Java依赖注入

时间:2022-06-28 ifredom 人气:0

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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