Handling Unknown Properties with Jackson JSON Parsing

When working with JSON data and Java objects, it’s common to encounter situations where the JSON contains properties that don’t match any fields in your Java class. This can happen when you’re consuming a third-party API or dealing with dynamic JSON data. In such cases, Jackson, a popular JSON parsing library for Java, throws an UnrecognizedPropertyException. In this tutorial, we’ll explore how to handle unknown properties using Jackson’s features and annotations.

Understanding the Problem

To illustrate the problem, let’s consider an example where we have a simple Java class Student with fields id and name, and another class Wrapper that contains a list of Student objects. We’re trying to deserialize a JSON string into a Wrapper object using Jackson.

public class Student {
    private String id;
    private String name;

    // Getters and setters
}

public class Wrapper {
    private List<Student> students;

    // Getters and setters
}

The JSON string we’re trying to deserialize looks like this:

{
    "wrapper": [
        {
            "id": "13",
            "name": "Fred"
        }
    ]
}

However, when we try to deserialize this JSON using Jackson’s ObjectMapper, we get an UnrecognizedPropertyException because the Wrapper class doesn’t have a field named wrapper.

Using @JsonIgnoreProperties Annotation

One way to handle unknown properties is by using the @JsonIgnoreProperties annotation on the Java class. This annotation tells Jackson to ignore any unknown properties in the JSON data.

@JsonIgnoreProperties(ignoreUnknown = true)
public class Wrapper {
    private List<Student> students;

    // Getters and setters
}

By adding this annotation, we’re telling Jackson to ignore the wrapper property in the JSON data and only consider the fields that are defined in the Wrapper class.

Configuring ObjectMapper

Another way to handle unknown properties is by configuring the ObjectMapper instance. We can do this by setting the DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES feature to false.

ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

This configuration tells Jackson not to throw an exception when it encounters unknown properties in the JSON data.

Using @JsonProperty Annotation

In some cases, we might need to rename a field in our Java class to match the property name in the JSON data. We can use the @JsonProperty annotation to specify the property name.

public class Wrapper {
    @JsonProperty("wrapper")
    private List<Student> students;

    // Getters and setters
}

This way, Jackson knows that the students field in the Wrapper class corresponds to the wrapper property in the JSON data.

Best Practices

When working with unknown properties, it’s essential to consider the following best practices:

• Use @JsonIgnoreProperties annotation or configure ObjectMapper to ignore unknown properties only when necessary.
• Make sure to document your API and specify which properties are expected and which ones can be ignored.
• Consider using a schema validation library like JSON Schema to validate the structure of your JSON data.

By following these best practices and using Jackson’s features and annotations, you can effectively handle unknown properties in your JSON parsing code and ensure robustness and reliability in your applications.