Spring Bean 生命周期常见错误

案例 1 ：构造器内抛空指针异常

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class LightMgrService {
  @Autowired
  private LightService lightService;
  public LightMgrService() {
    lightService.check();
  }
}

@Service
public class LightService {
    public void start() {
        System.out.println("turn on all lights");
    }
    public void shutdown() {
        System.out.println("turn off all lights");
    }
    public void check() {
        System.out.println("check all lights");
    }
}

在LightMgrService类初始化时，调用LightService的check方法去执行检查业务，从而输出"check all lights"。但结果确会抛出空指针。如下图所示：

image.png

Spring以上面这种写法去初始化对象，其过程是先实例化bean，注入bean依赖，再初始化bean。而默认构造器是再实例化的时候被自动调用的，此时，还未注入bean依赖，LightMgrService的属性LightService还是null，空指针异常也是正常的。有三种解决办法：
一是可以隐式注入，@Autowired注解引发的装配行为是在调用构造器之后的，隐式注入很好的解决了这个问题。

@Component
public class LightMgrService {

    private LightService lightService;

    public LightMgrService(LightService lightService) {
        this.lightService = lightService;
        lightService.check();
    }
}

二是可以使用@PostConstruct注解。

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class LightMgrService {
  @Autowired
  private LightService lightService;
  
  @PostConstruct
  public void init() {
       lightService.check();
  }
}

三是实现InitializingBean接口，重写其afterPropertiesSet()方法。

import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
public class LightMgrService implements InitializingBean {
    @Autowired
    private LightService lightService;
  
    @Override
    public void afterPropertiesSet() throws Exception {
        lightService.check();
    }
}