Question

With GSON, I can write this

JsonStreamParser parser = new JsonStreamParser(reader);
System.out.println(parser.next());

and if the stream consists of JSON objects, it will print the entire object out as a string.

Is there a simple way to do that with Jackson or do I need to use the while loop pattern that I see used in examples?

So if the first object was:
{
“id”: 1,
“name”: “Foo”,
“price”: 123,
“tags”: [ “Bar”, “Eek” ],
“stock”: {
“warehouse”: 300,
“retail”: 20
}
}

The first line printed would be :

{“id”: 1,”name”: “Foo”,”price”: 123,”tags”: [ “Bar”, “Eek” ],”stock”: {“warehouse”: 300,”retail”: 20}}

Answer 1

In streaming mode, every JSON “string” is consider as a single token, and each tokens will be processed incremental, that why we call it “incremental mode”. For example,

{
   "name":"mkyong"
}
Token 1 = “{“
Token 2 = “name”
Token 3 = “mkyong”
Token 4 = “}”

You can write the following code for parsing the above json text

import java.io.File;
import java.io.IOException;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.JsonToken;
import org.codehaus.jackson.map.JsonMappingException;

public class JacksonStreamExample {
   public static void main(String[] args) {

     try {

    JsonFactory jfactory = new JsonFactory();

    /*** read from file ***/
    JsonParser jParser = jfactory.createJsonParser(new File("c:\\user.json"));

    // loop until token equal to "}"
    while (jParser.nextToken() != JsonToken.END_OBJECT) {

        String fieldname = jParser.getCurrentName();
        if ("name".equals(fieldname)) {

          // current token is "name",
                  // move to next, which is "name"'s value
          jParser.nextToken();
          System.out.println(jParser.getText()); // display mkyong

        }

        if ("age".equals(fieldname)) {

          // current token is "age", 
                  // move to next, which is "name"'s value
          jParser.nextToken();
          System.out.println(jParser.getIntValue()); // display 29

        }

        if ("messages".equals(fieldname)) {

          jParser.nextToken(); // current token is "[", move next

          // messages is array, loop until token equal to "]"
          while (jParser.nextToken() != JsonToken.END_ARRAY) {

                     // display msg1, msg2, msg3
             System.out.println(jParser.getText()); 

          }

        }

      }
      jParser.close();

     } catch (JsonGenerationException e) {

      e.printStackTrace();

     } catch (JsonMappingException e) {

      e.printStackTrace();

     } catch (IOException e) {

      e.printStackTrace();

     }

  }

}

Output

mkyong 29

msg 1

msg 2

msg 3