REST Web Services

REST stands for REpresentational State Transfer. REST is an architectural style. HTTP is a protocol which contains the set of REST architectural constraints.

Fundamentals:

·         Everything in REST is considered as a resource.

·         Every resource is identified by an URI.

·         Uses uniform interfaces. Resources are handled using POST, GET, PUT, DELETE operations which are similar to create, read, update and delete (CRUD) operations.

·         Be stateless. Every request is an independent request. Each request from client to server must contain all the information necessary to understand the request.

·         Communications are done via representations. E.g. XML, JSON

Implementations:

Jersey framework is the reference implementation JAX-RS API.

The Jersey implementation provides a library to implement Restful webservices in a Java servlet container.

On the server side Jersey provides a servlet implementation which scans predefined classes to identify RESTful resources. In your web.xml configuration file your register this servlet for your web application.

This servlet analyzes the incoming HTTP request and selects the correct class and method to respond to this request. This selection is based on annotations in the class and methods.

A REST web application consists, therefore, out of data classes (resources) and services. These two types are typically maintained in different packages as the Jersey servlet will be instructed via the web.xml to scan certain packages for data classes.

JAX-RS supports the creation of XML and JSON via the Java Architecture for XML Binding (JAXB).

Important annotations in JAX-RS:

Annotations	Description
@PATH(your_path)	Sets the path to base URL + /your_path. The base URL is based on your application name, the servlet and the URL pattern from the web.xml configuration file.
@POST	Indicates that the following method will answer to an HTTP POST request.
@GET	Indicates that the following method will answer to an HTTP GET request.
@PUT	Indicates that the following method will answer to an HTTP PUT request.
@DELETE	Indicates that the following method will answer to an HTTP DELETE request.
@Produces(MediaType.TEXT_PLAIN[, more-types])	Defines which MIME type is delivered by a method annotated with @GET. Like “text/plain”, “application/xml”, “application/json”
@Consumes(type[, more-types])	Defines which MIME type is consumed by this method.
@PathParam	Used to inject values from the URL into a method parameter. This way you inject, for example, the ID of a resource into the method to get the correct object.

@QueryParam

Used to inject query parameters into method parameter

@Provider

The @Provider annotation is used for anything that is of interest to the JAX-RS runtime, such as MessageBodyReader and MessageBodyWriter. For HTTP requests, the MessageBodyReader is used to map an HTTP request entity body to method parameters. On the response side, a return value is mapped to an HTTP response entity body by using a MessageBodyWriter.

@DefaultValue

To give default value for a method path or query parameter if it is not present in URI

@MatrixParam, @HeaderParam, @CookieParam, and @FormParam

Cookie parameters, indicated by decorating the parameter with javax.ws.rs.CookieParam, extract information from the cookies declared in cookie-related HTTP headers.Header parameters, indicated by decorating the parameter with javax.ws.rs.HeaderParam, extract information from the HTTP headers.Matrix parameters, indicated by decorating the parameter with javax.ws.rs.MatrixParam, extract information from URL path segments. Form parameters, indicated by decorating the parameter with javax.ws.rs.FormParam, extract information from a request representation that is of the MIME media type application/x-www-form-urlencoded

@Encoded

The @javax.ws.rs.Encoded annotation can be used on a class, method, or param. By default, inject @PathParam and @QueryParams are decoded. By additionally adding the @Encoded annotation, the value of these params will be provided in encoded form. public String get(@PathParam("param") @Encoded String param) {...}

@HEAD

The HEAD verb is used to issue a request for a resource without actually retrieving it. It is a way for a client to check for the existence of a resource and possibly discover metadata about it.

@OPTIONS

The OPTIONS verb is also used to interrogate a server about a resource by asking what other verbs are applicable to the resource.

For Example refer: https://github.com/dinesh028/RESTfulWS.git

Other implementations of JAX-RS:

• Apache cxf
• Restlet
• Resteasy