Issue
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.
Solution
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;
...
}
Answered By - Mark Bramnik
Answer Checked By - Katrina (JavaFixing Volunteer)