spring中少見的註解@primary註解，例子

@Component  
public class MetalSinger implements Singer{  

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

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

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這兩個類都實現了singer介面

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

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那麼來個注入的：

@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");  
    }  
}  

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那麼最後輸出應該是：I am singing with DIO voice: song lyrics.

原因是隻有MetalSinger使用了註解@component，那麼autowird會只尋找這個標記的 
進行注入: 
如果也在OperaSinger 中使用了註解，則spring會報異常，不知道到底應該用 
哪個： 
org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [main.service.Singer] is defined: expected single matching bean but found 2: metalSinger,operaSinger 
而如果要讓spring知道必須注入的是OperaSinger ，則可以用@primary註解告訴 
spring:

@Primary  
@Component  
public class OperaSinger implements Singer{  

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