Using Gson with Interface Types

Question

I am working on some server code, where the client sends requests in form of JSON. My problem is, there are a number of possible requests, all varying in small implementation details. I therefore thought to use a Request interface, defined as:

public interface Request {     Response process ( ); } 

From there, I implemented the interface in a class named LoginRequest as shown:

public class LoginRequest implements Request {     private String type = "LOGIN";     private String username;     private String password;      public LoginRequest(String username, String password) {         this.username = username;         this.password = password;     }      public String getType() {         return type;     }     public void setType(String type) {         this.type = type;     }     public String getUsername() {         return username;     }     public void setUsername(String username) {         this.username = username;     }     public String getPassword() {         return password;     }     public void setPassword(String password) {         this.password = password;     }      /**      * This method is what actually runs the login process, returning an      * appropriate response depending on the outcome of the process.      */     @Override     public Response process() {         // TODO: Authenticate the user - Does username/password combo exist         // TODO: If the user details are ok, create the Player and add to list of available players         // TODO: Return a response indicating success or failure of the authentication         return null;     }      @Override     public String toString() {         return "LoginRequest [type=" + type + ", username=" + username             + ", password=" + password + "]";     } } 

To work with JSON, I created a GsonBuilder instance and registered an InstanceCreator as shown:

public class LoginRequestCreator implements InstanceCreator<LoginRequest> {     @Override     public LoginRequest createInstance(Type arg0) {         return new LoginRequest("username", "password");     } } 

which I then used as shown in the snippet below:

GsonBuilder builder = new GsonBuilder(); builder.registerTypeAdapter(LoginRequest.class, new LoginRequestCreator()); Gson parser = builder.create(); Request request = parser.fromJson(completeInput, LoginRequest.class); System.out.println(request); 

and I get the expected output.

The thing I wish to do is replace the line Request request = parser.fromJson(completeInput, LoginRequest.class); with something similar to Request request = parser.fromJson(completeInput, Request.class); but doing that will not work, since Request is an interface.

I want my Gson to return the appropriate type of request depending on the received JSON.

An example of the JSON I passed to the server is shown below:

{     "type":"LOGIN",     "username":"someuser",     "password":"somepass" } 

To reiterate, I am looking for a way to parse requests (In JSON) from clients and return objects of classes implementing the Request interface

Answer 1

Polymorphic mapping of the type described is not available in Gson without some level of custom coding. There is an extension type adapter available as an extra that provides a bulk of the functionality you are looking for, with the caveat that the polymorphic sub-types need to be declared to the adapter ahead of time. Here is an example of its use:

public interface Response {}  public interface Request {     public Response process(); }  public class LoginRequest implements Request {     private String userName;     private String password;      // Constructors, getters/setters, overrides }  public class PingRequest implements Request {     private String host;     private Integer attempts;      // Constructors, getters/setters, overrides }  public class RequestTest {      @Test     public void testPolymorphicSerializeDeserializeWithGSON() throws Exception {         final TypeToken<List<Request>> requestListTypeToken = new TypeToken<List<Request>>() {         };          final RuntimeTypeAdapterFactory<Request> typeFactory = RuntimeTypeAdapterFactory                 .of(Request.class, "type")                 .registerSubtype(LoginRequest.class)                 .registerSubtype(PingRequest.class);          final Gson gson = new GsonBuilder().registerTypeAdapterFactory(                 typeFactory).create();          final List<Request> requestList = Arrays.asList(new LoginRequest(                 "bob.villa", "passw0rd"), new LoginRequest("nantucket.jones",                 "crabdip"), new PingRequest("example.com", 5));          final String serialized = gson.toJson(requestList,                 requestListTypeToken.getType());         System.out.println("Original List: " + requestList);         System.out.println("Serialized JSON: " + serialized);          final List<Request> deserializedRequestList = gson.fromJson(serialized,                 requestListTypeToken.getType());          System.out.println("Deserialized list: " + deserializedRequestList);     } } 

Note that you don't actually need to define the type property on the individual Java objects - it exists only in the JSON.