How to parse JSON in Spring Boot using ObjectMapper

In this tutorial we will learn how to parse JSON using the ObjectMapper API in a Spring Boot application.

The Jackson com.fasterxml.jackson.databind.ObjectMapper class is a simple way to parse and create JSON. The Jackson ObjectMapper can parse JSON from a string, stream or file, and create a Java object or object graph representing the parsed JSON.

Let’s see quick example:

package com.example.demojson;
import java.io.File;
import java.util.List;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import com.fasterxml.jackson.core.JsonParser.Feature;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;
@SpringBootApplication
public class DemoApplication {
  public static void main(String[] args) {
    SpringApplication.run(DemoApplication.class, args);
  }
  @Bean CommandLineRunner runner() {
    return args -> {
      String json = " {n" + "    "
      name ": "
      Edmund lronside ",n" + "    "
      city ": "
      United Kingdom ",n" + "    "
      house ": "
      House of Wessex ",n" + "    "
      years ": "
      1016 "n" + "  }";
      ObjectMapper mapper = new ObjectMapper();King c = mapper.readValue(json, King.class);
      System.out.println(c);
    };
  }
}

In the above example, we are parsing a simple JSON String and converting it into a Java class:

package com.example.demojson;
public class King {
  String name;
  String city;
  String house;
  String years;
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getCity() {
    return city;
  }
  public void setCity(String city) {
    this.city = city;
  }
  public String getHouse() {
    return house;
  }
  public void setHouse(String house) {
    this.house = house;
  }
  public String getYears() {
    return years;
  }
  public void setYears(String years) {
    this.years = years;
  }
  public King() {
    super();
  }
  public King(String name, String city, String house, String years) {
    super();
    this.name = name;
    this.city = city;
    this.house = house;
    this.years = years;
  }
  @Override public String toString() {
    return "King [name=" + name + ", city=" + city + ", house=" + house + ", years=" + years + "]";
  }
}

In the following example, we are parsing a JSON external file and converting it into a List of Java objects:

TypeReference < List < King >> typeReference = new TypeReference < List < King >> () {};
List < King > list = mapper.readValue(new File("/tmp/sample.json"), typeReference);
for (King k: list) System.out.println(k);

Notice the TypeReference parameter passed to readValue(). This parameter tells Jackson to read a List of King objects.

You can also use the ObjectMapper API to produce JSON Strings into a File, as in the following example:

ObjectMapper objectMapper = new ObjectMapper();
King king = new King("Edward the Martyr", "United Kingdom", "House of Wessex", "975-978");
objectMapper.writeValue(new File("target/king.json"), king);

Check fields which are null

It is possible to configure the Jackson ObjectMapper to fail if a JSON string contains a field with its value set to null:

objectMapper.configure(DeserializationFeature.FAIL_ON_NULL_FOR_PRIMITIVES, true); 

In the above example, you will get an exception when trying to parse a null JSON field into a primitive Java field.