Can one module installed twice?

[Hanlu Chen]

There are four modules A, B, C, D. 

Their dependency is like graph below

   A   D

  |   \ /

  |   B

  |  /

  C

Could I install C & B in A and then install C in B again?

The experiment would just work, but I failed to find any doc supporting this. 

Thanks.

[Thomas Broyer]

IIRC, the modules need to compare equal (i.e. C.equals(C) == true), otherwise C would be installed twice and lead to duplicate bindings.

Not sure there's any doc, but I remember fixing something similar in GIN (Guice for/in GWT) a while ago.

[Tavian Barnes]

This is true, but brittle to rely on (anything using the SPI like Modules.override() will break equality).  It's better to rely on the fact that equivalent bindings get de-duplicated.

The one caveat is that to get @Provides methods to de-duplicate, the modules have to compare equal anyway.

[Andrés Testi]

You can check the ServletModule.configure() method to see a pattern to deal with twice installations: https://github.com/google/guice/blob/master/extensions/servlet/src/com/google/inject/servlet/ServletModule.java#L52