Jersey - The @Context annotation for injection. How does it work?

Question

I was looking at a good REST tutorial using Jersey. Down the page, there is a web resource that is built which is entitled TodoResource which itself contains two instance variables

public class TodoResource {
    @Context
    UriInfo uriInfo;

    @Context
    Request request;

    String id;

    public TodoResource(UriInfo uriInfo, Request request, String id) {
        this.uriInfo = uriInfo;
        this.request = request;
        this.id = id;
    }
}

I was wondering exactly how the UriInfo and Request instance variables are initialized? I know that using the @Context annotation allows for information to be injected, but at what point does this happen? Will this be handled automatically by Jersey?

I don't know the details, but yes, Jersey will automagically initialize those variables for you behind the scenes. If you need to know the details of how it works, in Eclipse, you could put a "watchpoint" on one of those variables, which tells the debugger to break whenever that variable is modified. Then you can look at the stack trace and see what Jersey class is modifying it. — Tyler, May 26 '11 at 21:24

score 27 · Answer 1 · edited Nov 13 '19 at 13:39

27

I've run into some interesting results with the Rules of Injection, here's what I've found:

public class TodoResource{
  @Context
  UriInfo uriInfo; // Set second
  public TodoResource(@Context UriInfo value){
    uriInfo = value; // Set first (makes sense)
  }
  @Context
  public void setUriInfo(UriInfo value){
    uriInfo = value; // Set third
  }
}

I hope this helps.

edited Nov 13 '19 at 13:39

Gary Sheppard

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answered Jan 20 '12 at 14:47

Mike Summers

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score 23 · Accepted Answer · answered Jul 22 '11 at 11:25

Jersey doesn't modify the class, but it creates it on every request from the client.

After the class constructor was invoked, the context fields are injected.
(Should you try to access those fields inside the constructor, they will be null)

In your case, the class wouldn't need a specific constructor, so just:

public TodoResource () {
    // in most cases the ctor stays empty.
    // don't do much work here, remember: the ctor is invoked at every client request
}

But inside methods (which represent web-resources) annotated with @POST, @GET, ... you would have access to context fields.

Kinjeiro · Answer 3 · 2012-08-03T10:46:06.183

Use @PostConstruct method annotation:

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

@Path("foo")
public class AuthResource {
    @Context
    HttpServletRequest request;

    public AuthResource() {
        //request is null
    }

    @PostConstruct
    public void postConstruct() {
        //request is NOT null
    }

    @PreDestroy
    public void preDestroy() {
       //after rest method executing
    }
}

Jersey - The @Context annotation for injection. How does it work?

3 Answers3

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