Resolution Rules

When DDI encounters an @Inject-annotated field, it walks a deterministic decision tree to pick one class or producer to instantiate. This page is the exact rule set, lifted from the project README and grouped by the kind of target you injected.

The rules use four building blocks:

Concept	What it is
Impl	A class implementing the injected interface.
Producer	A method or class annotated `@Produces`, returning the interface or an impl.
ClassResolver	A `ClassResolver<T>` annotated `@ResponsibleFor(T.class)` — runs on every lookup and returns the class to use.
ProducerResolver	A resolver that picks between multiple producers for the same impl.

When you inject an interface

@Inject Service service;

Setup	Resolution
0 Impl	Exception — nothing to inject.
0 Impl, 1 Producer for the interface	The producer is used.
1 Impl, 0 Producer	The single impl is used.
1 Impl, 1 Producer for the impl	The producer for the impl is used.
1 Impl, n Producers for the impl	Exception — ambiguous.
1 Impl, 1 Producer for the interface	The producer for the interface is used.
1 Impl, n Producers for the interface	Exception — ambiguous.
1 Impl, 1 Producer for the interface + 1 Producer for the impl	Exception — ambiguous.
n Impls, no resolver	Exception — ambiguous.
n Impls, 1 responsible ClassResolver	The class returned by the resolver is used.
n Impls, 1 Producer for the interface	The producer for the interface is used.
n Impls, 1–n Producers for an impl	Exception — ambiguous.
n Impls, 1 ClassResolver + 1 Producer for the interface	The producer for the interface is used.
n Impls, 1 ClassResolver + 1–n Producers for impls	The class returned by the resolver is used, or its matching producer if one exists.
n Impls, n responsible ClassResolvers	Exception — conflicting responsibility.
1 Impl, n Producers for impl, 1 ProducerResolver	The producer selected by the ProducerResolver is used.
n Impls, 1 ClassResolver, 0–n Producers per impl, 1 ProducerResolver per impl	The producer selected by the ProducerResolver is used, for the impl selected by the ClassResolver.

When you inject an implementation directly

@Inject FriendlyGreeter greeter;

Setup	Resolution
Impl, no producer	The impl is used.
Impl, 1 producer	The producer is used.

Reading the table

The two principles behind every row:

One responsibility per interface. Two ClassResolvers claiming the same interface is an error, not a merge. Same for ambiguous producer sets.
Producers win over plain instantiation; resolvers win over producers. A ClassResolver chooses which class; if that class has a producer, the producer runs. A ProducerResolver only enters the picture when multiple producers exist for the same impl.

If your setup hits one of the “Exception” rows, the resolution model is ambiguous on purpose — DDI refuses to guess. Add a ClassResolver, ProducerResolver, or remove the redundant producer to make the choice explicit.

Where the rules apply

These rules run every time DDI resolves an injection point — bootstrap, test, hot-swap, retry. That’s what makes the resolver path useful for pricing-tier routing, feature flags and runtime mocking.