@Qualifier注解作用

@Qualifier 注解作用就是为了给Bean打上一个标记，用来查找bean，代码示例:

1. 创建一个java bean

package com.gupaoedu.springcloud.example.demo;

public class TestClass {
     
    private String name; // getter setter 略
}

2. 编写配置类，将这个bean装载到IOC容器

package com.gupaoedu.springcloud.example.demo;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class TestConfiguration {
     

    @Bean("testClass1")
    TestClass testClass1(){
     
        return new TestClass("TestClass1");
    }
    @Qualifier
    @Bean("testClass2")
    TestClass testClass2(){
     
        return new TestClass("TestClass2");
    }
}

3. 启动类，配置扫描包

package com.gupaoedu.springcloud.example.springclouduserservice;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@ComponentScan(basePackages = "com.gupaoedu")
@SpringBootApplication
public class SpringCloudUserServiceApplication {
     
    public static void main(String[] args) {
     
        SpringApplication.run(SpringCloudUserServiceApplication.class, args);
    }
}

4. 测试controller 对testClassList 加上@Qualifier注解

package com.gupaoedu.springcloud.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Collections;
import java.util.List;
@RestController
public class TestController {
     

    @Qualifier
    @Autowired
    List<TestClass> testClassList= Collections.emptyList();

    @GetMapping("/test")
    public Object test(){
     
        return testClassList;
    }

}

然后启动，访问/test， 看到只打印了 TestClass2

然后对testClass1 Bean的注入，也加上@Qualifier注解后再请求/test

@Qualifier
@Bean("testClass1")
    TestClass testClass1(){
     
        return new TestClass("TestClass1");
    }

结果如下，可以看到，只有加了@Qualifier标记的Bean, 才会被注入给同样加了@Qualifier标记的 testClassList 这个list中

注： 本文是为了给ribbon负载均衡原理中， @LoadBalanced注解实现负载均衡的原理做铺垫；