Issue registering custom string deserializer via custom module

I have a custom string deserializer that only accepts string tokens.

public class Deserializers extends com.fasterxml.jackson.databind.deser.Deserializers.Base {
    @Override
    public @Nullable JsonDeserializer<?> findBeanDeserializer(JavaType type, DeserializationConfig config, BeanDescription description) throws JsonMappingException {
        if (type.hasRawClass(String.class)) {
            new StringDeserializer();
        }

        return null;
    }

    public static class StringDeserializer extends JsonDeserializer<String> {
        @Override
        public String deserialize(JsonParser parser, DeserializationContext context) throws IOException {
            System.out.println("StringDeserializer!");

            if (!parser.hasToken(JsonToken.VALUE_STRING)) {
                throw context.wrongTokenException(parser, String.class, JsonToken.VALUE_STRING, "");
            }

            return parser.getText();
        }
    }
}

When I register the custom deserializer in an ObjectMapper via SimpleModule it gets called.

try {
    ObjectMapper objectMapper = new ObjectMapper();
    SimpleModule module = new SimpleModule();
    module.addDeserializer(String.class, new Deserializers.StringDeserializer());
    objectMapper.registerModule(module);
    objectMapper.readValue("{ \"string\": true }", Foo.class);
} catch (Exception exception) {
    System.out.println(exception);
}

public class Foo {
    public String string;
}

Results in

StringDeserializer!
com.fasterxml.jackson.databind.exc.MismatchedInputException: Unexpected token (VALUE_TRUE), expected VALUE_STRING:
 at [Source: (String)"{ "string": true }"; line: 1, column: 13] (through reference chain: com.booking.demandapi.system.Initializer$Foo["string"])

as expected. However, if I register the custom deserializer with a custom Module

try {
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new Module());
    objectMapper.readValue("{ \"string\": true }", Foo.class);
} catch (Exception exception) {
    System.out.println(exception);
}

public class Module extends com.fasterxml.jackson.databind.Module {
    @Override
    public String getModuleName() {
        return getClass().getName();
    }

    @Override
    public void setupModule(com.fasterxml.jackson.databind.Module.SetupContext context) {
        context.addDeserializers(new Deserializers());
    }

    @Override
    public Version version() {
        return Version.unknownVersion();
    }
}

the custom deserializer is not being run. Is this the right way of registering it?

I have other custom serializers and deserializers registered in a similar way (findEnumDeserializer and findSerializer) and they are working.

I have considered using SimpleModule instead of a custom Module but I have generic Enum serializers and haven’t been able to register them via addSerializer with type = Enum.class since they are JsonSerializer<Enum<?>>.

[joca-bt]

That’s embarrassing, I’ll go hide in a cave and never leave. Our static analysis is supposed to flag these things.

[cowtowncoder]

Oh. But before that, looking at original code, is this actually what you have?

        if (type.hasRawClass(String.class)) {
            new StringDeserializer();
        }

… because it is not actually returning the deserializer… 😃