Main.java

import com.fasterxml.jackson.annotation.*;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.ObjectCodec;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.module.SimpleModule;

import java.io.IOException;
import java.util.*;

public class Main {
    private static String json =
            "[{\"email\": \"user@email\",\"roles\": [\"REGISTERED_USER\"]}]";

    public static void main(String[] args) throws IOException{
        System.out.println(json);
        ObjectMapper mapper = new ObjectMapper();

        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addDeserializer(Role.class, new RoleDeserializer());
        simpleModule.addSerializer(Role.class, new RoleSerializer());
        mapper.registerModule(simpleModule);

        List<User> users = Arrays.asList(mapper.readValue(json, User[].class));
        System.out.println(users);
        String jsonInString = mapper.writeValueAsString(users);
        System.out.println(jsonInString);
    }

    public static class RoleSerializer extends JsonSerializer {
        @Override
        public void serialize(Object value, JsonGenerator gen,
                              SerializerProvider serializers)
                throws IOException, JsonProcessingException {
            gen.writeString(((Role) value).getRole());
        }
    }

    public static class RoleDeserializer extends JsonDeserializer {
        @Override
        public Role deserialize(JsonParser jsonParser,
                                DeserializationContext deserializationContext)
                throws IOException {
            ObjectCodec oc = jsonParser.getCodec();
            JsonNode node = oc.readTree(jsonParser);
            Role role =  new Role();
            role.setRole(node.asText());
            return role;
        }
    }

    @JsonInclude(JsonInclude.Include.NON_NULL)
    @JsonIgnoreProperties(ignoreUnknown = true)
    public static class User {

        @JsonProperty(value = "id")
        private Long id;

        @JsonProperty(value = "email")
        private String email;

        @JsonProperty(value = "password")
        private String password;
        @JsonProperty(value = "roles")
        private List<Role> roles;

        @Override
        public String toString() {

            String listString = "";
            String delim = "";
            for (Role s : roles) {

                listString += delim + s ;
                delim = ", ";
            }

            return "User{" +
                    "id=" + id +
                    ", email='" + email + '\'' +
                    ", password='" + password + '\'' +
                    ", roles=" + listString +
                    '}';
        }
    }



    public static class Role {
        private String role;
        public String getRole() {return role;}
        public void setRole(String role) {this.role = role;}

        @Override
        public String toString() {
            return "Role{role='" + role + "'}";
        }
    }
}

Output:

[{"email": "user@email","roles": ["REGISTERED_USER"]}]
[User{id=null, email='user@email', password='null', roles=Role{role='REGISTERED_USER'}}]
[{"email":"user@email","roles":["REGISTERED_USER"]}]