基于运行时条件的Spring开关实现

Question

This is a simplified version of what I am trying to achieve. I have multiple implementations of the same interface. Based on the user input at runtime I want to pick the correct implementation.

For example suppose I an interface called Color. There are many classes that implement this interface, the Red class, the Blue class, the Green class and so on.

At run time I need to pick implementations based on the user input. One way to achieve this would be something like this

 @Autowired
 @Qualifier("Red")
 private Color redColor;

 @Autowired
 @Qualifier("Green")
 private Color greenColor;


private Color getColorImplementation()
{
if(userInput=="red")
{
return redColor;

}
else if(userInput=="green")
{
return greenColor;
}
else
{
return null;
}

}

But the problem with this is that everytime a new implementation is added, I would have to update the code that picks the implementation, which beats the whole purpose of inversion of control part of spring. What is the right way to do this using spring?

Answer 1

You could autowire all implementations of the interface in question and then decide based on properties provided by interface which to use.

@Autowired
private List<Color> colors;

public void doSomething(String input) {
    colors.stream().filter(c -> c.getName().contains(input)).findFirst().ifPresent(c -> {
        // something
    }
}

This is also less magical and more in line with OO principles. Dependency injection is to wire up things initially, not for dynamic switching at runtime.

Answer 2

You want to Autowire the ApplicationContext , then you can get all the Color beans with Map<String, Color> colors = appContext.getBeansOfType(Color.class); . This presumes that the userInput and the bean name are identical.

If that isn't the case, a solution would be to add a getName() to the Color interface; then you can autowire a List<Color> and construct the Map yourself.

Can't you make the Color an Enum?

Answer 3

The Spring ServiceLocatorFactoryBean (scroll down to the middle) API was built just for this purpose:

1. Create a dummy interface ( ColorFactory ) that provides a single method such as Color getColor(String color)
2. Create the proxy bean instance for org.springframework.beans.factory.config.ServiceLocatorFactoryBean passing ColorFactory as the serviceLocatorInterface parameter
3. Define beans for all of your color implementations with names matching the parameter you'd like to pass to getColor
4. Inject the factory into the collaborators and invoke getColor as needed

You could contrive this with similar APIs on the ApplicationContext , but the advantage of this approach is that it abstracts Spring from your Java implementation (for XML configured projects).

Answer 4

Same issue happen in my implementation where in, the scenario was based on user input, where the respective interface implementation needs to be invoked.

This solve my problem:

        **Base Interface**
        @Service
        public interface ParentInterface {
                public String doThis(ClassA param);
        }

        **First Implementation**

        @Component("FirstImp")
        public class FirstServiceImp implements ParentInterface {
        public String doThis(ClassA param){

        }
        **Second Implementation**

        @Component("SecondImp")
        public class SecondServiceImp implements ParentInterface {
        public String doThis(ClassA param){

        }

        **Factory**
        @Service
        public class ServiceResolver {

        @Autowired
        @Qualifier("FirstImp")
        private ParentInterface firstImpl;

        @Autowired
        @Qualifier("SecondImp")
        private ParentInterface secondImpl;

        public ParentInterface getInstance(String condition){
            switch(condition) {
            case "X": return firstImpl;
            case "Y": return secondImpl;
            default:
                throw new IllegalArgumentException(condition);
        }
        }
        }
        **Controller**
        @RestController
        public class UserController {
        @Resource
        private ServiceResolver serviceresolver;

        @PostMapping("/userbase/{inp1}/messages/{inptype}")
        public ResponseEntity<String> sendData(@PathVariable String 
        inp1,@PathVariable String inptype, @RequestBody XYZBean msg) 
        {
        for(ABC data : msg.getSubData())
            serviceresolver.getInstance(data.getType()).doThis(msg);
         return new ResponseEntity<String>("created",HttpStatus.OK);
        }

        }