Jackson: Override primitive type deserialization?

Question

We need to process some broken JSON from a legacy server here that wrongly encodes null values as literal "null" strings in its output.

I already found that I probably want to override https://github.com/FasterXML/jackson-core/blob/master/src/main/java/com/fasterxml/jackson/core/base/ParserMinimalBase.java#L368 to "fix" this, but this seems to be so deep inside Jackson that I'd rather do it differently. Are there alternatives, for example by using the ObjectMapper to add a custom deserializer for the String.class or am I lost?

Answer 1

Ok, it worked by overriding the standard String deserializer. Unfortunately I had to copy the complete implementation over because org/codehaus/jackson/map/deser/std/StringDeserializer.java is final and cannot be extended.

public class FixesModule extends SimpleModule {

    public FixesModule() {
        super();
        addDeserializer(String.class, new CustomStringDeserializer());
    }
}

and

public class CustomStringDeserializer extends StdScalarDeserializer<String> {

    private static final String NULL_STRING = "null";

    public CustomStringDeserializer() {
        super(String.class);
    }

    @Override
    public String deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonToken curr = jp.getCurrentToken();
        // Usually should just get string value:
        if (curr == JsonToken.VALUE_STRING) {
            // BEGIN NULL_STRING fix
            if (NULL_STRING.equals(jp.getText())) {
                return null;
            }
            // END NULL_STRING fix
            return jp.getText();
        }
        // [JACKSON-330]: need to gracefully handle byte[] data, as base64
        if (curr == JsonToken.VALUE_EMBEDDED_OBJECT) {
            Object ob = jp.getEmbeddedObject();
            if (ob == null) {
                return null;
            }
            if (ob instanceof byte[]) {
                return Base64Variants.getDefaultVariant().encode((byte[]) ob, false);
            }
            // otherwise, try conversion using toString()...
            return ob.toString();
        }
        // Can deserialize any scalar value, but not markers
        if (curr.isScalarValue()) {
            return jp.getText();
        }
        throw ctxt.mappingException(_valueClass, curr);
    }

    // 1.6: since we can never have type info ("natural type"; String, Boolean,
    // Integer, Double):
    // (is it an error to even call this version?)
    @Override
    public String deserializeWithType(JsonParser jp, DeserializationContext ctxt, TypeDeserializer typeDeserializer)
            throws IOException, JsonProcessingException {
        return deserialize(jp, ctxt);
    }
}