@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;
}
}