JAX-RS: getClasses vs getSingletons

Question

I'm using JAX-RS (Jersey implementation) to build a web service.

The entry point to the web application is a class that extends Application and overrides the getClasses method. I understand that JAX-RS will create a new instance for each new request.

It is possible to override getSingleton instead. My understanding is that this will reuse the same instance for all requests. However, won't this approach (assuming I'm correct) destroy concurrency within the web application? That is, since the same instance is always used, incoming requests will be processed one at a time (FIFO?).

Answer 1

No, it won't. Multiple threads in JVM are able to access one instance of a class at the same time (invoke a resource method on your resource in this case). You just need to make sure your resource is thread-safe.

To illustrate the difference between getClasses() and getSingletons() lets assume we have a HelloWorldResource like:

@Path("helloworld")
public class HelloWorldResource {

    private volatile int counter = 0;

    @GET
    @Produces("text/plain")
    public String getHello() {
        return "Hello World! " + counter++;
    }

}

Multiple invocations of getHello resource method would return you:

• in case you register your resource via getClasses

  Hello World! 0
  Hello World! 0
  Hello World! 0
  ...
  

• in case you use getSingletons to register your resource

  Hello World! 0
  Hello World! 1
  Hello World! 2
  ...