Handle de-duping of Modules (the "diamond" problem)

[gissuebot]

From kevinb9n on February 26, 2007 22:17:14

if module A installs B and C and B and C both install D, you'll get errors
and you won't be able to create the container.  We should be able to detect
this situation somehow and handle it correctly.

Original issue: http://code.google.com/p/google-guice/issues/detail?id=31

[gissuebot]

From dhanji on April 17, 2007 04:44:51

how is this any different from A installing D twice?

Isnt that supposed to be disallowed? Or would the contract change to re-registering
registered modules is silently ignored? If so, what is the strategy for comparing
module D at time t1 vs t2?

An unwitting user would think he has "overridden" D-t1 by D-t2.

I think Im missing something here...

[gissuebot]

From kevinb9n on April 19, 2007 11:26:42

A installing D twice should be harmless.  It's not essential for it to be harmless,
but it would become harmless if we fix this issue, and I think making it harmless
is... uhh, harmless.

I believe it's going to be too burdensome on someone with a large and complex
codebase like we have in my company to keep his/her module inclusion graph as a
strict hierarchy/tree.  I think it would cause a lot of ugliness to happen to work
around the restriction.

Luckily I think the solution to this is simple; we'll just keep a hash set of Modules
we've installed.  Now, if a user in one place installs "new MyModule()" and in
another place also installs "new MyModule()", it will still fail.  But at least now
we have a simple fix -- we advise the user to add an equals() method to the module
(or, I suppose, share the instance somehow).

I think this is the best we can do since we never know when someone might have a
module class which is actually parameterized with some constructor args that control
what stuff gets bound.

[gissuebot]

From kevinb9n on April 20, 2007 12:04:40

Well, I have this working now.  I'm not sure if it's awesome, though.  You, of
course, have to override both equals() and hashCode().  So I dunno if anyone else has
any better ideas... if not, I'll submit what I've got.

[gissuebot]

From kevinb9n on April 29, 2007 11:39:35

Fixed r320 .

Status: Fixed