How to censor custom annotated fields in java dtos?

Question

I need a handy way to censore DTO fields/secrets when converting them into a json string (for logging).

Im using objectmapper and I heard about @JsonProperty(access = JsonProperty.Access.WRITE_ONLY) but it completely hides the annotated field and it can be confusing because if I want to trace an error in the logs I might think that it was just null, therefore It would be better if the property's value was simply replaced with something like ***.

Defining a list of field names to replace values is also not suitable because i might censore fields that i dont want to. Also refactoring would complicate things too.

My idea is to annotate fields with a custom annotation and implement some json postprocessing with objectmapper but i couldnt find a way to do it. For example:

public record Person(String name, @Censored String secret) {
}

// and later
log.info(objectMapper.writeValueAsString(person));

The output should be

{"name":"John","secret":"***"}

Is there a way to configure ObjectMapper to censore fields that are annotated with my custom @Censored annotation?

Im also open to other suggestions to implement log censoring logic.

I tried to use @JsonProperty(access = JsonProperty.Access.WRITE_ONLY) but it didnt achieve the result that i wanted.

Answer 1

The simplest way I can think to implement this via Jackson would be with a custom serializer:

public class CensoredSerializer<T> extends StdSerializer<T> {
    public CensoredSerializer() {
        this(null);
    }

    public CensoredSerializer(Class<T> t) {
        super(t);
    }

    @Override
    public void serialize(T value, JsonGenerator gen, SerializerProvider provider)
            throws IOException
    {
        gen.writeString("***");
    }
}

You would use this like so:

public record Person(String name, @JsonSerialize(using = CensoredSerializer.class) String secret) {
}

If that's too verbose for you, you can use it as the basis for a custom annotation:

@Retention(RetentionPolicy.RUNTIME)
@JacksonAnnotationsInside
@JsonSerialize(using = CensoredSerializer.class)
public @interface Censored {
}

This lets you use @Censored as a shorthand for @JsonSerialize(using = CensoredSerializer.class), like so:

public record Person(String name, @Censored String secret) {
}

---

As mentioned in the comments, it is also possible to do this by masking values in the logging config. I personally feel like there's some value in having it defined directly in the class instead, but your milage may vary!