How to convert JSON array to Java POJO using Jackson

To convert a JSON array into a list of custom Java objects using Jackson, follow these steps:

1. Add Jackson Dependency: Ensure you have Jackson library included in your project dependencies.

Maven:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.13.3</version>
</dependency>  

Gradle:

implementation 'com.fasterxml.jackson.core:jackson-databind:2.13.3'

Example:

Address.java

public class Address {
    private String apartment;
    private String street;
    private String pinCode;

    // Getters and setters
    public String getApartment() {
        return apartment;
    }

    public void setApartment(String apartment) {
        this.apartment = apartment;
    }

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public String getPinCode() {
        return pinCode;
    }

    public void setPinCode(String pinCode) {
        this.pinCode = pinCode;
    }
}

Person.java

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import java.util.List;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Person {
    @JsonProperty("name")
    @JsonInclude(JsonInclude.Include.NON_NULL)
    private String name;

    @JsonProperty("age")
    @JsonInclude(JsonInclude.Include.NON_DEFAULT)
    private int age;

    @JsonProperty("address")
    private List<Address> address;

    // Getters and setters
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public List<Address> getAddress() {
        return address;
    }

    public void setAddress(List<Address> address) {
        this.address = address;
    }
}

Main Class to Convert JSON to List of Person Objects:

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.List;

public class Main {
    public static void main(String[] args) {
        String jsonArray = "[{\"name\":\"John\", \"age\":30, \"address\":[{\"apartment\":\"A1\",\"street\":\"Main St\",\"pinCode\":\"12345\"}], \"unknownField\":\"value\"}, {\"name\":\"Alice\", \"age\":25, \"address\":[{\"apartment\":\"B2\",\"street\":\"Second St\",\"pinCode\":\"67890\"}]}]";

        try {
            ObjectMapper objectMapper = new ObjectMapper();
            List<Person> personList = objectMapper.readValue(jsonArray, new TypeReference<List<Person>>() {});

            for (Person person : personList) {
                System.out.println("Name: " + person.getName() + ", Age: " + person.getAge());
                for (Address address : person.getAddress()) {
                    System.out.println("  Apartment: " + address.getApartment() + ", Street: " + address.getStreet() + ", PinCode: " + address.getPinCode());
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Common Errors and Fixes

1. UnrecognizedPropertyException:

• Error: Occurs when JSON contains fields not present in the POJO.
• Fix: Ignore unknown properties using @JsonIgnoreProperties(ignoreUnknown = true).

2. JsonMappingException:

• Error: Occurs when there’s a mismatch between JSON structure and Java POJO.
• Fix: Ensure JSON keys match POJO field names or use @JsonProperty annotation to map JSON keys to POJO fields.

3. InvalidFormatException:

• Error: Occurs when JSON value types do not match POJO field types.
• Fix: Ensure JSON values match the expected types in POJO.

4. Missing required properties:

• Error: Occurs when JSON lacks fields that are required in the POJO.
• Fix: Ensure JSON includes all necessary fields or make fields optional in POJO using @JsonInclude.

Example with Fixes Applied

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.List;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Person {
    @JsonProperty("name")
    @JsonInclude(JsonInclude.Include.NON_NULL)
    private String name;

    @JsonProperty("age")
    @JsonInclude(JsonInclude.Include.NON_DEFAULT)
    private int age;

    // Getters and setters
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

public class Main {
    public static void main(String[] args) {
        String jsonArray = "[{\"name\":\"John\", \"age\":30, \"unknownField\":\"value\"}, {\"name\":\"Alice\", \"age\":25}]";

        try {
            ObjectMapper objectMapper = new ObjectMapper();
            List<Person> personList = objectMapper.readValue(jsonArray, new TypeReference<List<Person>>() {});

            for (Person person : personList) {
                System.out.println("Name: " + person.getName() + ", Age: " + person.getAge());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

In this example:

• The @JsonIgnoreProperties(ignoreUnknown = true) annotation ensures that any unknown properties in the JSON are ignored, preventing UnrecognizedPropertyException.
• The TypeReference<List<Person>>(){} is used to inform Jackson about the type information for deserialization, enabling it to convert the JSON array into a list of Person objects.