问题出现

问题描述：在Spring Boot环境下使用策略模式，调用 Spring 的 Bean 出现了空指针异常。

// 这是一个很经典的策略模式实现，但是如果这么些就一定会出现上面所述BUG
@Service
public class BussinessServiceImpl implements BussinessService {
    private final Map<Integer, Strategy> strategyMap;

    @Autowired
    public BussinessServiceImpl() {
        this.searchMap = new HashMap<>();
        // 添加具体的策略实现类
        searchMap.put(1, new ConcreteStrategy1());
        searchMap.put(2, new ConcreteStrategy2());
    }

    @Override
    public void search(Integer type, DTO dto) {
        Strategy strategy = strategyMap.get(type);
        if (ObjUtil.isNull(strategy)) {
            return;
        }
        strategy.options(dto);
    }

}

/**
 * 在抽象策略，尽量使用抽象类，不要使用接口，原因在于，有些方法可以提取到父类中使用
 */
@Component
public abstract class Strategy  {
    @Resource
    private BussinessMapper bussinessMapper; // 这里的bussinessMapper一定为空
    // 抽象的业务方法
    public abstract void options(DTO dto);

    // 公共的方法，这里如果是接口就没办法做这种实现了
    protected void commonMethod(){
        bussinessMapper.do();
    }

}

/**
 * 具体策略实现类
 */
@Component
public class ConcreteStrategy1 extends Strategy {
    @Resource
    private BussinessMapper bussinessMapper; // 这里的bussinessMapper一定为空

    @Override
    public void options(DTO dto) {
       // do something
        bussinessMapper.do();
    }
}

解决方案

方案一：使用 @Autowired 注入 Strategy 的实现类

@Service
@Slf4j
public class BussinessServiceImpl implements BussinessService {
    private final Map<Integer, Strategy> strategyMap;

    @Autowired
    public BussinessServiceImpl(ConcreteStrategy1 concreteStrategy1, ConcreteStrategy2 concreteStrategy2) {
        this.strategyMap = new HashMap<>();
        strategyMap.put(1, concreteStrategy1);
        strategyMap.put(2, concreteStrategy2);
    }
}

方案二：使用 @PostConstruct 初始化 strategyMap

@Service
@Slf4j
public class BussinessServiceImpl implements BussinessService {
    private final Map<Integer, Strategy> strategyMap;

    @Autowired
    private ConcreteStrategy1 concreteStrategy1;

    @Autowired
    private ConcreteStrategy2 concreteStrategy2;

    @Autowired
    public BussinessServiceImpl() {
        this.strategyMap = new HashMap<>();
    }

    @PostConstruct
    public void init() {
        strategyMap.put(1, concreteStrategy1);
        strategyMap.put(2, concreteStrategy2);
    }    
}

方案三：使用 @Component 和 @Qualifier

@Service
@Slf4j
public class BussinessServiceImpl implements BussinessService {
    private final Map<Integer, Strategy> strategyMap;

    @Autowired
    public BussinessServiceImpl(@Qualifier("concreteStrategy1") ConcreteStrategy1 concreteStrategy1,
                                @Qualifier("concreteStrategy2") ConcreteStrategy2 concreteStrategy2) {
        this.strategyMap = new HashMap<>();
        strategyMap.put(1, concreteStrategy1);
        strategyMap.put(2, concreteStrategy2);
    }
}