Spring中@Primary的应用

首先看一下在Spring中这个注解的解释

 Indicates that a bean should be given preference when multiple candidates
 are qualified to autowire a single-valued dependency. If exactly one
 'primary' bean exists among the candidates, it will be the autowired value.

意思是：当注入了多个符合条件的bean时，被@Primary注解的bean将作为默认的注入被使用。

下面看一个例子：

package com.springDemodemo.primaryDemo;

public interface ITest {

     String test();
}

ITest接口的实现一，带Primary注解：

package com.springDemodemo.primaryDemo;

import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Service;

@Primary
@Service
public class TestDemo1 implements ITest{
    @Override
    public String test() {
        return "hello";
    }
}

ITest接口的实现二，不带Primary注解：

package com.springDemodemo.primaryDemo;

import org.springframework.stereotype.Service;

@Service
public class TestDemo2 implements ITest {
    @Override
    public String test() {
        return "world";
    }
}

测试，取出ITest接口的默认实现：

package com.springDemodemo;

import com.springDemodemo.primaryDemo.ITest;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.ApplicationContext;

@SpringBootTest
public class PrimaryTest {

    @Autowired
    ApplicationContext context;

    @Test
    public void test(){
        ITest bean = context.getBean(ITest.class);
        System.out.println(bean);
        System.out.println(bean.test());
    }

}

打印结果如下：

com.springDemodemo.primaryDemo.TestDemo1@34cf294c
hello

结论：当有多个符合条件的bean时，@Primary注解的bean将被默认注入。