Jackson Streaming API examples

By mkyong | Updated: April 26, 2024

Viewed: 4,331 (+97 pv/w)

Tags: jackson json streaming JsonGenerator JsonParser parse json

This article shows how to use Jackson’s Streaming API for both reading from and writing to JSON.

Jackson’s Streaming API

• JsonGenerator – Write JSON data
• JsonParser – Read and Parse JSON data

Table of contents:

• 1. Download Jackson
• 2. Write JSON using JsonGenerator
• 2. Write JSON Array using JsonGenerator
• 3. Read JSON using JsonParser
• 4. Read JSON Array using JsonParser
• 5. Download Source Code
• 6. References

P.S Tested with Jackson 2.17.0

1. Download Jackson

Declares jackson-databind at pom.xml.

  <dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.17.0</version>
  </dependency>

2. Write JSON using JsonGenerator

The following example demostrate the use of Jackson’s JsonGenerator object to write a simple JSON to a String and print them out.

package com.mkyong.json.jackson.streaming;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;

import java.io.IOException;
import java.io.StringWriter;

public class JsonGeneratorExample1 {

    public static void main(String[] args) {

        JsonFactory factory = new JsonFactory();
        StringWriter writer = new StringWriter();

        try (JsonGenerator generator = factory.createGenerator(writer)) {
            generator.writeStartObject();  // start root object
            generator.writeStringField("name", "mkyong");
            generator.writeNumberField("age", 42);
            generator.writeEndObject();    // end root object
            generator.close();

            String jsonOutput = writer.toString();
            System.out.println(jsonOutput);

        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }
}

Output

{"name":"mkyong","age":42}

2. Write JSON Array using JsonGenerator

The following example demostrate the use of Jackson’s JsonGenerator object to write JSON array to a file named user.json.

package com.mkyong.json.jackson.streaming;

import com.fasterxml.jackson.core.JsonEncoding;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.ObjectMapper;

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

public class JsonGeneratorExample2 {

    public static void main(String[] args) {

        ObjectMapper mapper = new ObjectMapper();

        try (JsonGenerator generator =
                     mapper.getFactory().createGenerator(
                        new File("user.json"), JsonEncoding.UTF8)) {

            // pretty print
            generator.useDefaultPrettyPrinter();

            // start array
            generator.writeStartArray();                    // [

            generator.writeStartObject();                   // {
            generator.writeStringField("name", "mkyong");   // "name" : "mkyong"
            generator.writeNumberField("age", 42);          // "age" : 42

            generator.writeFieldName("messages");           // "messages" :
            generator.writeStartArray();                    // [
            generator.writeString("msg 1");                 // "msg 1"
            generator.writeString("msg 2");                 // "msg 2"
            generator.writeString("msg 3");                 // "msg 3"
            generator.writeEndArray();                      // ]

            generator.writeEndObject();                     // }

            // next object

            generator.writeStartObject();                   // {
            generator.writeStringField("name", "ah pig");   // "name" : "ah pig"
            generator.writeNumberField("age", 30);          // "age" : 30

            generator.writeFieldName("messages");           // "messages" :
            generator.writeStartArray();                    // [
            generator.writeString("a");                     // "a"
            generator.writeString("b");                     // "b"
            generator.writeString("c");                     // "c"
            generator.writeEndArray();                      // ]

            generator.writeEndObject();                     // }

            generator.writeEndArray();                      // ]

        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }
}

Output

[ 
    {
        "name" : "mkyong",
        "age" : 42,
        "messages" : [ "msg 1", "msg 2", "msg 3" ]
    }, 
    {
        "name" : "ah pig",
        "age" : 30,
        "messages" : [ "a", "b", "c" ]
    } 
]

3. Read JSON using JsonParser

The following example demostrates the use of Jackson’s JsonParser object to Read a JSON and print it out.

{
    "name":"mkyong",
    "age":42
}

package com.mkyong.json.jackson.streaming;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;

import java.io.IOException;
import java.io.StringReader;

public class JsonParserExample1 {

    public static void main(String[] args) {

        String json = "{\"name\":\"mkyong\",\"age\":42}";

        JsonFactory factory = new JsonFactory();
        try (JsonParser parser = factory.createParser(new StringReader(json))) {
            while (parser.nextToken() != JsonToken.END_OBJECT) {

                // Deprecated alias of currentName().
                // Deprecated Since 2.17 use currentName instead.
                // String fieldName = parser.getCurrentName();

                String fieldName = parser.currentName();

                if ("name".equals(fieldName)) {
                    parser.nextToken();
                    System.out.println("Name: " + parser.getText());
                } else if ("age".equals(fieldName)) {
                    parser.nextToken();
                    System.out.println("Age: " + parser.getIntValue());
                }

            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }

}

Output

Name: mkyong
Age: 42

Token
In the Jackson streaming mode, it splits JSON string into a list of tokens, and each token will be processed incremental. For example,

{
   "name":"mkyong"
}

• Token 1 = {
• Token 2 = name
• Token 3 = mkyong
• Token 4 = }

4. Read JSON Array using JsonParser

The following example demostrate the use of Jackson’s JsonParser object to read JSON from a file named user.json and parse the JSON and print the token one by one.

[ 
    {
        "name" : "mkyong",
        "age" : 42,
        "messages" : [ "msg 1", "msg 2", "msg 3" ]
    }, 
    {
        "name" : "ah pig",
        "age" : 30,
        "messages" : [ "a", "b", "c" ]
    } 
]

package com.mkyong.json.jackson.streaming;

import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonToken;

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

public class JsonParserExample2 {

    public static void main(String[] args) {

        try (JsonParser jParser = new JsonFactory()
                .createParser(new File("user.json"));) {

            // JSON array?
            if (jParser.nextToken() == JsonToken.START_ARRAY) {

                while (jParser.nextToken() != JsonToken.END_ARRAY) {

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

                        String fieldName = jParser.currentName();
                        if ("name".equals(fieldName)) {
                            // current token is "name",
                            // move to next, which is "name"'s value
                            jParser.nextToken();
                            System.out.println(jParser.getText());
                        }

                        if ("age".equals(fieldName)) {
                            jParser.nextToken();
                            System.out.println(jParser.getIntValue());
                        }

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

                            //jParser.nextToken(); // current token is "[", move next
                            if (jParser.nextToken() == JsonToken.START_ARRAY) {
                                // messages is array, loop until token equal to "]"
                                while (jParser.nextToken() != JsonToken.END_ARRAY) {
                                    System.out.println(jParser.getText());
                                }
                            }

                        }

                    }

                }
            }

        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }

}

Output

mkyong
42
msg 1
msg 2
msg 3
ah pig
30
a
b
c

5. Download Source Code

6. References

• Jackson Streaming Api
• Jackson Github
• Parse JSON string with Jackson
• Parse JSON Array with Jackson
• Convert Java Objects to JSON with Jackson
• Jackson and Lombok examples