S313265 - Advanced Java API for RESTful Web Services at JavaOne Brazil 2010

2. <Insert Picture Here>
Advanced Java API for
RESTful Web Services
Lee Chuk Munn
chuk-munn.lee@oracle.com

3. <Insert Picture Here>
Objective
Examine some “advance” features
in JAX-RS

4. Architectural Style
• Music styles – baroque, romantic, rap, jazz, etc
– There is no Jazz (music) note
– How you put the notes that makes the style
• Computing styles – client/server, object oriented, etc.
– There is no client/server API
• “... a coordinated set of architectural constraints that
restricts the roles/features of architectural elements
and the allowed relationships among those elements
within any architecture that conforms to that style” -
Dr. Roy Fielding

5. What is REST?
• REpresentation State Transfer
– Architectural Styles and the Design of Network-based
Software Architecture – Roy Fielding PhD thesis
• Major components
– Nouns (resources) are identified by URIs
– A small subset of verbs to manipulate the nouns
– State of the data
– Representation is how you would like to view the state
• Use verbs to exchange application states and
representation
• Hypermedia is the engine for application state
transition

6. REST Example
• HTTP is one of the most RESTful protocol
• Example: the web browser
– The URL is the noun (resource)
– GET (verb) the page from the server
– State of the page is transferred from the server to the browser
• Page maybe static or dynamic
• State of page now exists on the client
– Page may be represented as HTML, RSS, JSON, etc.
– Click on a link in page (hypermedia) the process is repeated

7. What is REST?
Request
GET /music/artists/magnum/recordings HTTP/1.1
Host: media.example.com
Accept: application/xml
Verb Response Noun
HTTP/1.1 200 OK
Date: Tue, 08 May 2007 16:41:58 GMT
Server: Apache/1.3.6
Content-Type: application/xml; charset=UTF-8
State <?xml version="1.0"?>
<recordings xmlns="…">
transfer <recording>…</recording>
… Representation
</recordings>

8. JAX-RS in One Slide
@Path(“/music”)
@Produces(“application/xml”)
public class Music { www.mymusic.com/music
@GET //Returns List<Genre>
public Response getGenreList() {
...
}
@GET @Path(“{genre}”) //Returns List<Song>
public Response getSongs(
@PathParam(“genre”) String genreId) {
...
} www.mymusic.com/music/jazz
}

9. Selected Topics
• Runtime resource resolution
• Integration with EJB and CDI
• Runtime content negotiation
• Conditional HTTP request
• Dealing with type erasure
• Pluggable exception handling

10. Terms
• Root resource
http://server/department/eng
@Path(“department”)
public class Department {
• Sub resource method
– Handles the request
@GET @Path(“{id}”)
public Department get(
@PathParam(“id”) String deptId) {
• Sub resource locator
– Returns an object that will handle the request
@Path(“{id}”)
public Department get(
@PathParam(“id”) String deptId) {

11. Runtime Resource Resolution
@Path(“department”)
public class Department {
@Path(“{id}”)
public Object get(
@PathParam(“id”) String id) {
//Determine department type and return resource
return (new ...);
}
public class Marketing extends Department {
@GET @Path(“{type}”)
public Response get(
@PathParam(“type”) String deptType) { ... }

12. Runtime Resource Resolution
@Path(“department”)
public class Department {
@Path(“{id}”)
public Object get(
@PathParam(“id”) String id) { ... }
GET /department/marketing/tele
public class Marketing extends Department {
@GET @Path(“{type}”)
public Response get(
@PathParam(“type”) String deptType) { ... }

13. Demo

14. More Resource Resolution
• Sub resource locators determines what type of
resource to return dynamically
– Eg. Use JPA to query data and return that as a resource
– Can be used with CDI or EJBs
– All parameters must be annotated
@Path(“department”)
public class Department {
@Path(“{id}”)
public Object get(@PathParam(“id”) String id) {
EntityManager em = ... //Get an instance
Department dept = em.find(Department.class, id);
return (dept);
}

15. Resource Methods and Locators
• Easy to get confuse with sub-resource locator and
sub-resource methods
• Both are annotated with @Path
• Sub-resource methods has resource method
designators
– Eg. @GET
• Sub-resource locator do not have method designators

16. Integration with EJB
• Annotate @Path to convert into a root resource
– Stateless session bean
– Singleton bean
@Path("stateless-bean")
@Stateless
public class StatelessResource { ... }
@Path("singleton-bean")
@Singleton
public class SingletonResource { ... }

17. CDI One Pager
@Stateless public class ShoppingService {
@Inject private Cart myCart;
public void addToCart(Item someItem) {
myCart.add(someItem);
}
…
}
public interface Cart {
public void add(Item item);
}
@ConversationScoped
public class DefaultShoppingCart implements Cart {
@PersistenceContext private EntityManager em;
public void add(Item item) {
…
}
}

18. Using CDI with Root Resources
• Perfect world – use CDI to manage REST resources
– CDI providing lifecycle management, dependency, etc
• Not very well specified
• See http://www.mentby.com/paul-sandoz/jax-rs-on-
glassfish-31-ejb-injection.html

19. JAX-RS/CDI Component Models
• Root resources are managed in the request scope
– Default
• CDI requires “normal” scoped beans to be proxyable
– Annotated with @RequestScoped or @ApplicationScoped
– Unproxyable beans
• No non-private constructor with no argument
• Final class
• Root resources needs to be annotated with CDI
scoped to be managed by CDI

20. Using CDI with JAX-RS
• Request scoped, JAX-RS managed
@Path(“customer/{id}”)
public class Customer {
public Customer(@PathParam(“id”) String id) {
• Application scoped, CDI managed, will fail
@Path(“customer/{id}”) @ApplicationScoped
public class Customer {
@Inject
public Customer(@PathParam(“id”) String id) {
• Application scoped, provide a no-args constructor
– Use resource method to get id
@Path(“customer/{id}”) @ApplicationScoped
public class Customer {
public Customer() { }

21. Demo

22. Representation
Request
GET /music/artists/magnum/recordings HTTP/1.1
Host: media.example.com
Accept: application/xml
Format
Response
HTTP/1.1 200 OK
Date: Tue, 08 May 2007 16:41:58 GMT
Server: Apache/1.3.6
Content-Type: application/xml; charset=UTF-8
<?xml version="1.0"?>
<recordings xmlns="…">
<recording>…</recording>
…
</recordings>

23. Supported Media Types
• Out-of-the-box support for
– */* - byte[], InputStream, DataSource
– text/* - String
– text/xml, application/xml, application/*+xml –
JAXBElement
– application/x-www-form-urlencoded –
MultivalueMap
• Use @Produces or @Consumes to match with HTTP
headers
– Accept, Content-Type

24. Runtime Content Negotiation
• Static content/media type negotiation of
representation supported with @Produces
• Runtime content negotiation of representation
supports 4 dimensions
– Media type, character set, language, encoding
• Each representation has a Variant which is a point
in the 4 dimension space
List<Variant> variant = Variant
.mediaType(MediaType.APPLICATION.JSON
, MediaType.APPLICATION.XML)
.languages(Locale.ENGLISH, Locale.CHINESE);
assert variant.size() == 4

25. Select Most Acceptable Variant
@GET public Response get(@Context Request r) {
List<Variant> vs = ...
Variant v = r.selectVariant(vs);
if (v == null)
return (Response.notAcceptable(vs).build());
else {
Object rep = selectRepresentation(v);
return (Response.ok(rep, v));
}
}
• Selection will compare the list of variants with the
correspond acceptable values in the client request
– Accept, Accept-Langauge, Accept-Encoding, Accept-
Charset

26. Conditional HTTP Request
• Save bandwidth and client processing
– A GET can return a 304 (not modified) if representation has
not changed since previous request
• Avoid the lost update problem
– A PUT can return 412 (precondition failed) if the resource state
has been modified since previous request
• A date and/or entity tag can be used
– Last-Modified and Etag headers
– HTTP dates have granularity of 1 second
– Etags are better for use with PUT

27. Processng Etags
@GET
public Response get(@Context Request r) {
EntityTag et = ... //Calculate etag
Response.ResponseBuilder rb =
r.evaluatePreconditions(et);
if (rb != null) {
// Return 304 (Not Modified)
return rb.build();
}
String rep = ...
return Response.ok(rep).tag(et).build();
}

28. Dealing with Type Erasure – 1
• Resources can return an entity
@GET List<Customer> getCustomers() { ...
• Type information is lost when returning Response
@GET Response getCustomers() {
List<Customer> list = ...
return (Response.ok(list).build());
• MessageBodyWriter support for List<Customer>
will not work when type information is lost
– Converts Java object to stream

29. Dealing with Type Erasure – 2
• Use GenericEntity to preserve type information at
runtime
@GET Response getCustomers() {
List<Customer> list = ...
GenericEntity<List<Customer>> ge =
new GenericEntity<List<Customer>>(list){};
return (Response.ok(list).build());

31. Pluggable Exception Handling
• Propagation on unmapped exceptions to web
container
– A runtime exception thrown by the JAX-RS container or
application is propagated as is
– A checked exception is thrown by the application is
propagated as the cause of a ServletException
– Propagated exceptions can be mapped to error pages
• Runtime/checked exceptions can be “caught” and
mapped to Response using ExceptionMapper

32. Exception Classes
Class A extends RuntimeException { … }
Class B extends A { … }
Class C extends B { … }

33. Exception Mapper
@Provider
public class AMapper
implements ExceptionMapper<A> {
public Response toResponse(A a) { … }
}
@Provider
public class BMapper
implements ExceptionMapper<B> {
public Response toResponse(B b) { … }
}

34. Throwing Exceptions
// throwing A maps to Response of AMapper
@GET public String a() throws A { ... }
// throwing B maps to Response of BMapper
@GET public String b() throws B { ... }
// throwing C maps to Response of BMapper
@GET public String c() throws C { ... }

35. ExceptionMapper
• ExceptionMapper “closest” to exception class is
selected to map exception
• Can map Throwable
@Provider public class CatchAll
implements ExceptionMapper<Throwable> {
public Response toResponse(Throwable t) {
// Internal Server Error
return Response.status(500).build();
}
}
• Inject @Provider to delegate
– Map the cause of the exception

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“Software. Hardware. Complete.” at the end of all your presentations.
This message should replace any reference to our previous corporate
tagline “Oracle Is the Information Company.”

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38. Alternate Title with Subhead
Arial Size 18 (Gray)
• First-level bullet
– Second-level bullet
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