How to fix JSON decoding error in Spring?

OCPi :

I'm sending a User object through REST which contains a Set of SimpleGrantedAuthority objects. On the receiver side, I'm getting an exception:

org.springframework.core.codec.DecodingException: JSON decoding error: Cannot construct instance of org.springframework.security.core.authority.SimpleGrantedAuthority (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator);

I'm using the default JSON mapper which Spring Boot 2.1.2 provides. On the receiving side, I'm using WebFlux's WebClient (WebTestClient in this case).

Can anyone explain to me why am I getting this error and how to solve it?

rustyx :

SimpleGrantedAuthority is not suitable for automatic mapping with Jackson; it has no no-arg constructor and no setter for the authority field.

So it needs a custom deserializer. Something like this:

class SimpleGrantedAuthorityDeserializer extends StdDeserializer<SimpleGrantedAuthority> {
    public SimpleGrantedAuthorityDeserializer() {
        super(SimpleGrantedAuthority.class);
    }
    @Override
    public SimpleGrantedAuthority deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode tree = p.getCodec().readTree(p);
        return new SimpleGrantedAuthority(tree.get("authority").textValue());
    }
}

Register it with Jackson globally like this:

objectMapper.registerModule(new SimpleModule().addDeserializer(
                      SimpleGrantedAuthority.class, new SimpleGrantedAuthorityDeserializer()));

Or annotate the field(s) with:

@JsonDeserialize(using = SimpleGrantedAuthorityDeserializer.class)

Note: you don't need a serializer because SimpleGrantedAuthority has the getAuthority() method, which is usable by Jackson.