how to handle multiple beans of same type

In Spring, it’s common to have multiple beans of the same type. When you do, you may encounter issues with @Autowired or dependency injection because Spring doesn’t know which bean to inject. Here are several ways to handle multiple beans of the same type in Spring:

1. Using @Qualifier

• You can use @Qualifier with @Autowired to specify which bean you want to inject by its name.

  @Component public class MyServiceOne implements MyService { /* ... */ } @Component public class MyServiceTwo implements MyService { /* ... */ } @Service public class MyClientService { private final MyService myService; @Autowired public MyClientService(@Qualifier("myServiceOne") MyService myService) { this.myService = myService; } }

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  @Component public class MyServiceOne implements MyService { /* ... */ }   @Component public class MyServiceTwo implements MyService { /* ... */ }   @Service public class MyClientService {    private final MyService myService;      @Autowired    public MyClientService(@Qualifier("myServiceOne") MyService myService) {        this.myService = myService;    } }

Here, @Qualifier("myServiceOne") specifies that myServiceOne bean should be injected into myService.

2. Using @Primary

• If you want one bean to be injected by default, you can use the @Primary annotation. Spring will inject the primary bean when no @Qualifier is specified.

  @Component @Primary public class MyPrimaryService implements MyService { /* ... */ } @Component public class MySecondaryService implements MyService { /* ... */ } @Service public class MyClientService { @Autowired private MyService myService; // Will inject MyPrimaryService }

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  @Component @Primary public class MyPrimaryService implements MyService { /* ... */ }   @Component public class MySecondaryService implements MyService { /* ... */ }   @Service public class MyClientService {    @Autowired    private MyService myService;  // Will inject MyPrimaryService }

In this example, MyPrimaryService is marked with @Primary, so it’s used as the default bean unless another specific one is requested.

3. Injecting All Beans of a Type with @Autowired and @Qualifier

• You can inject a collection of all beans of a specific type using @Autowired on a List or Map. This approach is helpful if you want to access all beans of a certain type.

  @Service public class MyClientService { @Autowired private List<MyService> myServices; // All MyService beans will be injected here }

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  @Service public class MyClientService {    @Autowired    private List<MyService> myServices;  // All MyService beans will be injected here }

Alternatively, you can inject a map with bean names as keys and the beans as values:

4. Using Custom Qualifiers

• You can create custom qualifiers to make the code more readable and intuitive, especially when there are many beans of the same type.

  @Qualifier @Retention(RetentionPolicy.RUNTIME) @Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD }) public @interface MyServiceOne {} @Component @MyServiceOne public class MyServiceOneImpl implements MyService { /* ... */ } @Service public class MyClientService { @Autowired @MyServiceOne private MyService myService; // Will inject MyServiceOneImpl }

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  @Qualifier @Retention(RetentionPolicy.RUNTIME) @Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD }) public @interface MyServiceOne {}   @Component @MyServiceOne public class MyServiceOneImpl implements MyService { /* ... */ }   @Service public class MyClientService {    @Autowired    @MyServiceOne    private MyService myService;  // Will inject MyServiceOneImpl }

5. Using @Resource

• You can use @Resource to specify the bean by its name. @Resource works similarly to @Autowired with @Qualifier, but it’s part of Java’s javax.annotation package.

  @Service public class MyClientService { @Resource(name = "myServiceOne") private MyService myService; // Injects the myServiceOne bean }

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  @Service public class MyClientService {    @Resource(name = "myServiceOne")    private MyService myService;  // Injects the myServiceOne bean }

6. Using Conditional Beans with @Conditional

• Sometimes, you only want one of the beans to be instantiated based on certain conditions. You can use @Conditional with a custom condition or Spring’s built-in conditions to create conditional beans.

  @Component @Conditional(MyCustomCondition.class) public class ConditionalServiceOne implements MyService { /* ... */ } @Component @ConditionalOnProperty(name = "myapp.feature.enabled", havingValue = "true") public class ConditionalServiceTwo implements MyService { /* ... */ }

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  @Component @Conditional(MyCustomCondition.class) public class ConditionalServiceOne implements MyService { /* ... */ }   @Component @ConditionalOnProperty(name = "myapp.feature.enabled", havingValue = "true") public class ConditionalServiceTwo implements MyService { /* ... */ }

In this example, ConditionalServiceOne or ConditionalServiceTwo beans will only be instantiated if certain conditions are met.

7. Using Profiles

• If different beans should be active in different environments, you can annotate them with @Profile.

  @Component @Profile("dev") public class DevService implements MyService { /* ... */ } @Component @Profile("prod") public class ProdService implements MyService { /* ... */ } @Service public class MyClientService { @Autowired private MyService myService; // Will inject either DevService or ProdService based on active profile }

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  @Component @Profile("dev") public class DevService implements MyService { /* ... */ }   @Component @Profile("prod") public class ProdService implements MyService { /* ... */ }   @Service public class MyClientService {    @Autowired    private MyService myService;  // Will inject either DevService or ProdService based on active profile }

Here, only one of the beans will be loaded depending on the active Spring profile (e.g., dev or prod).

Choosing the Right Strategy

The best approach depends on your needs:

• Use @Primary for a single default implementation.
• Use @Qualifier or custom qualifiers to specify a particular bean.
• Use @Profile for environment-specific beans.
• Use @Conditional for beans that should only be created under specific conditions.