1：yml文件换成properties文件

2：pom.xml添加

<dependency>
    <groupId>org.springframework.bootgroupId>
    <artifactId>spring-boot-configuration-processorartifactId>
    <optional>trueoptional>
dependency>

3：添加插件

<plugin>
    <groupId>org.springframework.bootgroupId>
    <artifactId>spring-boot-maven-pluginartifactId>
plugin>

4：启动类添加@EnableConfigurationProperties({MailProperties.class})