optional表示可选依赖,比如项目依赖了dubbo-spring-boot-starter
<dependency>
<groupId>org.apache.dubbo</groupId>
<artifactId>dubbo-spring-boot-starter</artifactId>
<version>${dubbo.version}</version>
</dependency>
上面的依赖又依赖了spring-boot-starter
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
<optional>true</optional>
</dependency>
此时如果项目不显示添加依赖spring-boot-starter,那么项目是不会依赖到
spring-boot-starter的。