Building Restful Web Services with Java

© 2010 VMware Inc. All rights reserved
Building RESTful Web Services with Java
Vassil Popovski (vpopovski@vmware.com)
Staff Engineer, R&D
VMware, Inc.
10/07/2010

2
Agenda
 REST Principles and examples
 Java (JAX-RS) and REST
 Live Demo
 JAX-RS Advanced concepts

5
What is REST ? (now seriously)
 Representational State Transfer
 Introduced in 2000 by Roy Fielding:
• in his PhD thesis (chapter 5)
http://www.ics.uci.edu/~fielding/pubs/dissertation/rest_arch_style.htm
• Captured his interpretation of the WWW architecture
 Architectural style for distributed system such as World Wide Web
• Note: REST is *not* a standard, just a set of principles
 Positioned as an alternative to WS-* implementations

6
What is REST ? (now seriously)

7
REST Principles (RESTful systems)
Everything is a resource
• Examples: Customer, Locations, Item, List of users
Resources have identifiers
• Examples: URIs for web
Uniform interfaces to access the resources
• Examples: HTTP methods (GET, POST, PUT, DELETE, HEAD)
Resources have representations
• Examples: XML, JSON, Binary
Link resources together
• Hypermedia as the Engine of Application State (HATEOAS)
• Examples: Hyperlinks
1
2
3
4
5

9
Real life example
 Bug tracking system (over simplified)
• Bugs (summary, priority, user who reported it)
• Notes (description, user who created the note)
• Users (user name)
UML Notation:
“whole-part” relationship (composition)
“part” is exclusively owned by “whole”
UML Notation:
One-way association
UML Notation:
Generalization / inheritance

10
Real life example – resources and identifies
 Resources:
• User, Bug, Note,
• All users, All bugs, All notes for a particular bug
 Identifiers
Identifiers Note
http://<host>/users All users
http://<host>/users/{userId} A particular user
http://<host>/bugs All bugs
http://<host>/bugs/{bugId} A particular bug
http://<host>/bugs/{bugId}/notes All notes for a particular bug
http://<host>/bugs/{bugId}/notes/{noteId} A particular note for a particular bug
1 Everything is a resource
2 Resources have identifiers

11
Uniform interfaces (in the context of REST)
 HTTP verbs (CRUD interfaces)
HTTP verb Meaning Safe? Idempotent? Cacheable?
POST Create*
GET Retrieve
PUT Update*
DELETE Delete
f(f(x)) = f(x)
Does not cause
side effects
(*) POST and PUT may also have other meanings
POST – can be used for partial updates and also to add something to a
resource (PASTE AFTER)
PUT – can be used to create if sending the full content of the specified
resource and you know the URI of the resource (PASTE OVER)

12
Real life example – uniform interfaces
Operation Description
GET http://<host>/users List all users
POST http://<host>/users Creates a new user
GET http://<host>/users/345 Retrieves a particular user
PUT http://<host>/users/345 Modifies a particular user
DELETE http://<host>/users/345 Deletes a particular user
Operation Description
GET http://<host>/bugs/234/notes List all notes for bug 234
POST http://<host>/bugs/234/notes Creates a new note for bug 234
[GET | PUT | DELETE] .../bugs/234/notes/34 [retrieves | modifies | deletes]
note 34 in bug 234
3 Uniform interfaces to access the resources

13
Real life example – representations and linking resources
 XML representation for note:
 JSON representation for bug:
GET http://localhost:9000/bugs/1/notes/1
Content-type: application/xml
<Note href=“http://localhost:9000/bugs/1/notes/1">
<description>It is really broken</description>
<owner>http://localhost:9000/users/1</owner>
</Note>
POST http://localhost:9000/bugs
Content-type: application/json
{
"Bug" : {
"priority" : "P1",
"reporter" : "http://localhost:9000/users/1",
"summary" : "Something is wrong"
}
}
4 Resources have representations
5 Link resources together
Note: the client can request
different representations
using “Accept” http header
Note: the client declares what
representation it sends to
server with “Content-type”
http header

15
JAX-RS: Java API for RESTful Web Services
 JSR 311 (https://jsr311.dev.java.net/)
• Current version of JAX-RS spec is 1.1
• The spec is “readable”!
• Roy Fielding is an expert group member for this JSR
• javax.ws.rs.*
 JAX-RS in 1 sentence:
• JAX-RS = POJOs + Annotations
 JAX-RS capabilities:
• Dispatch URIs to specific classes and methods that can handle requests
• Methods deal with POJOs (nicely integrates with JAXB)
• Allows to map HTTP requests to method invocations
• URI manipulation functionality

16
JAX-RS implementations
 Jersey (reference implementation)
 Apache CXF (the one used in the demo)
 Apache Wink
 eXo
 RESTEasy
 Restlet
 Triaxrs

17
JAX-RS key concepts
 Resource classes
• Java classes that have at least one method annotated with @Path or a request method designator
(@GET, @PUT, @POST, @DELETE)
• Lifecycle:
• By default a new resource class instance is created for each request
• The JSR does not restrict other implementation specific lifecycles
 Resource methods
• A public method of a resource class annotated with a request method designator (@GET, @PUT,
@POST, @DELETE, @OPTIONS, @HEAD)
 Provider classes
• Classes annotated with @Provider and implementing one or more JAX-RS interfaces:
• MessageBodyReader/MessageBodyWriter
• ExceptionMapper
• ContextResolver
• Lifecycle:
• By default single instance of each provider class is instantiated for each JAX-RS application
• The JSR does not restrict other implementation specific lifecycles

18
JAX-RS annotations (most commonly used)
Annotation Target Description
@Path Class or Method Relative path for a resource
@Consumes
@Produces
Class or Method List of media types that can be consumed /
produced
@GET
@POST
@PUT
@DELETE
@OPTIONS
@HEAD
Method HTTP verb handled by the annotated method
@PathParam Parameter (also
field, POJO method)
Value that can be extracted from URI
@Context Parameter (also
field, POJO method)
Inject contextual information – UriInfo,
HttpHeaders, Request, ContextResolver, etc.

19
JAX-RS complete example
@Path("/users")
public class UserHandler {
@GET
@Path("{id}")
@Produces("application/xml”)
public JaxbUser getUser(@PathParam("id") long id) {
...
}

20
@Path("/users")
@GET
@Path("{id}")
...
}
Base URI path to resource

21
@Path("/users")
@GET
@Path("{id}")
...
}
The HTTP method for
getUser() method

22
@Path("/users")
@GET
@Path("{id}")
...
}
URI path
segment/parameter

23
@Path("/users")
@GET
@Path("{id}")
...
}
GET …/users/{id}

24
@Path("/users")
@GET
@Path("{id}")
...
}
Returned Content-Type

25
@Path("/users")
@GET
@Path("{id}")
...
}
Injects the value of URI
segment into the id
parameter

26
@Path("/users")
@GET
@Path("{id}")
...
}
GET http://localhost:9000/users/2
Content-Type: application/xml
<?xml version="1.0" encoding="UTF-8"?>
<User> … </User>

29
Advanced topics
 Regular expressions in @Path
 @*Params
 ExceptionMapper
 MessageBodyReader/MessageBodyWriter

30
@Path and regular expression mappings
 @Path(path-expression)
"{" variable-name [ ":" regular-expression ] "}“
 Examples
• @Path("example2/{var:.+}") matches following:
• …/example2/a
• …/example2/a/b/c
• @Path("example3/{var:d+}") matches following:
• …/example3/345
• …/example3/23
• @Path(“example4/{name}-{id}”) matches following:
• …/example4/a-1
• …/example4/a----1

31
@[Query|Header|Matrix|Cookie|Form]Param
 @QueryParam
• For example …/query?sorted=true
@GET
@Path("query")
public String queryParamExample(@QueryParam(“sorted") String var) {
return var;
}
 @MartixParam
• For example …/users;sorted=true
 @HeaderParam
• For example “Date” header
 @CookieParam / @FormParam
 @DefaultValue – can be used with any @*Param to specify the default
value

32
Exception Mappers
package javax.ws.rs.ext
public interface ExceptionMapper<E extends Throwable> {
Response toResponse(E exception);
}

33
MessageBodyReader
 MessageBodyReader
 MessageBodyWriter
public interface MessageBodyReader<T> {
boolean isReadable(Class<?> type, Type genericType,
Annotation annotations[], MediaType mediaType);
T readFrom(Class<T> type, Type genericType,
Annotation annotations[], MediaType mediaType,
MultivaluedMap<String, String> httpHeaders,
InputStream entityStream) throws IOException, WebApplicationException;
}

34
MessageBodyWriter
public interface MessageBodyWriter<T> {
boolean isWriteable(Class<?> type, Type genericType,
Annotation annotations[], MediaType mediaType);
long getSize(T t, Class<?> type, Type genericType, Annotation annotations[],
MediaType mediaType);
void writeTo(T t, Class<?> type, Type genericType, Annotation annotations[],
MediaType mediaType,
MultivaluedMap<String, Object> httpHeaders,
OutputStream entityStream) throws IOException, WebApplicationException;
}

35
JAX-RS features that were not covered
 PathSegment
 @Encoded
 @ApplicationPath
 Cookies
 Complex negotiation / Variant processing (javax.ws.rs.core.Variant):
 @OPTIONS, @HEAD, @HttpMethod
 SecurityContext
 CacheControl (+ Etags, conditional GET/PUT)
 Application (+ how to deploy in servlet container, EE6, EJB)
 Integration with Spring
 Security

36
Several other topics not covered
 Restful java clients
 WADL
 REST support in Spring 3.0

37
Additional materials
 Apache CXF (JAX-RS part): http://cxf.apache.org/docs/jax-rs.html
 RESTEasy users guide:
http://docs.jboss.org/resteasy/docs/1.1.GA/userguide/pdf/RESTEas
y_Reference_Guide.pdf
 WizTools REST Client: http://code.google.com/p/rest-client/

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