Jackson UnrecognizedPropertyException: field Ignorable

1. Overview

In this article, we will discuss the scenarios that would cause Jackson UnrecognizedPropertyException error and how we can fix it.

2. Jackson UnrecognizedPropertyException

Let’s understand the primary reason for getting this error.

Assume you are using the Jackson library in your project to deserialize the JSON to Java object. If the JSON contains fields that are not defined in the Java object, then the application would throw UnrecognizedPropertyException.

3. Ignore new or unknown fields Jackson

Consider that the JSON input contains fields that are not required in your project. Since we don’t need those fields in our code, we rarely define them in our Java class.

Assume we have a Student class with id property.

public class Student {
	public Student(long id) {
		this.id = id;
	}	
	private long id;
	public long getId() {
		return id;
	}
	public void setId(long id) {
		this.id = id;
	}
	@Override
	public String toString() {
		return "Student [id=" + id + "]";
	}
}

In the below code, the json input contains id, studentName and dept fields. Since we don’t require the fields studentName and dept in our code, we haven’t defined them in the Student class.

ObjectMapper mapper = new ObjectMapper();
String json = "{\"id\": 1001, \"studentName\": \"John\", \"dept\": \"IT\"}";
Student student = mapper.readValue(json, Student.class);
System.out.println(student);

If you run the above code, then it produces the following UnrecognizedPropertyException error.

Caused by: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "studentName" (class com.tedblob.jackson.fieldname.models.Student), not marked as ignorable (one known property: "id"])
 at [Source: (String)"{"id": 1001, "studentName": "John", "dept": "IT"}"; line: 1, column: 30] (through reference chain: com.tedblob.jackson.fieldname.models.Student["studentName"])

3.1. Ignore unknown fields at class level

You can use @JsonIgnoreProperties annotation at the class level to ignore any unknown fields in Jackson.

This annotation ignores processing of JSON properties during deserialization. If you know the JSON specific fields that is not required in your Java class, then you can prevent those by defining them explicitly:

@JsonIgnoreProperties({ "studentName", "dept" })

To ignore any unknown properties in JSON input with no exception:

@JsonIgnoreProperties(ignoreUnknown = true)
public class Student {
    private long id;
}

Depending on your Jackson version, you must use a different import statement:

import com.fasterxml.jackson.annotation.JsonIgnoreProperties; 


import org.codehaus.jackson.annotate.JsonIgnoreProperties;

3.2. Ignore unknown fields using ObjectMapper

You can configure your ObjectMapper to ignore unknown properties:

ObjectMapper objectMapper = new ObjectMapper();


import org.codehaus.jackson.map.DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES;
objectMapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);



import com.fasterxml.jackson.databind.DeserializationFeature;

objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false) 

If you run the deserialization code now, then it outputs:

Student [id=1001]

3.3. Avoid Jackson UnrecognizedPropertyException using properties

Alternatively, you can define spring.jackson.deserialization.fail-on-unknown-properties as false in your project properties file to ignore unknown JSON fields during deserialization.

application.properties file:

 spring.jackson.deserialization.fail-on-unknown-properties=false

application.yml file:

spring:
  jackson:
    deserialization:
      fail-on-unknown-properties:false

3.4. Avoid UnrecognizedPropertyException globally using Jackson2ObjectMapperBuilder

You can use Jackson2ObjectMapperBuilder to build your ObjectMapper instance. By default, this builder disables DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES. So whenever you convert your JSON to Java object using its ObjectMapper, there won’t be any UnrecognizedPropertyException failure.

@Autowired
Jackson2ObjectMapperBuilder objectBuilder;

For example, the below code creates an ObjectMapper instance by calling the build method of Jackson2ObjectMapperBuilder.

import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder

@SpringBootApplication
public class JacksonFieldNameApplication implements CommandLineRunner {
	 
	public static void main(String[] args) {
		SpringApplication.run(JacksonFieldNameApplication.class, args);
	}
	
	@Autowired
	Jackson2ObjectMapperBuilder objectBuilder;
	@Override
	public void run(String... args) throws Exception {
		
		ObjectMapper mapper = objectBuilder.build();
		String json = "{\"id\": 1001, \"studentName\": \"John\", \"dept\": \"IT\"}";
		Student student = mapper.readValue(json, Student.class);
		System.out.println(student);
	}
}

If you run the above code, then it produces the following result:

Student [id=1001]

4. Jackson UnrecognizedPropertyException: private fields without setter or getter

If your property is private, then Jackson uses the setter and getter methods to update the value during deserialization.

Assume you have defined all the fields or properties in your Java class so that both input JSON and output Java class have identical fields. But missing setter and getter methods of private fields still results in the UnrecognizedPropertyException error.

The following Student class contains a private id field without any setter or getter methods.

public class Student {
	public Student(long id) {
		this.id = id;
	}
	private long id;
	@Override
	public String toString() {
		return "Student [id=" + id + "]";
	}
}

If you execute the following code, then it would produce the UnrecognizedPropertyException error:

Caused by: com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "id" (class com.tedblob.jackson.fieldname.models.Student), not marked as ignorable (0 known properties: ])
 at [Source: (String)"{"id": 1001}"; line: 1, column: 12] (through reference chain: com.tedblob.jackson.fieldname.models.Student["id"])

@SpringBootApplication
public class JacksonFieldNameApplication implements CommandLineRunner {
	 
	public static void main(String[] args) {
		SpringApplication.run(JacksonFieldNameApplication.class, args);
	}
	@Override
	public void run(String... args) throws Exception {
		
		ObjectMapper mapper = new ObjectMapper();
		String json = "{\"id\": 1001}";
		Student student = mapper.readValue(json, Student.class);
		System.out.println(student);
	}
}

To avoid this error, you can add the setter or getter method to your private field. However, there are alternatives to perform deserialization without adding a setter or getter. Refer to this section for the same.

5. Conclusion

To sum up, we have discussed the various scenarios where we could get the UnrecognizedPropertyException error and also the solutions for each one of them.