原文链接 https://bigosg.github.io/spring/2015/05/05/spring-annotation-primary.html
注：以下为加速网络访问所做的原文缓存，经过重新格式化，可能存在格式方面的问题，或偶有遗漏信息，请以原文为准。
假定，有如下两个类，OperaSinger and MetalSinger。
MetalSinger类
{% highlight java %} @Component public class MetalSinger implements Singer {
@Override
public String sing(String lyrics) {
return "I am singing with DIO voice: "+lyrics;
}
} {% endhighlight %}
OperaSinger类
{% highlight java %} public class OperaSinger implements Singer { @Override public String sing(String lyrics) { return "I am singing in Bocelli voice: "+lyrics; } } {% endhighlight %}
Singer接口
{% highlight java %} public interface Singer { String sing(String lyrics); } {% endhighlight %}
现在定义SingerService并且注入Singer
SingerService类
{% highlight java %} @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");
}
} {% endhighlight %}
你觉得哪个Singer实现类会被注入？结果如下：
{% highlight java %} I am singing with DIO voice: song lyrics {% endhighlight %}
这是因为OperaSinger类未定义@Component或者@Service注解，所以Spring对于OperaSinger类完全没想法，现在加上@Component注解：
{% highlight java %} @Component public class OperaSinger implements Singer { @Override public String sing(String lyrics) { return "I am singing in Bocelli voice: "+lyrics; } } {% endhighlight %}
然后，会得到异常信息:
{% highlight java %} org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [main.service.Singer] is defined: expected single matching bean but found 2: metalSinger,operaSinger {% endhighlight %}
原因显而易见，如果有不止一个bean的相同类型，并且使用了@Autowired注解根据类型绑定bean，就会得到这个异常信息，Spring不知道如何去选择。 现在给OperaSinger类加上@Primary注解。
{% highlight java %} @Primary @Component public class OperaSinger implements Singer {
@Override
public String sing(String lyrics) {
return "I am singing in Bocelli voice: "+lyrics;
}
} {% endhighlight %}
结果如下：
{% highlight java %} I am singing in Bocelli voice: song lyrics {% endhighlight %}
这是因为OperaSinger类加上了@Primary注解，Primary主要的，从单词意思就可以理解。 另外解决方法是，使用@Qualifier注解使用value属性指定bean的名称。
{% highlight java %} @Component public class SingerService { private static final Logger logger = LoggerFactory.getLogger(SingerService.class);
@Autowired
@Qualifier(value = "operaSinger")
private Singer singer;
public String sing(){
return singer.sing("song lyrics");
}
} {% endhighlight %}
英文原文链接{:target="_blank"}