spring 注解@primary解析

文章引自：http://jackyrong.iteye.com/blog/2208000

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
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;  
    }  
}