How to deserialize a float value with a localized decimal separator with Jackson

Question

The input stream I am parsing with Jackson contains latitude and longitude values such as here:

{
    \"name\": \"pr         


        

Answer 1

A more general solution than the other proposed answers, which require registering individual deserializers for each type, is to provide a customized DefaultDeserializationContext to ObjectMapper.

The following implementation (which is inspired by DefaultDeserializationContext.Impl) worked for me:

class LocalizedDeserializationContext extends DefaultDeserializationContext {
    private final NumberFormat format;

    public LocalizedDeserializationContext(Locale locale) {
        // Passing `BeanDeserializerFactory.instance` because this is what happens at
        // 'jackson-databind-2.8.1-sources.jar!/com/fasterxml/jackson/databind/ObjectMapper.java:562'.
        this(BeanDeserializerFactory.instance, DecimalFormat.getNumberInstance(locale));
    }

    private LocalizedDeserializationContext(DeserializerFactory factory, NumberFormat format) {
        super(factory, null);
        this.format = format;
    }

    private LocalizedDeserializationContext(DefaultDeserializationContext src, DeserializationConfig config, JsonParser parser, InjectableValues values, NumberFormat format) {
        super(src, config, parser, values);
        this.format = format;
    }

    @Override
    public DefaultDeserializationContext with(DeserializerFactory factory) {
        return new LocalizedDeserializationContext(factory, format);
    }

    @Override
    public DefaultDeserializationContext createInstance(DeserializationConfig config, JsonParser parser, InjectableValues values) {
        return new LocalizedDeserializationContext(this, config, parser, values, format);
    }

    @Override
    public Object handleWeirdStringValue(Class targetClass, String value, String msg, Object... msgArgs) throws IOException {
        // This method is called when default deserialization fails.
        if (targetClass == float.class || targetClass == Float.class) {
            return parseNumber(value).floatValue();
        }
        if (targetClass == double.class || targetClass == Double.class) {
            return parseNumber(value).doubleValue();
        }
        // TODO Handle `targetClass == BigDecimal.class`?
        return super.handleWeirdStringValue(targetClass, value, msg, msgArgs);
    }

    // Is synchronized because `NumberFormat` isn't thread-safe.
    private synchronized Number parseNumber(String value) throws IOException {
        try {
            return format.parse(value);
        } catch (ParseException e) {
            throw new IOException(e);
        }
    }
}

Now set up your object mapper with your desired locale:

Locale locale = Locale.forLanguageTag("da-DK");
ObjectMapper objectMapper = new ObjectMapper(null,
                                             null,
                                             new LocalizedDeserializationContext(locale));

If you use Spring RestTemplate, you can set it up to use objectMapper like so:

RestTemplate template = new RestTemplate();
template.setMessageConverters(
    Collections.singletonList(new MappingJackson2HttpMessageConverter(objectMapper))
);

Note that the value must be represented as a string in the JSON document (i.e. {"number": "2,2"}), since e.g. {"number": 2,2} is not valid JSON and will fail to parse.