Jackson: Deserialize map with non String keys

I’m trying to deserialize a map that contains keys that are not Strings. They do not seem to be deserialized properly.

For instance the library I am interfacing with, treats all strings as the binary type. (The encoding of the bytes not guaranteed to be utf-8 which is why they are not using the string type.)

So assuming I have a map of {a:1, b:2, c:3} where a, b and c are binary payloads. The following code tries to deserialize that into a Map<byte[], Integer>

ByteArrayOutputStream out = new ByteArrayOutputStream();
MessagePacker messagePacker = MessagePack.newDefaultPacker(out).packMapHeader(3);
for (int i = 0; i < 3; i++) {
    messagePacker
            .packBinaryHeader(1).writePayload(new byte[]{(byte) (i + 'a')})
            .packInt(i);
}
messagePacker.close();

ObjectMapper objectMapper = new ObjectMapper(new MessagePackFactory());
Map<byte[], Integer> map = objectMapper.readValue(
        out.toByteArray(), new TypeReference<Map<byte[], Integer>>() {});

This results in an exception

com.fasterxml.jackson.databind.JsonMappingException: Can not find a (Map) Key deserializer for type [array type, component type: [simple type, class byte]]
    at com.fasterxml.jackson.databind.deser.DeserializerCache._handleUnknownKeyDeserializer(DeserializerCache.java:608)
    at com.fasterxml.jackson.databind.deser.DeserializerCache.findKeyDeserializer(DeserializerCache.java:169)
    at com.fasterxml.jackson.databind.DeserializationContext.findKeyDeserializer(DeserializationContext.java:462)
    at com.fasterxml.jackson.databind.deser.std.MapDeserializer.createContextual(MapDeserializer.java:231)
    at com.fasterxml.jackson.databind.DeserializationContext.handleSecondaryContextualization(DeserializationContext.java:653)
    at com.fasterxml.jackson.databind.DeserializationContext.findRootValueDeserializer(DeserializationContext.java:444)
    at com.fasterxml.jackson.databind.ObjectMapper._findRootDeserializer(ObjectMapper.java:3666)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3558)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2688)

A quick search of this on google looks like I would need to add a KeyDeserializer. http://stackoverflow.com/questions/11246748/deserializing-non-string-map-keys-with-jackson. So I created a SimpleModule and registered it with the ObjectMapper.

ObjectMapper objectMapper = new ObjectMapper(new MessagePackFactory());
SimpleModule module = new SimpleModule();
module.addKeyDeserializer(byte[].class, new KeyDeserializer() {
            @Override
            public Object deserializeKey(String key, DeserializationContext ctxt) throws
                    IOException,
                    JsonProcessingException {
                throw new IOException("KeyDeserializer Called");
            }
        });
objectMapper.registerModule(module);

Map<byte[], Integer> map = objectMapper.readValue(
        out.toByteArray(), new TypeReference<Map<byte[], Integer>>() {});

However it doesn’t appear the KeyDeserializer ever called called and the returned map is empty.

---

This is also a problem with Integer keys (and I assume all other non string keys)

ByteArrayOutputStream out = new ByteArrayOutputStream();
MessagePacker messagePacker = MessagePack.newDefaultPacker(out).packMapHeader(3);
for (int i = 0; i < 3; i++) {
    messagePacker
            .packInt(i)
            .packInt(i);
}
messagePacker.close();

ObjectMapper objectMapper = new ObjectMapper(new MessagePackFactory());
Map<Integer, Integer> map = objectMapper.readValue(
        out.toByteArray(), new TypeReference<Map<Integer, Integer>>() {});

Exception is thrown without the KeyDeserializer. Adding the KeyDeserializer the map is empty.

(Side comment (and possibly another issue): Serializing a Map<Integer, Integer> with jackson actually outputs a map of Map<String, Integer> on the msgpack side which is undesirable.)

[komamitsu]

@fdinoff In https://github.com/msgpack/msgpack-java/issues/271, jackson-dataformat-msgpack supports Boolean, Integer and Float type key while byte array key seemed to work before the change.

BTW, you tried to use Map<byte[], Integer> map but I don’t think it’s easy to use byte[] as a key of Map since byte[]#equals doesn’t match other byte arrays which have the same values.

[fdinoff]

@komamitsu Sorry, didn’t realize that byte[] was a valid key now.

The problem now is what happens if the bytes weren’t valid utf8? is the conversion to String and back to a byte array lossless? The doc for String#getBytes() says its unspecified, so the conversion isn’t lossless. Losing valid data would be undesirable.

Also I don’t think you can deserialize into an actual object. (Is there?)

Example: You have the following class

class SomeObject {
    public Integer hello;
}

I would expect something like the following to work (the byte array is utf16 encoded.

ByteArrayOutputStream out = new ByteArrayOutputStream();
MessagePacker messagePacker = MessagePack.newDefaultPacker(out).packMapHeader(1);
byte[] bytes = "Hello".getBytes(StandardCharsets.UTF_16); 
messagePacker.packBinaryHeader(bytes.length).writePayload(bytes);
messagePacker.packInt(42);
messagePacker.close();

SomeObject o = objectMapper.readValue(out.toByteArray(), SomeObject.class);
assertEquals(42, o.hello);