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From: Steven Simpson <ss@domain.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Wormholes
Date: Tue, 04 Sep 2012 14:18:33 +0100
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On 04/09/12 02:12, Roedy Green wrote:
> Somehow I must pass information down the long chain from x to y.  This
> means changing the signatures of all the intermediate methods, and
> adjusting code to the new way.  This can cause ripples incommensurate
> with the triviality of the change.

Assuming that you can't improve your structure or refactor, a 
ThreadLocal might be appropriate as your wormhole.

I've had to do this lately.  The intermediate steps were recursive calls 
through a hierarchy.  y was a custom implementation of a node nested in 
that hierarchy.  x started off the recursive call.

If I was only using the hierarchy once, for one call by x, I would have 
simply embedded the context in y.  But the hierarchy was to be re-used, 
and y needed a different context for each call by x.  Also, calls by x 
could be concurrent.

However, I could guarantee that any single call by x would invoke y by 
the same thread.  That allowed the context to be passed by a 
ThreadLocal, set by x just before the call.

I suppose you could do something similar without threads, e.g.:

   class Context {
     Map<ContextLocal<?>, Object> items = new IdentityHashMap<>();
   }

   class ContextLocal<T> {
     void set(Context ctxt, T val) {
       ctxt.put(this, val);
     }

     T get(Content ctxt) {
       return (T) ctxt.get(this);
     }
   }

Then you'd refactor once to pass Context through the chain.  x creates a 
static ContextLocal<MyContext> myContext, and one Context ctxt for each 
call, and sets up myContext.set(ctxt, value) before the call.  y calls 
myContext.get(ctxt) to get the value.

-- 
ss at comp dot lancs dot ac dot uk