Creating a guice injector with two different modules having similar dependencies

Question

Suppose I have two modules ModuleA and ModuleB which are self-sufficient and can be used as stand-alone modules. Both the modules have a dependency on a third module ModuleC like

install(new ModuleC());

Now, I have a use case where I need to create an injector with both modules, A and B. I did this:

Guice.createInjector(new ModuleA(), new ModuleB());

It threw a CreationException as expected which said that a binding to a certain class was already configured at one of the modules. Keeping in mind that I do not have the power to alter ModuleA and ModuleB, how do I make it work?

I tried using Modules.combine(Modules... modules) but that did not solve my problem. Is there any solution out there for this?

colossatr0n · Accepted Answer · 2020-03-12 00:21:52Z

10

Consider you have the following bindings defined:

ModuleC:

C1

ModuleA

A1
A2

ModuleB

B1
B2

Now when you do Guice.createInjector(new ModuleA(),new ModuleB()) or Modules.combine(..), your final list of bindings will be:

A1
A2
B1
B2
C1 (as inherited from A)
C1 (!) (as inherited from B)

as C1 binding is listed twice, that creates a conflict and results in CreationException.

But if you use Modules.override() instead:

Returns a builder that creates a module that overlays override modules over the given modules. If a key is bound in both sets of modules, only the binding from the override modules is kept.

E.g. by doing

Modules.override(new ModuleA()).with(new ModuleB())

Your final list of bindings will be as follows:

A1
A2
B1
B2
~~C1 (as inherited from A)~~
C1 (as inherited from B)

The C1 binding inherited from ModuleA will be dropped in favor of the C1 binding as defined in ModuleB, thus resolving the conflict.

edited Mar 12, 2020 at 0:21

colossatr0n

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answered Dec 5, 2016 at 22:37

zeppelin

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3 Comments

rrrocky Over a year ago

I tried Guice.createInjector(Modules.override(new ModuleA(), new ModuleB()).with(new ModuleC()));

rrrocky Over a year ago

This worked. Guice.createInjector(Modules.override(new ModuleA()).with(new ModuleB()). Thanks. But can you elaborate your answer a bit?

MageWind Over a year ago

Downvoted because Modules.override() says "Prefer to write smaller modules that can be reused and tested without overrides."

MageWind · Accepted Answer · 2022-09-23 15:59:52Z

-1

The documentation for Modules.override() says "Prefer to write smaller modules that can be reused and tested without overrides." For production code, here are a few alternatives:

Refactor ModuleA and ModuleB to not install ModuleC and install ModuleC when you create the injector. This makes it more clear that bindings in ModuleC are shared between modules A and B.
Make ModuleA and ModuleB each a PrivateModule so that the ModuleC bindings are not shared and, therefore, cannot conflict. The potential downside is that now ModuleA and ModuleB will not share bindings (e.g. could be problematic for Singletons and Multibinders).
Annotate the bindings in ModuleA &or ModuleB so that they do not share bindings. Then, when they install ModuleC, they tell ModuleC (through its constructor) what the binding annotation should be. This option would be more suitable if you think the bindings for ModuleA and ModuleB could diverge in the future or if the bindings must not be shared. You could also make ModuleC a PrivateModule so that it only needs to annotate the bindings it exposes.

answered Sep 23, 2022 at 15:59

MageWind

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1 Comment

Spacejockey Over a year ago

The poster specifically said he can not alter the modules.

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