java - Spring Boot 应用程序无法实例化类，除非使用 Autowired 注解

Question

我有一个 Spring Boot 应用程序，它有一个工厂类，用于根据字符串确定要实例化的策略。策略工厂类有 3 个构造函数。每个策略对应一个。这是该类的通用版本:

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");
        }
    }
}

当尝试实例化其中一种策略时，只有 @Autowired 策略才会被实例化。如果我尝试实例化其他的，它会返回 null。

如何实例化其他策略？

Answer 1

因为只有用 @Autowired 注解的构造函数才会被处理并注入(inject)依赖项。 Strategy2 和 Strategy3 的构造函数将被忽略，因为上面没有 @Autowired。

您有两个选择:

(1) 更改为使用字段注入(inject)而不是构造函数注入(inject):

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

    StrategyFactory(){}
}

(2) 继续使用构造函数注入(inject)。由于所有策略都是 GenericStrategy ，因此您可以在构造函数中自动连接 GenericStrategy 列表。然后检查其类以返回实际实例。

代码如下所示:

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;
    }
}