@Autowired

About

In the Spring Framework, @Autowired is an annotation used for automatic dependency injection. It tells Spring to resolve and inject a bean into the marked field, constructor, or method based on the bean’s type, without requiring explicit configuration in XML or Java configuration classes.

Spring performs this injection at runtime by scanning the application context for a matching bean. If exactly one bean of the required type exists, it is injected automatically. If more than one candidate exists, Spring requires additional disambiguation - typically via @Primary, @Qualifier, or by matching bean names.

@Autowired is used whenever a class depends on another Spring-managed bean, and we want Spring to handle the wiring without manual instantiation. It promotes loose coupling and easier testing by allowing bean dependencies to be defined declaratively rather than imperatively in code.

Attributes

The @Autowired annotation has one primary attribute and one meta-behavior that affects how it interacts with the injection process.

Attribute

Type

Default

Description

required

boolean

true

Indicates whether the dependency is mandatory. If true and no suitable bean is found, Spring throws an exception. If false, Spring will inject null (for objects) or the default value (for primitives/wrappers).

Attribute Details

required

Default behavior (true):
- If no bean of the required type (or matching qualifiers) exists in the application context, Spring throws a NoSuchBeanDefinitionException during context initialization.
- Ensures the application fails fast when a critical dependency is missing.
Optional injection (false):
- The annotated field, parameter, or method argument can remain unset if no candidate is found.
- Commonly used when injecting optional collaborators or when certain beans are only conditionally loaded.
- Useful for plugin-based or feature-flag-driven systems.
Caution: Optional injection may lead to NullPointerException if we forget to check for null before using the dependency.

Meta-Behavior

@Autowired as a meta-annotation

While @Autowired itself has only required as a configurable attribute, its behavior is influenced by other annotations when combined:

With @Qualifier: Narrows bean selection when multiple candidates exist.
With @Lazy: Delays bean instantiation until the dependency is first accessed.
With @Primary: Selects a default candidate when multiple matches exist.

How It Works ?

The @Autowired annotation triggers Spring’s dependency resolution and injection process during the bean creation phase of the application context lifecycle.

At a high level, Spring’s AutowiredAnnotationBeanPostProcessor detects @Autowired annotations on bean fields, constructors, and methods, and then attempts to resolve and inject a suitable bean from the application context.

Step-by-Step Internal Process

1. Bean Scanning and Detection

When Spring starts, it scans our classes for annotations like @Component, @Service, @Controller, and @Configuration (or beans defined via XML/Java config).
It also registers BeanPostProcessors, including AutowiredAnnotationBeanPostProcessor, which specifically looks for @Autowired (and similar annotations like @Inject).

2. Identifying Injection Points

For each bean being created, the post-processor:

Inspects all fields, constructors, and methods.
Marks those annotated with @Autowired as injection points.
Tracks metadata so the injection can be performed after the bean is instantiated but before it’s used.

3. Injection Type Resolution

Spring matches dependencies by type first:

Single Match → Inject directly.
Multiple Matches → Apply narrowing rules in order:
- @Qualifier match (if present at injection point).
- Bean name match (field name or parameter name matches a bean id).
- @Primary preference (if exactly one primary exists).
No Match →
- If required=true, throw NoSuchBeanDefinitionException.
- If required=false, inject null (or default primitive value).

4. Constructor Injection Priority

If a bean has a single constructor, Spring will use it for injection automatically (even without @Autowired).
If multiple constructors exist, we must annotate exactly one with @Autowired so Spring knows which one to use.
Constructor injection ensures all required dependencies are available at instantiation time, making the bean immutable after creation.

5. Field Injection Mechanics

Spring uses reflection to set private/protected/public fields.
Field injection happens after the bean is constructed but before any @PostConstruct methods run.
This approach is convenient but makes dependencies less explicit — constructor injection is often preferred in complex systems.

6. Method (Setter) Injection Mechanics

Spring identifies setter methods (or arbitrary methods) annotated with @Autowired.
Parameters are resolved like constructor parameters — by type first, then by qualifier/name.
Useful for optional dependencies that may change or for legacy beans needing backward-compatible wiring.

7. Handling Optional Dependencies

If a dependency is optional, we can:
- Set @Autowired(required=false)

8. Circular Dependency Handling

For singleton beans, Spring can resolve circular dependencies between fields and setters via early references from the singleton cache.
However, constructor-based circular dependencies are not resolvable and will cause a BeanCurrentlyInCreationException.
Avoiding circular dependencies is generally recommended - it indicates tight coupling between components.

9. Lazy Injection

If combined with @Lazy, the dependency is injected as a proxy, and the target bean is created only when first used.
This can break dependency cycles or reduce startup time for expensive beans.

Best Practice

Correct usage of @Autowired can greatly improve code clarity and maintainability, while poor usage can lead to brittle, tightly coupled systems. The following guidelines help ensure @Autowired is used effectively in production applications.

1. Prefer Constructor Injection Over Field Injection

Why: Constructor injection makes dependencies explicit, supports immutability, and allows for easier unit testing without reflection.
Spring automatically injects a single constructor without needing @Autowired, but adding it explicitly improves readability.
Example:

@Component
class OrderService {
    private final PaymentService paymentService;

    @Autowired
    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

2. Avoid Field Injection in Complex Systems

Field injection is convenient for quick prototypes but hides dependencies, making code harder to test and refactor.
If we must use field injection (e.g., in legacy beans or configuration classes), keep it to simple, non-critical dependencies.

3. Keep Dependencies Minimal

If a bean requires many @Autowired dependencies, it may be violating the Single Responsibility Principle.
Consider splitting the bean into smaller components with clearer purposes.

4. Use `@Qualifier` When Multiple Beans Exist

When multiple beans of the same type exist, always disambiguate with @Qualifier or @Primary.
This avoids accidental bean resolution changes if another bean of the same type is introduced later.

5. Avoid Circular Dependencies

Circular dependencies indicate strong coupling and make code harder to maintain.
If unavoidable, use setter injection or @Lazy injection to break the cycle but redesign is usually the better fix.

6. Use `@Lazy` for Expensive Beans

If a dependency is costly to initialize or rarely used, annotate it with @Lazy to defer creation until needed.
This can speed up application startup and reduce memory footprint.

7. Limit @Autowired Usage in Configuration Classes

In @Configuration classes, prefer constructor injection for dependencies rather than field injection.
This ensures all dependencies are ready before bean creation and avoids subtle lifecycle issues.

8. Do Not Overuse `@Autowired`

Use it for true dependencies, not just for convenience to access a bean.
If a dependency can be passed as a method argument or handled locally, avoid injecting it.

9. Test Dependency Wiring Explicitly

For integration tests, ensure the Spring context loads with the intended dependencies wired in.
Use @MockBean or @TestConfiguration to override dependencies in test scenarios without affecting production beans.

10. Handle Optional Dependencies Safely

Instead of @Autowired(required=false), prefer:
- Optional<T> injection (since Spring 4.3)
- @Nullable injection (since Spring 5.0)
These approaches make optionality explicit and reduce null-handling errors.

Example

1. Field Injection (Basic Usage)

@Component
class OrderService {

    @Autowired
    private PaymentService paymentService;

    public void processOrder() {
        paymentService.pay();
    }
}

Explanation

Spring finds a bean of type PaymentService and injects it into the private field, even without a setter or constructor.
Convenient for small projects, but hides dependencies (less test-friendly).

2. Constructor Injection (Preferred)

@Component
class OrderService {

    private final PaymentService paymentService;

    @Autowired
    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

Explanation

Dependencies are made explicit and required at bean creation time.
If there’s only one constructor, Spring can inject it without the @Autowired annotation, but adding it makes intent clear.

3. Multiple Constructor Scenario

@Component
class OrderService {

    private final PaymentService paymentService;

    @Autowired
    public OrderService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    public OrderService() {
        this.paymentService = null;
    }
}

Explanation

If multiple constructors exist, we must annotate the one we want Spring to use.
Without @Autowired, Spring cannot decide which constructor to use.

4. Setter Injection

@Component
class OrderService {

    private PaymentService paymentService;

    @Autowired
    public void setPaymentService(PaymentService paymentService) {
        this.paymentService = paymentService;
    }
}

Explanation

Useful when we want to inject dependencies after object creation, or when they are optional.
More flexible, but less strict than constructor injection.

5. With `@Qualifier` to Resolve Ambiguity

@Component
class OrderService {

    @Autowired
    @Qualifier("fastPaymentService")
    private PaymentService paymentService;
}

Explanation

When multiple beans implement PaymentService, @Qualifier ensures the correct one is injected.
Without this, Spring throws NoUniqueBeanDefinitionException.

6. Optional Dependency with `required=false`

@Component
class OrderService {

    @Autowired(required = false)
    private DiscountService discountService;
}

Explanation

If DiscountService bean doesn’t exist, no error is thrown; discountService will be null.
Risk: Must check for null before use.

7. Optional Dependency with `Optional<T>`

@Component
class OrderService {

    @Autowired
    private Optional<DiscountService> discountService;
}

Explanation

Since Spring 4.3, Optional injection avoids null checks.
Use discountService.ifPresent(...) to safely run code when the bean exists.

8. Optional Dependency with `@Nullable`

@Component
class OrderService {

    @Autowired
    public void setDiscountService(@Nullable DiscountService discountService) {
        // discountService may be null
    }
}

Explanation

Since Spring 5.0, @Nullable can be applied to parameters to indicate the bean is optional.

9. Lazy Injection

@Component
class OrderService {

    @Autowired
    @Lazy
    private PaymentService paymentService;
}

Explanation

Bean creation is deferred until paymentService is first accessed.
Useful for breaking circular dependencies or speeding up startup.

10. Collection Injection

@Component
class OrderService {

    @Autowired
    private List<PaymentService> paymentServices;

    @Autowired
    private Map<String, PaymentService> paymentServiceMap;
}

Explanation

Injects all beans of a given type into a List or Map.
List is ordered by @Order or bean definition order.
Map keys are bean names.

11. Self-Defined Bean Selection with Collection Filtering

@Component
class FastPaymentService implements PaymentService { }

@Component
class SlowPaymentService implements PaymentService { }

@Component
class OrderService {

    @Autowired
    @Qualifier("fastPaymentService")
    private PaymentService paymentService;

    @Autowired
    private List<PaymentService> allPaymentServices;
}

Explanation

Injects one specific bean via qualifier, but also injects all available beans into a collection.

12. Injection in `@Configuration` Class

@Configuration
class AppConfig {

    private final PaymentService paymentService;

    @Autowired
    public AppConfig(PaymentService paymentService) {
        this.paymentService = paymentService;
    }

    @Bean
    public OrderService orderService() {
        return new OrderService(paymentService);
    }
}

Explanation

Constructor injection in configuration classes ensures dependencies are ready when defining beans.

13. Circular Dependency Example (Setter-Based Fix)

@Component
class ServiceA {

    private ServiceB serviceB;

    @Autowired
    public void setServiceB(ServiceB serviceB) {
        this.serviceB = serviceB;
    }
}

@Component
class ServiceB {

    private final ServiceA serviceA;

    @Autowired
    public ServiceB(ServiceA serviceA) {
        this.serviceA = serviceA;
    }
}

Explanation

Field or setter injection can resolve circular dependencies between beans of singleton scope.
Constructor-based circular dependencies cause errors.

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Last updated 3 months ago

About

Attributes

Attribute Details

Meta-Behavior

How It Works ?

Step-by-Step Internal Process

Best Practice

1. Prefer Constructor Injection Over Field Injection

2. Avoid Field Injection in Complex Systems

3. Keep Dependencies Minimal

4. Use @Qualifier When Multiple Beans Exist

5. Avoid Circular Dependencies

6. Use @Lazy for Expensive Beans

8. Do Not Overuse @Autowired

9. Test Dependency Wiring Explicitly

10. Handle Optional Dependencies Safely

Example

1. Field Injection (Basic Usage)

2. Constructor Injection (Preferred)

3. Multiple Constructor Scenario

4. Setter Injection

5. With @Qualifier to Resolve Ambiguity

6. Optional Dependency with required=false

7. Optional Dependency with Optional<T>

8. Optional Dependency with @Nullable

9. Lazy Injection

10. Collection Injection

11. Self-Defined Bean Selection with Collection Filtering

12. Injection in @Configuration Class

13. Circular Dependency Example (Setter-Based Fix)

4. Use `@Qualifier` When Multiple Beans Exist

6. Use `@Lazy` for Expensive Beans

8. Do Not Overuse `@Autowired`

5. With `@Qualifier` to Resolve Ambiguity

6. Optional Dependency with `required=false`

7. Optional Dependency with `Optional<T>`

8. Optional Dependency with `@Nullable`

12. Injection in `@Configuration` Class