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spring中少用的注解@primary解析

秦瑟读书 2022-12-05 阅读 79


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


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;
}
}

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