Inject

• Backported from jakarta inject -

Identifies injectable constructors, methods, and fields. May apply to static as well as instance members. An injectable member may have any access modifier (private, package-private, protected, public). Constructors are injected first, followed by fields, and then methods. Fields and methods in superclasses are injected before those in subclasses. Ordering of injection among fields and among methods in the same class is not specified.

Injectable constructors are annotated with {@code @Inject} and accept zero or more dependencies as arguments. {@code @Inject} can apply to at most one constructor per class.

@Inject ConstructorModifiersopt SimpleTypeName(FormalParameterListopt) Throwsopt ConstructorBody

{@code @Inject} is optional for public, no-argument constructors when no other constructors are present. This enables injectors to invoke default constructors.

{@literal @}Injectopt Annotationsopt public SimpleTypeName() Throwsopt ConstructorBody

Injectable fields:

• are annotated with {@code @Inject}.
• are not final.
• may have any otherwise valid name.

@Inject FieldModifiersopt Type VariableDeclarators;

Injectable methods:

• are annotated with {@code @Inject}.
• are not abstract.
• do not declare type parameters of their own.
• may return a result
• may have any otherwise valid name.
• accept zero or more dependencies as arguments.

@Inject MethodModifiersopt ResultType Identifier(FormalParameterListopt) Throwsopt MethodBody

The injector ignores the result of an injected method, but non-{@code void} return types are allowed to support use of the method in other contexts (builder-style method chaining, for example).

Examples:

public class Car {
    // Injectable constructor
    @Inject public Car(Engine engine) { ... }

    // Injectable field
    @Inject private Provider<Seat> seatProvider;

    // Injectable package-private method
    @Inject void install(Windshield windshield, Trunk trunk) { ... }
}

A method annotated with {@code @Inject} that overrides another method annotated with {@code @Inject} will only be injected once per injection request per instance. A method with no {@code @Inject} annotation that overrides a method annotated with {@code @Inject} will not be injected.

Injection of members annotated with {@code @Inject} is required. While an injectable member may use any accessibility modifier (including private), platform or injector limitations (like security restrictions or lack of reflection support) might preclude injection of non-public members.

Qualifiers

A {@linkplain Qualifier qualifier} may annotate an injectable field or parameter and, combined with the type, identify the implementation to inject. Qualifiers are optional, and when used with {@code @Inject} in injector-independent classes, no more than one qualifier should annotate a single field or parameter. The qualifiers are bold in the following example:

public class Car {
    @Inject private @Leather Provider<Seat> seatProvider;

    @Inject void install(@Tinted Windshield windshield,
        @Big Trunk trunk) { ... }
}

If one injectable method overrides another, the overriding method's parameters do not automatically inherit qualifiers from the overridden method's parameters.

Injectable Values

For a given type T and optional qualifier, an injector must be able to inject a user-specified class that:

1. is assignment compatible with T and
2. has an injectable constructor.

For example, the user might use external configuration to pick an implementation of T. Beyond that, which values are injected depend upon the injector implementation and its configuration.

Circular Dependencies

Detecting and resolving circular dependencies is left as an exercise for the injector implementation. Circular dependencies between two constructors is an obvious problem, but you can also have a circular dependency between injectable fields or methods:

class A {
    @Inject B b;
}
class B {
    @Inject A a;
}

When constructing an instance of {@code A}, a naive injector implementation might go into an infinite loop constructing an instance of {@code B} to set on {@code A}, a second instance of {@code A} to set on {@code B}, a second instance of {@code B} to set on the second instance of {@code A}, and so on.

A conservative injector might detect the circular dependency at build time and generate an error, at which point the programmer could break the circular dependency by injecting {@link Provider Provider<A>} or {@code Provider} instead of {@code A} or {@code B} respectively. Calling {@link Provider#get() get()} on the provider directly from the constructor or method it was injected into defeats the provider's ability to break up circular dependencies. In the case of method or field injection, scoping one of the dependencies (using {@linkplain Singleton singleton scope}, for example) may also enable a valid circular relationship.

See also