spring中少用的注解@primary解析

这次看下spring中少见的注解@primary注解，例子 

@Component
public class MetalSinger implements Singer{

    @Override
    public String sing(String lyrics) {
        return "I am singing with DIO voice: "+lyrics;
    }
}

 

public class OperaSinger implements Singer {
    @Override
    public String sing(String lyrics) {
        return "I am singing in Bocelli voice: "+lyrics;
    }
}

  这两个类都实现了singer接口 
 

public interface Singer {
    String sing(String lyrics);
}

  那么来个注入的： 

@Component
public class SingerService {
    private static final Logger logger = LoggerFactory.getLogger(SingerService.class);

    @Autowired
    private Singer singer;

    public String sing(){
        return singer.sing("song lyrics");
    }
}

   那么最后输出应该是：I am singing with DIO voice: song lyrics. 

原因是只有MetalSinger使用了注解@component，那么autowird会只寻找这个标记的 
进行注入: 
  如果也在OperaSinger 中使用了注解，则spring会报异常，不知道到底应该用 
哪个： 
  org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [main.service.Singer] is defined: expected single matching bean but found 2: metalSinger,operaSinger 

以上报错因为根据bean的值找到对应两个类导致冲突； 如果某个@component("singer") ,spring就能默认找到bean名称，而确定注入哪一个实现类   

   而如果要让spring知道必须注入的是OperaSinger ，则可以用@primary注解告诉 
spring: 
  

@Primary
@Component
public class OperaSinger implements Singer{

    @Override
    public String sing(String lyrics) {
        return "I am singing in Bocelli voice: "+lyrics;
    }
}