How to format java.util.Locale or any type of Object when serializing/deserializing using Jackson.

There are many tutorials out there about how to format LocalDate with Jackson, but what I was really missing was a custom Locale convertor. There are different formats and ISOs for displaying locale. Some accept underscores as separator (e.g. Apache LocaleUtil libraray) and some that are based on ISO 86?? really don’t. Hyphen is the accepted value for those. Like en-US. Besides it might be important for you to display the country in capital letter, etc.

Let’s jump to the code!

The @JsonDeserialize and @JsonSerialize annotations are our best friends here. :) Simply annotate the property:

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import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import java.util.Locale;

public class MyModel {

    @JsonSerialize(converter = LocaleCustomConverter.class)
    @JsonDeserialize(converter = StringToCustomLocaleConverter.class)
    private Locale myLocale;
}

The classes mentioned in front of the converter attribute our in charge of formatting. You can convert them to your desired format. Here I am trying to make sure there always be hyphen between language and country, whether I am converting to String or generating Locale object from a String. Therefore we create two classes that extend StdConverter<Locale, String>.

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import java.util.Locale;

/**
* Formats a Locale property when converting to String.
*/
public class LocaleCustomConverter extends StdConverter<Locale, String> {

    @Override
    public String convert(Locale locale) {
        //whatever custom format you dersire. Capitalize it, split it, whatever...
        return locale.toLanguageTag();
    }
}

Yeah, I know how you are feeling now. :) This StdConverter class was originally introduced for java.utilLocalDate formatting however, luckily it can also be used for other type of objects. Ok, let’s find out how we gonna accept only a standard-based locale String:

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import java.util.Locale;

public class StringToCustomLocaleConverter extends StdConverter<String, Locale> {
    @Override
    public Locale convert(String value) {
        return Locale.forLanguageTag(value);
    }
}

Aaand that’s it.

What’s happening underhood?

StdConverter class comes with a beautiful generic method, convert that needs to be overridden in our customized converter classes. In our first example, StdConverter<Locale, String>, Locale is the IN object and String is the return type of this method; OUT.

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abstract OUT convert(IN in);

And since IN and OUT are generic objects, basically we can convert back and forth our arbitrary objects.

Happy coding! :)