As applications grow in complexity, it is common to have different implementations or dependencies that need to be injected based on specific conditions or configurations. In Java dependency injection frameworks, such as Spring, conditional injection allows us to define rules or conditions for injecting dependencies.

Let’s take a look at how conditional injection can be implemented in Java using the Spring framework.

1. Using @Conditional Annotation

The @Conditional annotation provided by Spring allows us to define a condition class that determines whether a bean should be created and injected. Here’s an example:

@Configuration
public class AppConfig {

    @Bean
    @Conditional(WindowsCondition.class)
    public FileManager fileManagerForWindows() {
        return new WindowsFileManager();
    }

    @Bean
    @Conditional(MacCondition.class)
    public FileManager fileManagerForMac() {
        return new MacFileManager();
    }
}

In this example, we have defined two bean methods annotated with @Conditional. The WindowsCondition and MacCondition classes are responsible for defining the conditions for injection.

2. Implementing the Conditional Classes

To implement the conditional classes, we need to implement the Condition interface from the Spring framework. Here’s an example:

public class WindowsCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return System.getProperty("os.name").toLowerCase().contains("windows");
    }
}

public class MacCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        return System.getProperty("os.name").toLowerCase().contains("mac");
    }
}

In these examples, we simply check the value of the os.name system property to determine the operating system. Based on the condition, the respective bean will be injected.

3. Testing Conditional Injection

To test the conditional injection, we can create a dummy class that depends on the FileManager bean:

@Component
public class FileProcessor {

    private final FileManager fileManager;

    public FileProcessor(FileManager fileManager) {
        this.fileManager = fileManager;
    }

    // ...
}

Spring will automatically inject the appropriate implementation of the FileManager bean based on the condition.

Conclusion

Conditional injection in Java dependency injection frameworks provides flexibility in selecting the right implementation or dependency based on specific conditions. By using the @Conditional annotation and implementing custom condition classes, we can achieve more control over dependency injection in our applications.

#java #dependencyinjection