java - Serialize a Double to 2 decimal places using Jackson

Question

I'm using Jackson, with Spring MVC, to write out some simple objects as JSON. One of the objects, has an amount property, of type Double. (I know that Double should not be used as a monetary amount. However, this is not my code.)

In the JSON output, I'd like to restrict the amount to 2 decimal places. Currently it is shown as:

"amount":459.99999999999994

I've tried using Spring 3's @NumberFormat annotation, but haven't had success in that direction. Looks like others had issues too: MappingJacksonHttpMessageConverter's ObjectMapper does not use ConversionService when binding JSON to JavaBean propertiesenter link description here.

Also, I tried using the @JsonSerialize annotation, with a custom serializer.
In the model:

@JsonSerialize(using = CustomDoubleSerializer.class)
public Double getAmount()

And serializer implementation:

public class CustomDoubleSerializer extends JsonSerializer<Double> {
    @Override
    public void serialize(Double value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonGenerationException {
        if (null == value) {
            //write the word 'null' if there's no value available
            jgen.writeNull();
        } else {
            final String pattern = ".##";
            //final String pattern = "###,###,##0.00";
            final DecimalFormat myFormatter = new DecimalFormat(pattern);
            final String output = myFormatter.format(value);
            jgen.writeNumber(output);
        }
    }
}

The CustomDoubleSerializer "appears" to work. However, can anyone suggest any other simpler (or more standard) way of doing this.

Answer 1

I know that Double should not be used as a monetary amount. However, this is not my code.

Indeed, it should not. BigDecimal is a much better choice for storing monetary amounts because it is lossless and provides more control of the decimal places.

So for people who do have control over the code, it can be used like this:

double amount = 111.222;
setAmount(new BigDecimal(amount).setScale(2, BigDecimal.ROUND_HALF_UP));

That will serialize as 111.22. No custom serializers needed.