Understanding REST APIs - What are Context and @Context?

Question

I recently went through restful web services tutorial, but couldn't understand what a context is. Can someone explain what it it and also what @Context does?

Answer 1

JAX-RS provides the @Context annotation to inject 12 object instances related to the context of the HTTP request and they are:

• SecurityContext - Security context instance for the current HTTP request
• Request - Used for setting precondition request processing
• Application, Configuration, and Providers -> Provide access to the JAX-RS application, configuration, and providers instances
• ResourceContext - Resource contect aclass instances
• ServletConfig - The ServletConfig instance instance
• ServletContext - The ServletContext instance
• HttpServletRequest - The HttpServletRequest instance for the current request
• HttpServletResponse - The HttpServletResponse instance for the current request
• HttpHeaders - Maintains the HTTP header keys and values
• UriInfo - Query parameters and path variables from the URI called

It is a little confusing to have both an @Inject (or @Autowired in Spring) and @Context that does the same job, but it is hoped to bring more alignment to Java EE in the next edition. In the meantime, you will have to make do.

An interesting feature is that all of these instances can be injected as a field value or directly into the resource method.

An example of injection into the resource method parameter list:

@Path("/")
public class EndpointResource {

  @GET
  @Produces(MediaType.APPLICATION_JSON)
  public Response getAllHttpHeaders(final @Context HttpHeaders httpHeaders){
      // Code here that uses httpHeaders
  }
}

An example of injection into a field:

@Path("/")
public class EndpointResource {

  private final @Context HttpHeaders httpHeaders;

  @GET
  @Produces(MediaType.APPLICATION_JSON)
  public Response getAllHttpHeaders(){
      // Code here that uses httpHeaders
  }
}

If you want to know more, take a look at this series of articles answering the question What is @Conext in JAX-RS used for?