For situations where performance is most critical, Jackson comes with Streaming API which is a high performance, sequential access api, with very low memory footprints and processing overhead.

Due to it’s incremental processing/sequential access nature, all content must be read/write in order as it arrives, so it’s bit hard to use. This API is mainly used by middle-ware and frameworks internally. On application level, to remain simple, usually data-binding or tree-model api’s are preferred(which in fact uses the Streaming API internally for JSON reading/writing).

On reading front, API comes with Parsers which works like a pointer moving through individual tokens in JSON string and provide access to data of those tokens.
key, value, brackets [ { } ] are all tokens.

On writing front, Generators are available which writes JSON content. They provide methods to write individual tokens.

Let’s see how it is used exactly:

Step 1: Include JACKSON dependency in pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.websystique.xml</groupId>
  <artifactId>JsonJacksonStreamingExample</artifactId>
  <version>1.0.0</version>
  <packaging>jar</packaging>

  <name>JsonJacksonStreamingExample</name>

  <dependencies>
 <dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.5.3</version>
 </dependency>
  </dependencies>
 </project>

Step 2: Write JSON using JsonGenerator

package com.websystique.json.jackson;

import java.io.File;
import java.io.IOException;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.databind.JsonMappingException;

public class JsonJacksonStreamingExample {

 public static void main(String args[]) throws JsonParseException, JsonMappingException, IOException{

  JsonFactory factory = new JsonFactory();

  /**
   * Write values in JSON format to a file
   */  JsonGenerator generator = factory.createGenerator(new File("result.json"), JsonEncoding.UTF8);

  generator.writeStartObject();     // write opening brace "{"

  generator.writeStringField("car", "Bentley"); //write key:value pair   "car":"bentley"
  generator.writeNumberField("model", 2010); 
  generator.writeNumberField("price", 98.700); 

  generator.writeFieldName("colors");    
   
  generator.writeStartArray();     //write array opening brace "["
  
  generator.writeString("gray");     //write individual array item
  generator.writeString("white");
  generator.writeString("black");
 
  generator.writeEndArray();      //write array closing brace "]"
  generator.writeEndObject();      // write closing brace "}"
  generator.close();
  
 }
}

Below is the output(result.json content)

{"car":"Bentley","model":2010,"price":98.7,"colors":["gray","white","black"]}

Step 3: Read JSON using JsonParser

package com.websystique.json.jackson;

import java.io.File;
import java.io.IOException;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParseException;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;
import com.fasterxml.jackson.databind.JsonMappingException;

public class JsonJacksonStreamingExample {

 public static void main(String args[]) throws JsonParseException, JsonMappingException, IOException{
  JsonFactory factory = new JsonFactory();
  /**
   * Read values in json format
   */  JsonParser parser = factory.createParser(new File("result.json"));
  parser.nextToken();          //start reading the file
  while (parser.nextToken() != JsonToken.END_OBJECT) { //loop until "}"
      
      String fieldName = parser.getCurrentName();
      if (fieldName.equals("car")) {
       parser.nextToken();
       System.out.println("car : " + parser.getText());
      } else if (fieldName.equals("model")) {
       parser.nextToken();
       System.out.println("model : " + parser.getIntValue());
      } else if (fieldName.equals("price")) {
       parser.nextToken();
       System.out.println("price : " + parser.getFloatValue());
      } else if (fieldName.equals("colors")) {
       parser.nextToken();
       while (parser.nextToken() != JsonToken.END_ARRAY){ //loop until "]"
        System.out.println(parser.getText());
       }
      } else { // unexpected token, generate error
       throw new IOException("Unrecognized field '"+fieldName+"'");
      }
  }
     parser.close(); 
 }
}

Below is the output

car : Bentley
model : 2010
price : 98.7
gray
white
black

That’s it. In the next post we will learn about frequently used Jackson Json annotations.

References

• Jackson Streaming API
• Jackson Reference
• Json Official Reference