This document discusses REST (REpresentational State Transfer) and how to implement RESTful services on Android. It begins by defining REST and describing its core concepts like client-server architecture, statelessness, uniform interface, and CRUD (create, read, update, delete) operations. It then covers how to make HTTP requests in Android using libraries like HttpURLConnection and Apache HTTP Client. Helpful libraries for working with REST APIs are also presented, including Gson for JSON parsing and CRest for declarative REST clients. The document emphasizes best practices like performing HTTP calls in a background thread, persisting data to content providers, and minimizing network usage.

1. Android and REST
Roman Woźniak

2. Roman Woźniak
SignlApps, Lead Android Developer
roman@signlapps.com
@wozniakr

3. Agenda
• REST – what is it?
• REST – how to use it
• HTTP communication on Android
• Libraries to write apps faster
• HTTP communication and databases

4. REST
REST
imgsrc: https://sites.google.com/site/sleepasandroid/

5. REST – what is it?
REpresentational State Transfer (REST) is a style of
software architecture for distributed systems such as
the World Wide Web
Roy Fielding

6. REST – what is it?
RESTful service features
• client-server architecture
• stateless
• resposne caching
• layered system
• uniform interface

7. REST – what is it?
URI structure
• collections
• resources
• controllers
https://api.rwozniak.com/users
https://api.rwozniak.com/users/20
https://api.rwozniak.com/export

8. REST – what is it?
CRUD operations
POST = Create
GET = Read
PUT = Update
DELETE = Delete

9. REST – what is it?
Create
$ curl -v -X POST -d "..." https://api.rwozniak.com/users
> POST /users HTTP/1.1
>
< HTTP/1.1 201 Created
< Location /users/113
< Content-Length: 57
< Content-Type: application/json
<
[{"id":113,"firstname":"John","lastname":"Doe","age":31}]

10. REST – what is it?
Read
$ curl -v https://api.rwozniak.com/users/113
> GET /users/113 HTTP/1.1
>
< HTTP/1.1 200 OK
< Content-Length: 57
< Content-Type: application/json
<
{"id":113,"firstname":"John","lastname":"Doe","age":31}

11. REST – what is it?
Update
$ curl -v -X PUT -d "..." https://api.rwozniak.com/users/113
> PUT /users/113 HTTP/1.1
>
< HTTP/1.1 200 OK
< Content-Length: 57
< Content-Type: application/json
<
{"id":113,"firstname":"John","lastname":"Doe","age":18}

12. REST – what is it?
Delete
$ curl -v -X DELETE https://api.rwozniak.com/users/113
> DELETE /users/113 HTTP/1.1
>
< HTTP/1.1 204 No Content
< Content-Length: 0
< Content-Type: application/json
<

13. REST – more
REST - more
• HTTP resposne codes
• usage of HTTP headers

14. REST – more
HTTP response codes
• 2XX - correct
• 3XX - redirects
• 4XX - client fault (request)
• 5XX - server fault (response)

15. REST – more
Errors
• 400 (Bad Request)
• 401 (Unauthorized)
• 403 (Forbidden)
• 404 (Not Found)
• 500 (Internal Server Error)
• 503 (Service Unavailable)
• 418 (I’m a teapot (RFC 2324))

16. REST – more
Programmers are people too
imgsrc: http://thefuturebuzz.com/wp-content/uploads/2011/10/data.jpg

17. REST – more
Programmer-friendly
{
"http_code": 400,
"error": "validation_errors",
"error_title": "Set Creation Error",
"error_description": "The following validation errors occurred:nYou must have a
titlenBoth the terms and definitions are mandatory",
"validation_errors": [
"You must have a title",
"Both the terms and definitions are mandatory",
"You must specify the terms language",
"You must specify the definitions language"
]
}
src: https://quizlet.com/api/2.0/docs/api_intro/

18. REST – more
Headers
• Accept: application/json
Accept: application/json Accept: application/xml
{ <user>
"id":113, <id>113</id>
"firstname":"John", <firstname>John</firstname>
"lastname":"Doe", <lastname>Doe</lastname>
"age":18 <age>18</age>
} </user>
• If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
• ETag i If-None-Match: 12331-erfwe-123331
• Accept-Language: ES-CT
• Accept-Encoding: gzip

19. Android and HTTP
Android and HTTP
• Apache HTTP Client
– DefaultHttpClient, AndroidHttpClient
– stable, minor bugs
– lack of active development from Android team
• HttpURLConnection
– bugs in the beginning, but improved over time
– transparent response compression (gzip)
– response cache
– active development over Android versions

20. Android and HTTP
HttpClient
public DefaultHttpClient createHttpClient() {
final SchemeRegistry supportedSchemes = new SchemeRegistry();
supportedSchemes.register(new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
supportedSchemes.register(new Scheme("https", SSLSocketFactory.getSocketFactory(), 443));
final HttpParams params = new BasicHttpParams();
HttpConnectionParams.setConnectionTimeout(params, TIMEOUT * 1000);
HttpConnectionParams.setSoTimeout(params, TIMEOUT * 1000);
HttpConnectionParams.setSocketBufferSize(params, 8192);
HttpClientParams.setRedirecting(httpParams, false);
final ClientConnectionManager ccm = new ThreadSafeClientConnManager(httpParams, supportedSchemes);
return new DefaultHttpClient(ccm, httpParams);
}

21. Android and HTTP
Simple GET
String url = "https://api.rwozniak.com/users/1303";
HttpGet httpGet = new HttpGet( url );
//setCredentials( httpGet );
//addHeaders( httpGet );
DefaultHttpClient httpClient = createHttpClient();
HttpResponse httpResponse = httpClient.execute( httpGet );
String responseContent = EntityUtils.toString( httpResponse.getEntity() );
{"id":131,"firstname":"John","lastname":"Doe","age":18}

22. Android and HTTP
JSON parser
{"id":131,"firstname":"John","lastname":"Doe","age":18}
public class User {
public long id;
public String firstname;
public String lastname;
public int age;
}

23. Android and HTTP
JSON parser
public User parse( String response ) throws JSONException {
User user = new User();
JSONObject json = new JSONObject( response );
user.id = json.getLong( "id" );
user.firstname = json.getString( "firstname" );
user.lastname = json.getString( "lastname" );
user.age = json.getInt( "age" );
return user;
}

24. Android and HTTP
imgsrc: http://www.quickmeme.com/meme/3rxotv/

25. Helpful libraries - Gson
Gson
• library from Google
• conversion of Java objects to JSON representation
• and the other way around
• annotations
• support for complex objects
src: https://sites.google.com/site/gson/

26. Helpful libraries - Gson
Comparison
public User parse( String response ) throws JSONException {
User user = new User();
JSONObject json = new JSONObject( response );
user.id = json.getLong( "id" );
user.firstname = json.getString( "firstname" );
user.lastname = json.getString( "lastname" );
user.age = json.getInt( "age" );
return user;
}
public User parse( String response ) {
Gson gson = new Gson();
return gson.fromJson( response, User.class );
}
public User parse( String response ) {
return new Gson().fromJson( response, User.class );
}

27. Helpful libraries - Gson
More complex class
public class User {
public String username;
@SerializedName("account_type") public AccountType accountType;
@SerializedName("sign_up_date") public long signupDate;
@SerializedName("profile_image") public String profileImage;
public List<Group> groups;
@Since(1.1) public String newField;
}

28. Helpful libraries - CRest
CRest
• just Client REST
• makes communication with RESTful services easier
• annotations
• rich configurability
src: http://crest.codegist.org/

29. Helpful libraries - CRest
Example
@EndPoint("http://api.twitter.com")
@Path("/1/statuses")
@Consumes("application/json")
public interface StatusService {
@POST
@Path("update.json")
Status updateStatus(
@FormParam("status") String status,
@QueryParam("lat") float lat,
@QueryParam("long") float longitude);
@Path("{id}/retweeted_by.json")
User[] getRetweetedBy(
@PathParam("id") long id,
@QueryParam("count") long count,
@QueryParam("page") long page);
@Path("followers.json")
User[] getFollowers(@QueryParam("user_id") long userId);
}
CRest crest = CRest.getInstance();
StatusService statusService = crest.build(StatusService.class);
User[] folowers = statusService.getFollowers(42213);

30. Helpful libraries - CRest
One-time configuration
CRestBuilder crestInstance = new CRestBuilder()
.property( MethodConfig.METHOD_CONFIG_DEFAULT_ENDPOINT, "https://api.rwozniak.com/1.0" )
.property( MethodConfig.METHOD_CONFIG_DEFAULT_ERROR_HANDLER, MyErrorHandler.class )
.setHttpChannelFactory( HttpClientHttpChannelFactory.class )
.placeholder( "auth.token", getUserAuthToken() )
.deserializeJsonWith( CustomGsonDeserializer.class );

31. Helpful libraries - CRest
Error handler
public class MyErrorHandler implements ErrorHandler {
@Override
public <T> T handle(Request request, Exception e) throws Exception {
if( e instanceof RequestException ) {
RequestException ex = (RequestException) e;
Response resp = ex.getResponse();
throw new ApiException( resp.to( com.rwozniak.api.entities.ErrorResponse.class ) );
}
return null;
}
}
public class ErrorResponse {
@SerializedName("http_code") public int httpCode;
public String error;
@SerializedName("error_title") public String errorTitle;
@SerializedName("error_description") public String errorDescription;
@SerializedName("validation_errors") public ArrayList<String> validationErrors;
}

32. Helpful libraries - CRest
Custom deserializer
public class CustomGsonDeserializer implements Deserializer {
@Override
public <T> T deserialize(Class<T> tClass, Type type, InputStream inputStream, Charset charset) throws Exception {
GsonBuilder builder = new GsonBuilder();
JsonParser parser = new JsonParser();
JsonElement element = parser.parse( new InputStreamReader(inputStream) );
return builder.create().fromJson( element, type );
}
}

33. Helpful libraries - CRest
Service factory
public class ServiceFactory {
private static CRestBuilder crestInstance;
private static UserService userService;
private static void init( boolean authenticated ) {
crestInstance = new CRestBuilder();
}
private static CRestBuilder getInstance() {
if( crestInstance==null ) {
init();
}
return crestInstance;
}
public static UserService getUserService() {
if( userService==null ) {
userService = getInstance().placeholder( "userid", getUserId() ).build().build( UserService.class );
}
return userService;
}
}

34. Helpful libraries - CRest
Final
User user = ServiceFactory.getUserService().create( "John", "Doe", 25 );
user.age = 32;
ServiceFactory.getUserService().update( user );
ServiceFactory.getUserService().delete( user.id );

35. Persistence and REST
Persistence and REST
Presentation from GoogleIO 2010
– author: Virgil Dobjanschi
– source: http://goo.gl/R15we

36. Persistence and REST
Incorrect implementation
Activity 1. Get, create, update, delete CursorAdapter
Thread
3. Processing
REST method Processor 5. Requery
4. Save
2. GET/POST/PUT/DELETE
Memory

37. Persistence and REST
Incorrect implementation
• Application process can be killed by system
• Data is not presisted for future use

38. Persistence and REST
Service API
Activity CursorAdapter
1. execute 11. callback to Activity
8'. notify
ContentOberver 8''. requery
Service Helper
2. startService(Intent) 10. Binder callback
Service
3. start(param) 9. callback
4. insert/update
Processor Content Provider
8. insert/update
5. start(param)
7. REST resposne
REST method
6. GET/POST/PUT/DELETE

39. Persistence and REST
Processor - POST and PUT
POST
4. insert
(set STATE_POSTING)
Processor Content Provider
8. update
(clear STATE_POSTING)
REST method
PUT
4. insert
(set STATE_POSTING)
Processor Content Provider
8. update
(clear STATE_POSTING)
REST method

40. Persistence and REST
Processor - DELETE and GET
DELETE
4. insert
(set STATE_DELETING)
Processor Content Provider
8. delete
REST method
GET
Processor Content Provider
8. insert new resource
REST method

41. Summary
Things to keep in mind
• always make HTTP calls in a separate thread (eg.
IntentService)
• persist before and after communication
• minimize HTTP communication (compression, headers)
• libraries - make things easier
• StackOverflow 

42. Questions
Questions?
imgsrc: http://presentationtransformations.com/wp-content/uploads/2012/03/Question.jpg

43. Thankful I am
Thank you
imgsrc: http://memegenerator.net/instance/29015891