How does Grails make `params` available to a Singleton controller? (Grails internals)

Question

If I make a Grails controller a singleton via:

static scope = "singleton"

... how does Grails expose a params variable to my controller's actions, where the params are request specific?

I would understand if params were passed into my action methods as a variable, but here params are just available and in-scope (and different for simultaneous requests, despite the fact that there's only one instance of my controller).

How is this implemented under the hood?

Answer 1

Each request gets its own thread, so the request, response, params, session, etc. are available independent of whether the controller is a singleton or created new for every request. It would be different if these variables were fields in the class, but they're not.

Under the hood this is implemented by an AST transform that mixes in org.codehaus.groovy.grails.plugins.web.api.ControllersApi into the controller classes, which adds methods such as getParams() (which you can use as the params property). These call RequestContextHolder.currentRequestAttributes() to get the thread-local information.