Java详细讲解依赖注入的方式

2022-11-13 10:11:26 依赖 注入 讲解

Spring中的三种依赖注入方式

  • Field Injection :@Autowired注解的一大使用场景就是Field Injection
  • Constructor Injection :构造器注入,是我们日常最为推荐的一种使用方式Setter Injection:
  • Setter Injection也会用到@Autowired注解,但使用方式与Field Injection有所不同,Field Injection是用在成员变量上,而Setter Injection的时候,是用在成员变量的Setter函数上
// Field Injection
@Service("uploadService")
public class UploadServiceImpl extends ServiceImpl<UploadDao, UploadEntity> implements UploadService {
    @Autowired
    private UploadDao uploadDao;
}
// Constructor Injection
@Service("uploadService")
public class UploadServiceImpl extends ServiceImpl<UploadDao, UploadEntity> implements UploadService {
    private UploadDao uploadDao;
	UploadServiceImpl(UploadDao uploadDao){
		this.uploadDao = uploadDao;
	}
}
// Setter Injection
@Service("uploadService")
public class UploadServiceImpl extends ServiceImpl<UploadDao, UploadEntity> implements UploadService {
    private UploadDao uploadDao;
	@Autowired
	public void setUploadDao(UploadDao uploadDao){
		this.uploadDao =uploadDao
	}
}

1.是否进行循环依赖检测

  • Field Injection:不检测
  • Constructor Injection:自动检测
  • Setter Injection:不检测

可能遇到的问题

循环依赖报错: 当服务A需要用到服务B时,并且服务B又需要用到服务A时,Spring在初始化创建Bean时,不知道应该先创建哪一个,就会出现该报错。

This is often the result of over-eager type matching - consider using 'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example

class ServerA{
	@Autowired
	private ServerB b;
}
class ServerB{
	@Autowired
	private ServerA a;
}

如果使用构造方式注入,能够精准的提醒你是哪两个类产生了循环依赖 .异常报错信息能够迅速定位问题:

循环报错解决办法是使用 @Lazy注解 ,对任意一个需要被注入Bean添加该注解,表示延迟创建即可。

class ServerA{
	@Autowired
	@Lazy
	private ServerB b;
}
class ServerB{
	@Autowired
	@Lazy
	private ServerA a;
}

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