Spring choose bean implementation at runtime

You can move the bean injection into the configuration, as:

@Configuration
public class AppConfig {

    @Bean
    public MyService getMyService() {
        if(windows) return new MyServiceWin();
        else return new MyServiceLnx();
    }
}

Alternatively, you may use profiles windows and linux, then annotate your service implementations with the @Profile annotation, like @Profile("linux") or @Profile("windows"), and provide one of this profiles for your application.

1. Implement a custom Condition

public class LinuxCondition implements Condition {
  @Override
  public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    return context.getEnvironment().getProperty("os.name").contains("Linux");  }
}

Same for Windows.

2. Use @Conditional in your Configuration class

@Configuration
public class MyConfiguration {
   @Bean
   @Conditional(LinuxCondition.class)
   public MyService getMyLinuxService() {
      return new LinuxService();
   }

   @Bean
   @Conditional(WindowsCondition.class)
   public MyService getMyWindowsService() {
      return new WindowsService();
   }
}

3. Use @Autowired as usual

@Service
public class SomeOtherServiceUsingMyService {

    @Autowired    
    private MyService impl;

    // ... 
}

Let's create beautiful config.

Imagine that we have Animal interface and we have Dog and Cat implementation. We want to write write:

@Autowired
Animal animal;

but which implementation should we return?

enter image description here

So what is solution? There are many ways to solve problem. I will write how to use @Qualifier and Custom Conditions together.

So First off all let's create our custom annotation:

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.METHOD, ElementType.FIELD, ElementType.TYPE})
public @interface AnimalType {
    String value() default "";
}

and config:

@Configuration
@EnableAutoConfiguration
@ComponentScan
public class AnimalFactoryConfig {

    @Bean(name = "AnimalBean")
    @AnimalType("Dog")
    @Conditional(AnimalCondition.class)
    public Animal getDog() {
        return new Dog();
    }

    @Bean(name = "AnimalBean")
    @AnimalType("Cat")
    @Conditional(AnimalCondition.class)
    public Animal getCat() {
        return new Cat();
    }

}

Note our bean name is AnimalBean. why do we need this bean? because when we inject Animal interface we will write just @Qualifier("AnimalBean")

Also we crated custom annotation to pass the value to our custom Condition.

Now our conditions look like this (imagine that "Dog" name comes from config file or JVM parameter or...)

   public class AnimalCondition implements Condition {

    @Override
    public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
        if (annotatedTypeMetadata.isAnnotated(AnimalType.class.getCanonicalName())){
           return annotatedTypeMetadata.getAnnotationAttributes(AnimalType.class.getCanonicalName())
                   .entrySet().stream().anyMatch(f -> f.getValue().equals("Dog"));
        }
        return false;
    }
}

and finally injection:

@Qualifier("AnimalBean")
@Autowired
Animal animal;

Autowire all your implementations into a factory with @Qualifier annotations, then return the service class you need from the factory.

public class MyService {
    private void doStuff();
}

My Windows Service:

@Service("myWindowsService")
public class MyWindowsService implements MyService {

    @Override
    private void doStuff() {
        //Windows specific stuff happens here.
    }
}

My Mac Service:

@Service("myMacService")
public class MyMacService implements MyService {

    @Override
    private void doStuff() {
        //Mac specific stuff happens here
    }
}

My factory:

@Component
public class MyFactory {
    @Autowired
    @Qualifier("myWindowsService")
    private MyService windowsService;

    @Autowired
    @Qualifier("myMacService")
    private MyService macService;

    public MyService getService(String serviceNeeded){
        //This logic is ugly
        if(serviceNeeded == "Windows"){
            return windowsService;
        } else {
            return macService;
        }
    }
}

If you want to get really tricky you can use an enum to store your implementation class types, and then use the enum value to choose which implementation you want to return.

public enum ServiceStore {
    MAC("myMacService", MyMacService.class),
    WINDOWS("myWindowsService", MyWindowsService.class);

    private String serviceName;
    private Class<?> clazz;

    private static final Map<Class<?>, ServiceStore> mapOfClassTypes = new HashMap<Class<?>, ServiceStore>();

    static {
        //This little bit of black magic, basically sets up your 
        //static map and allows you to get an enum value based on a classtype
        ServiceStore[] namesArray = ServiceStore.values();
        for(ServiceStore name : namesArray){
            mapOfClassTypes.put(name.getClassType, name);
        }
    }

    private ServiceStore(String serviceName, Class<?> clazz){
        this.serviceName = serviceName;
        this.clazz = clazz;
    }

    public String getServiceBeanName() {
        return serviceName;
    }

    public static <T> ServiceStore getOrdinalFromValue(Class<?> clazz) {
        return mapOfClassTypes.get(clazz);
    }
}

Then your factory can tap into the Application context and pull instances into it's own map. When you add a new service class, just add another entry to the enum, and that's all you have to do.

 public class ServiceFactory implements ApplicationContextAware {

     private final Map<String, MyService> myServices = new Hashmap<String, MyService>();

     public MyService getInstance(Class<?> clazz) {
         return myServices.get(ServiceStore.getOrdinalFromValue(clazz).getServiceName());
     }

      public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
          myServices.putAll(applicationContext.getBeansofType(MyService.class));
      }
 }

Now you can just pass the class type you want into the factory, and it will provide you back the instance you need. Very helpful especially if you want to the make the services generic.