Changing the Spring beans implementation at runtime

Question

I have an Interface and multiple implementation. I'm auto wiring the interface in classes for usage. I need to choose different implementation at runtime.

public class Util {
  public void getClient();
}

Implementations

public class UtilOne implements Util {
  public void getClient() {...}
}

public class UtilTwo implements Util {
  public void getClient() {...}
}

@Configuration
public class AppConfig {
  
  @Autowired
  @Bean
  @Primary
  public Util utilOne() {
    return new UtilOne();
  }

  @Autowired
  @Bean
  public Util utilTwo() {
    return new UtilTwo();
  }

}

@Component
public class DemoService {

  @Autowired
  private Util util;
}

For some reason if we are unable to get client in UtilOne, I want to switch to UtilTwo without restarting the app. I want to change the Util object in DemoService to UtilTwo object. Property active.util will come from DB and can we updated from UI.

Answer 1

It doesn't work this way - if you have a certain implementation of Util wired to, say, class SampleClass (which is a singleton) you can't really change the implementation of the Util to something different without restarting the application context.

So instead of going this way, I suggest an alternative. You say that under certain conditions that evaluate in runtime you want to switch implementations. What kind of condition it is? Is it possible to extract this condition decision logic?

If so, you can autowire a special DynamicUtil that will hold the reference to all the utils and will call the required util depending on the condition:

// represents all possible business 'runtime' outcomes
enum ConditionOutcome {
   A, B, C 
}
interface ConditionEvaluator {
   ConditionOutcome evaluate(); // when called in runtime will evaluate a condition that currently exists in the system
}

interface Util {
   void foo();
   ConditionOutcome relevantOfOutcome();
}

class Utill1Impl implements Util {
   public void foo() {...}
   public ConditionOutcome relevantOfOutcome() {return ConditionOutcome.A;}
}
class Utill2Impl implements Util {
   public void foo() {...}
   public ConditionOutcome relevantOfOutcome() {return ConditionOutcome.B;}
}
class Utill3Impl implements Util {
   public void foo() {...}
   public ConditionOutcome relevantOfOutcome() {return ConditionOutcome.C;}
}

class DynamicUtil {
   private final Map<ConditionOutcome, Util> possibleImpls;
   private final ConditionEvaluator evaluator;

   public class DynamicUtil(List<Util> allImplementations, ConditionEvaluator evaluator) {
      // create a map by calling the 'relevantOfOutcome' per util impl in a loop
    this.evaluator = evaluator;
   }
   
   public void foo() {
      ConditionOutcome key = evaluator.evaluate();
      // pick the relevant implementation based on evaluated key
      possibleImpls.get(key).foo();
   }
}

Now with such a design you can dynamically add new possible outcomes (along with utils that should implement them. You classes in the system will have to autowire DynamicUtil though, so effectively you'll introduce one additional level of indirection but will gain flexibility

class SampleClass { // a business class that will need to work with util capable of being changed during the runtime 
   @Autowired
   private DynamicUtil util;
   ... 
}

Answer 2

You can try approach with delegating proxy. Have a primary Util bean that is just wrapper around actual implementation and allow to change its internal delegate at runtime. In addition you can create something like manager/helper class that holds references to all actual implementation beans to simplify switching between them.

@Component
@Primary
public class DelegatingUtil implements Util {
  private Util delegate;

  public void setDelegate(Util delegate){ this.delegate = delegate; }
  public Util getDelegate(){ return delegate; }

  public void getClient() {
    return delegate.getClient();
  }
}

And where switching logic applies:

  // Use @Named or @Qualifier or any other way to obtain references to actual implementations 
  private Util defaultImpl;
  private Util fallbackImpl;

  @Autowired
  private DelegatingUtil switcher;

  public void switchToFallback(){
    this.switcher.setDelegate(this.fallbackImpl);
  }

Note, this is only schematic example, you should take care about details like bean creation order, injection with qualifiers (maybe conditional), initialization and so on.

Answer 3

Here is a simple approach based on your situation. The main idea is that read active.util property from DB by PropertyService and wrap your Utils into RouteUtil:

@Component
public class RouteUtil {

    @Autowired
    private PropertyService propertyService;  

    @Qualifier("one")
    @Autowired
    private Util utilOne;

    @Qualifier("two")
    @Autowired
    private Util utilTwo;

    public void getClient() {
        if ("one".equals(propertyService.read("active.util"))) {
            utilOne.getClient();
        } else {
            utilTwo.getClient();
        }
    }
}

and in DemoService:

@Service
public class DemoService {

  @Autowired
  private RouteUtil util;

  // RouteUtil.getClient() ...
}

You can change active.util to select which Util will be used at runtime without restarting the app.

Answer 4

Spring provides you a solution which I personally didn't like. What you can do is declare a

@MyInterface
List<MyIntercase> myimpls

Where MyInterface is your interface and list will contain all the implementations. However, I (since I didn't like this solution) wrote my own solution, where you can have a static factory that is self-populated by all implementations. So you don't have to inject all your implementations but choose them at run-time from a factory either by class name or by custom-defined name. An additional advantage is that the custom-defined name must be unique per factory. So lets say you have some staged process and for each stage you have your own interface and your own factory. So you can have the same custom defined names for your implementations of different interfaces. Say you working with text formats XML, JSON and CSV and have an interface (and related factory) for say stage-1 stage-2 stage-3. So for each stage-X inteface you can have implemetations named JSON, XML and CSV so all you have to do is have a variable called currentType that will hold one of the values - JSON, XML and CSV and for each stage you can use the factory to get the appropriate implementation:

Stage1Handler handler = stage-1-factory.getInstance(currentValue);
Stage2Handler handler = stage-2-factory.getInstance(currentValue);
Stage3Handler handler = stage-3-factory.getInstance(currentValue);

where Stage[X]Handler is your interface. But this is just an additional benifit. My solution is available in Open-source MgntUtils library. The article about this particular fiture could be found here: Non-intrusive access to "Orphaned" Beans in Spring framework Also, I describe this feature in my library javadoc here. The library could be found as Maven artifact and on Github including source code and Javadoc