• Purpose:
  • Parse, generate, transform and query JSON documents
• It Supports:
  • Element-Based Parsing
  • Document-Based Parsing
• A Popular Alternative:
  • Jackson

• Approach:
  • Process each element in the source JSON document individually (in a "streaming" fashion)
  • Ignore the hierarchical nature of the JSON document
• Rationale:
  • Very convenient when the hierarchical structure is not important

• Important Classes in the "Streaming" API:
  • Json
  • JsonParser
• Events:
  • Are returned by the JsonParser object's next() method

• Events:
  • START_ARRAY, END_ARRAY
  • START_OBJECT, END_OBJECT
  • KEY_NAME - the name in a name/value pair
  • VALUE_STRING, VALUE_NUMBER, VALUE_TRUE, VALUE_FALSE, VALUE_NULL
• Getters:
  • Must be called when the parser is in the appropriate state
  • One for each element (i.e., getString(), getObject(), getArray(), getInt(), getBigDecimal()

An Example

javaexamples/jsonp/ElementBasedExample.java

        import java.io.*;
import javax.json.*;
import javax.json.stream.*;


/**
 * An example of element-based parsing using JSON-P.
 *
 * @author  Prof. David Bernstein, James Madison University
 */
public class ElementBasedExample
{
    /**
     * The entry point of the application.
     *
     * @param args  The command line arguments (which are ignored)
     */
    public static void main(String[] args) throws Exception
    {
        BufferedReader in = new BufferedReader(
            new FileReader(
                new File("../../jsonexamples/basics/person.json")));

        // Create the JsonParser
        JsonParser parser = Json.createParser(in);

        // Process events
        while (parser.hasNext())
        {
            JsonParser.Event evt = parser.next();
            if (evt == JsonParser.Event.KEY_NAME)
            {
                String name = parser.getString();
                if (name.equals("firstName"))
                {
                    // Move to the next state
                    parser.next();           

                    // Get the String for the value
                    String value = parser.getString();
                    System.out.println(value);
                }
            }
        }
    }
}
        

• Approach:
  • Constructs a tree of objects from the JSON document
• Providing Access to the Tree:
  • Use an element of the tree

• Important Classes in the "Object Model" API:
  • Json
  • JsonReader
  • JsonObjectBuilder
  • JsonArrayBuilder
• The Objects:
  • JsonArray
  • JsonNumber
  • JsonObject
  • JsonString
  • JsonValue

An Example

javaexamples/jsonp/DocumentBasedExample.java

        import java.io.*;
import javax.json.*;

/**
 * An example of document-based parsing using JSON-P.
 *
 * @author  Prof. David Bernstein, James Madison University
 */
public class DocumentBasedExample
{
    /**
     * The entry point of the application.
     *
     * @param args  The command line arguments (which are ignored)
     */
    public static void main(String[] args) throws Exception
    {
        BufferedReader in = new BufferedReader(
            new FileReader(
                new File("../../jsonexamples/basics/person.json")));

        // Create the JsonReader
        JsonReader reader = Json.createReader(in);

        // Use the JsonReader to read the JsonObject
        JsonObject person = reader.readObject();

        // Access a top-level attribute of the JsonObject
        System.out.println(person.getString("firstName"));

        // Move into the hierarchy
        JsonObject address = person.getJsonObject("address");        
        System.out.println(address.getString("city"));
    }
}