Spring Boot应用程序无法实例化一个类，除非使用Autowired对其进行了注释

Question

I have a Spring Boot application that has a factory class with determines which strategy to instantiate based on a string. The strategy factory class has 3 constructors. One for each of the strategies. Here is a generalized version of the class:

public class StrategyFactory {
    private Strategy1 strategy1;
    private Strategy2 strategy2;
    private Strategy3 strategy3;

    @Autowired
    public StrategyFactory(Strategy1 strategy1) {
        this.strategy1 = strategy1;
    }

    public StrategyFactory(Strategy2 strategy2) {
        this.strategy2 = strategy2;
    }

    public StrategyFactory(Strategy3 strategy3) {
        this.strategy3 = strategy3;
    }

    public GenericStrategy getTrailerStrategy(String strategy) {
        LOGGER.info("Retrieving Strategy Class for {}", strategy);
        if (strategy.equals("CLOSE_DETAIL")) {
            return strategy1;
        } 
        else if(strategy.equals("LOAD_TRAILER")) {
            return strategy2;
        } 
        else if(strategy.equals("CLOSE_SUMMARY")) {
            return strategy3;
        } 
        else {
            throw new InvalidStrategyTypeException("Invalid Strategy Type");
        }
    }
}

When trying to instantiate one of the strategies, only the one that is @Autowired will be instantiated. If I try to instantiate the others, it returns null.

How can I get the other strategies to be instantiated?

Answer 1

Because only the constructors that are annotated with @Autowired will get processed and inject the dependencies . The constructor for Strategy2 and Strategy3 are ignored since there are no @Autowired on it.

You have two options :

(1) Change to use field injection rather than constructor injections:

public class StrategyFactory {
    @Autowired
    private Strategy1 strategy1;
    @Autowired
    private Strategy2 strategy2;
    @Autowired
    private Strategy3 strategy3;

    StrategyFactory(){}
}

(2) Keep to use constructor injection. As all Strategy is GenericStrategy , so you can auto-wire a list of GenericStrategy in constructor instead. Then check its class to return the actual instance.

The codes will look likes the followings:

public class StrategyFactory {
    private List<GenericStrategy> strategy; 

    @Autowired
    public StrategyFactory(List<GenericStrategy> strategy) {
        this.strategy = strategy1;
    }

    public GenericStrategy getTrailerStrategy(String strategy) {
        LOGGER.info("Retrieving Strategy Class for {}", strategy);
        GenericStrategy result = null; 
        if (strategy.equals("CLOSE_DETAIL")) {
            result = getStrategy(Strategy1.class);
        } 
        else if(strategy.equals("LOAD_TRAILER")) {
            result = getStrategy(Strategy2.class);
        } 
        else if(strategy.equals("CLOSE_SUMMARY")) {
           result = getStrategy(Strategy3.class);
        } 
        else {
            throw new InvalidStrategyTypeException("Invalid Strategy Type");
        }

        if(result == null){
          throw new RuntimeException("Fail to load the strategy....");
        }
    }

    private <T> GenericStrategy getStrategy(Class<T> clazz){
        for(GenericStrategy s : strategy){
          if(clazz.isInstance(s)){
            return s;
          }
        }
        return null;
    }
}