我正在使用关于@Autowired和@Primary annotation的Spring Boot教程
下面你可以找到我的主类

package com.example.springtest;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import service.Animal;

@SpringBootApplication
public class SpringtestApplication {
@Autowired
private Animal animal;

public static void main(String[] args) {
    SpringApplication.run(SpringtestApplication.class, args);

}

}

动物接口（我应该自动连接的接口）

package service;

public interface Animal {

      String characteristics();
}

Dog类（Animal接口的主要实现）

package service;

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

@Primary
@Service
public class Dog implements Animal {

    @Override
    public String characteristics() {
        return "Bark";
    }
}

Cat类（Animal接口的第二个实现）

package service;

import org.springframework.stereotype.Service;

@Service
public class Cat implements Animal {

    @Override
    public String characteristics() {
        return "Meow";
    }
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.1.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>springtest</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>springtest</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <exclusions>
                <exclusion>
                    <groupId>org.junit.vintage</groupId>
                    <artifactId>junit-vintage-engine</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

当我运行应用程序时，动物注射出现问题，我得到以下错误：

***************************
APPLICATION FAILED TO START
***************************

Description:

Field animal in com.example.springtest.SpringtestApplication required a bean of type 'service.Animal' that could not be found.

The injection point has the following annotations:
    - @org.springframework.beans.factory.annotation.Autowired(required=true)

Action:

Consider defining a bean of type 'service.Animal' in your configuration.

Spring不能自动装配的原因是什么？