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1. Overview

In this article, we try to understand how to use @JsonFormat in Jackson. It is a Jackson annotation that is used to specify how to format fields and/or properties for JSON output.

Specifically, this annotation allows you to specify how to format Date and Calendar values according to a SimpleDateFormat format.

2. Maven Dependency

@JsonFormat is defined in the jackson-databind package so we need the following Maven Dependency:

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.11.1</version>
</dependency>

3. Getting Started

3.1. Using the Default Format

To get started, we will demonstrate the concepts of using the @JsonFormat annotation with a class representing a user.

Since we are trying to explain the details of the annotation, the User object will be created on request (and not stored or loaded from a database) and serialized to JSON:

public class User {
    private String firstName;
    private String lastName;
    private Date createdDate = new Date();

    // standard constructor, setters and getters
}

Building and running this code example returns the following output:

{"firstName":"John","lastName":"Smith","createdDate":1482047026009}

As you can see, the createdDate field is shown as the number of seconds since epoch which is the default format used for Date fields.

3.2. Using the Annotation on a Getter

Let us now use @JsonFormat to specify the format that the createdDate field should be serialized. Here is the User class updated for this change. The createdDate field has been annotated as shown to specify the date format.

The data format used for the pattern argument is specified by SimpleDateFormat:

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "[email protected]:mm:ss.SSSZ")
private Date createdDate;

With this change in place, we build the project again and run it. The output is shown below:

{"firstName":"John","lastName":"Smith","createdDate":"[email protected]:53:34.740+0000"}

As you can see, the createdDate field has been formatted using the specified SimpleDateFormat format using the @JsonFormat annotation.

The above example demonstrates using the annotation on a field. It can also be used in a getter method (a property) as follows.

For instance, you may have a property which is being computed on invocation. You can use the annotation on the getter method in such a case. Note that the pattern has also been changed to return just the date part of the instant:

@JsonFormat(shape = JsonFormat.Shape.STRING, pattern = "yyyy-MM-dd")
public Date getCurrentDate() {
    return new Date();
}

The resultant output is as follows:

{ ... , "currentDate":"2016-12-18", ...}

3.3. Specifying the Locale

In addition to specifying the date format, you can also specify the locale to be used for serialization. Not specifying this parameter results in serialization being performed with the default locale:

@JsonFormat(
  shape = JsonFormat.Shape.STRING, pattern = "[email protected]:mm:ss.SSSZ", locale = "en_GB")
public Date getCurrentDate() {
    return new Date();
}

3.4. Specifying the Shape

Using @JsonFormat with shape set to JsonFormat.Shape.NUMBER results in the default output for Date types — as the number of seconds since the epoch. The parameter pattern is not applicable to this case and is ignored:

@JsonFormat(shape = JsonFormat.Shape.NUMBER)
public Date getDateNum() {
    return new Date();
}

The output is as shown below:

{ ..., "dateNum":1482054723876 }

4. Conclusion

In conclusion, @JsonFormat is used to control the output format of Date and Calendar types as demonstrated above.

The sample code shown above is available over on GitHub.

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