.
  2. The JSP:
    <%@page import="beans.MapHolder,net.sf.json.JSONObject"%>
    <html>
    <head>
    <title>Title</title>
    <script>
    function getInfo(o)
    {
    var obj=o;
    document.getElementById('test').innerHTML=obj.name1;
    }
    </script>
    </head>
    <body>
    <%
    JSONObject obj = (new MapHolder()).getJsonObject();
    %>
    <input type="button" value="submit" onclick='getInfo(<%=obj%>)' />

    <div id="test"></div>
    </body>
    </html>
    index.jsp
    • Note the "obj.name1" in the script to access the element from the associative array.

  3. The JAR files: In order to use JSON-lib, you will need to include the following jars in your classpath

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