Jackson：将 null 字符串反序列化为空字符串

Question

I have the following class, that is mapped by Jackson (simplified version):

public class POI {
    @JsonProperty("name")
    private String name;
}

In some cases the server returns "name": null and I would like to then set name to empty Java String.

Is there any Jackson annotation or should I just check for the null inside my getter and return empty string if the property is null ?

Answer 1

Again, this answer is for the SO users who happen to stumble upon this thread.

While the accepted answer stands accepted and valid in all its sense - it did not help me in the case where the decision to set null string values to empty string came only after we made our services available to iOS clients.

So, around 30-40 pojo's(increasing) and initializing them while instantiating the object in question or at the point of declaration was too much.

Here's how we did it.

public class CustomSerializerProvider extends DefaultSerializerProvider {

    public CustomSerializerProvider() {
        super();
    }

    public CustomSerializerProvider(CustomSerializerProvider provider, SerializationConfig config,
            SerializerFactory jsf) {
        super(provider, config, jsf);
    }

    @Override
    public CustomSerializerProvider createInstance(SerializationConfig config, SerializerFactory jsf) {
        return new CustomSerializerProvider(this, config, jsf);
    }

    @Override
    public JsonSerializer<Object> findNullValueSerializer(BeanProperty property) throws JsonMappingException {
        if (property.getType().getRawClass().equals(String.class))
            return Serializers.EMPTY_STRING_SERIALIZER_INSTANCE;
        else
            return super.findNullValueSerializer(property);
    }
}

And, the serializer

public class Serializers extends JsonSerializer<Object> {
    public static final JsonSerializer<Object> EMPTY_STRING_SERIALIZER_INSTANCE = new EmptyStringSerializer();

    public Serializers() {}

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException, JsonProcessingException {
        jsonGenerator.writeString("");
    }

    private static class EmptyStringSerializer extends JsonSerializer<Object> {
        public EmptyStringSerializer() {}

        @Override
        public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
                throws IOException, JsonProcessingException {
            jsonGenerator.writeString("");
        }
    }
}

And, then set the serializer in the ObjectMapper. ( Jackson 2.7.4 )

ObjectMapper nullMapper = new ObjectMapper();
nullMapper.setSerializerProvider(new CustomSerializerProvider());

Hoping, this will save someone some time.

Answer 2

Jackson 2.9 actually offers a new mechanism not yet mentioned: use of @JsonSetter for properties, and its equivalent "Config Overrides" for types like String.class . Longer explanation included in

https://medium.com/@cowtowncoder/jackson-2-9-features-b2a19029e9ff

but gist is that you can either mark field (or setter) like so:

@JsonSetter(nulls=Nulls.AS_EMPTY) public String stringValue;

or configure mapper to do the same for all String value properties:

mapper.configOverride(String.class)
 .setSetterInfo(JsonSetter.Value.forValueNulls(Nulls.AS_EMPTY));

both of which would convert incoming null into empty value, which for Strings is "".

This also works for Collection s and Map s as expected.

Answer 3

A simple solution using no Jackson specialities: Write a Getter for name which returns an empty String instead of null as Jackson uses those to serialize.

public String getName() {
  return name != null ? name : "";
}

Another way would be to write a custom deserializer. Look here: http://wiki.fasterxml.com/JacksonHowToCustomSerializers

Answer 4

You can either set it in the default constructor, or on declaration:

public class POI {
    @JsonProperty("name")
    private String name; 

    public POI() {
        name = "";
    }
}

OR

public class POI {
    @JsonProperty("name")
    private String name = "";
} 

Answer 5

In case you are looking for a global solution for spring boot, you can configure the ObjectMapper

@Configuration
public class JacksonConfiguration {

    @Bean
    public ObjectMapper objectMapper(Jackson2ObjectMapperBuilder mapperBuilder) {
        DefaultSerializerProvider sp = new DefaultSerializerProvider.Impl();
        sp.setNullValueSerializer(new JsonSerializer<Object>() {
            public void serialize(Object value, JsonGenerator jgen,
                                  SerializerProvider provider)
                    throws IOException, JsonProcessingException
            {
                jgen.writeString("");
            }
        });
        ObjectMapper mapper = mapperBuilder.build();
        mapper.setSerializerProvider(sp);
        return mapper;
    }

}