Sample code showing how to properly dedupe modules and bindings in Guice
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Guice Deduplication

Guice modules allow nesting through the install method. It is useful to have modules that are related/dependent install each other, to reduce the burden on the consumers of your library and/or application.

However you might have run into this very annoying problem:

Error while trying to start the application Unable to create injector, see the following errors: 
   1) A binding to SomeClass was already configured at SomeModule.someClassProvider() 
   (via modules: ModuleA -> SomeModule).

The code here shows the proper way to solve this problem, along with a broken implementation to demonstrate it.


Guice dedupes modules by using equals/hashcode. The problem with the default AbstractModule is that it uses the default equals/hashcode which does object reference only. That means multiple new instantiations of the same module are actually not equal and do not get deduped.

If you don't use @Provides, you'll find the deduplication works though because Guice will still do deduplication of the bindings. The problem is when you introduce @Provides cause now two modules provide the same interface through a method!

If you'd like to add deduplication to your codebase, simply change your AbstractModule to inherit from SingletonModule.