Question

How do i tell jackson to use a different serializer for each of the Generic collection type.
I have a CustomProductsSerializer and a CustomClientsSerializer

    public class ProductsListSerializer extends JsonSerializer<List<Products>>{

    // logic to insert __metadata , __count into json
    public void serialize(List<Products> value, JsonGenerator jgen, SerializerProvider provider) {
     .... .... .... 
     }
    // always returns ArrayList.class 
    public Class<List<Instrument>> handledType() {
    Class c = var.getClass();
    return c;
    }
    }


    public class CustomClientsSerializer extends JsonSerializer<List<Clients>>{

    // logic to insert __metadata , __count into json
    public void serialize(List<Clients> value, JsonGenerator jgen, SerializerProvider provider){
     .... .... .... 
     }
     .... .... .... 
    // always returns ArrayList.class 
    public Class<List<Clients>> handledType() {
    Class c = var.getClass();
    return c;
    }
    }

I have registered both the serializers using SimpleModule.
The problem is, since handledType in both cases returns ArrayList.class, serialization of clients below fail with a class cast exception.

    List<Clients> clients;
    // code to get clients from database. 
    String jsonString = mapper.writeValueAsString(clients);

Question is how to tell jackson about which serializer to use ?

I am using jackson as the json serializer with resteasy.

–Edit in response to @HiJon89
I have a business object which returns ‘List’ and another which returns List. The way I need is when the Busines Object returns a ‘List’ then ProductsListSerializer should be used for the whole List. and When Business Object returns ‘List the CustomClientsSerializer should be used for the whole List. In my use-case I append additional elements to the serialized json ‘eg:__count, __metadata , __references ‘ There is only one __count per Collection. Contentusing Can only be used on properties. Any suggestions ?

Answer 1

It possible by extending the simplemodule or by extending com.fasterxml.jackson.databind.Module. I have taken the second approach. Below are the summary of changes

added method to take JavaType as parameter.

    public <T> TypedModule addSerializer(JavaType type, JsonSerializer<T> ser)
{
    if (_serializers == null) {
        _serializers = new TypedSerializers();
    }
    _serializers.addSerializer(type, ser);
    return this;
}

Above method adds the new serilaizer to

protected TypedSerializers _serializers = null;

The class ‘TypedSerializers extends Serializers.Base’ stores all the typed serializers in a HashMap.

Changed the logic of findSerializer method in TypedSerializers

public JsonSerializer<?> findSerializer(SerializationConfig config,
        JavaType type, BeanDescription beanDesc) {

    Class<?> cls = type.getRawClass();
    ClassKey key = new ClassKey(cls);
    JsonSerializer<?> ser = null;
    if (_javaTypeMappings != null) {
        ser = _javaTypeMappings.get(type.toString());
        if (ser != null) {
            return ser;
        }
    }
 ..... process normal serilaizers ...

Below change is needed when registering a customserializer

    TypedModule simpleModule = new TypedModule("TypedModule", new Version(1, 0, 0, null));       
    JavaType productlist = this.getTypeFactory().constructCollectionType(List.class, Product.class);
    JavaType clientlist = this.getTypeFactory().constructCollectionType(List.class, Client.class);
    simpleModule.addSerializer(productlist,  new prodctlistserializer());
    simpleModule.addSerializer(clientlist, new clientlistserializer() );
    this.registerModule(simpleModule);

The resulting behaviour is, when ‘List of type Product’ is passed to ObjectMapper , it tries to locate a suitable serilaizer. We are maintaining the generic serializers in the map of _javaTypeMappings. The Objecmapper executes findSerializer. The findSerializer first checks for an avaliable serializer in _javaTypeMappings. In this case it returns prodctlistserializer. When ‘List of type Client ‘ is passed it returns clientlistserializer.

I have taken all source code under com.fasterxml.jackson.databind.Module to a package in my project and added above changes.