java - 从Gradle中的Spring Boot中排除Tomcat依赖项

我正在将Spring Boot与Jetty一起使用，看来我不能在Gradle构建文件中排除所有Tomcat依赖项。

build.gradle的相关部分：

compile("org.springframework.boot:spring-boot-starter") {
    exclude module: "tomcat-embed-el"
}
compile("org.springframework.boot:spring-boot-starter-jetty")

compile("org.springframework.boot:spring-boot-starter-web") {
    exclude module: "spring-boot-starter-tomcat"
}

但是，当我运行gradle dependencies部分tomcat仍然存在时，并导致WebSockets出现问题：

...
|    
+--- org.springframework.boot:spring-boot-starter-web: -> 1.4.1.RELEASE
|    +--- org.springframework.boot:spring-boot-starter:1.4.1.RELEASE (*)
|    +--- org.hibernate:hibernate-validator:5.2.4.Final
|    |    +--- javax.validation:validation-api:1.1.0.Final
|    |    +--- org.jboss.logging:jboss-logging:3.2.1.Final -> 3.3.0.Final
|    |    \--- com.fasterxml:classmate:1.1.0 -> 1.3.1
|    +--- com.fasterxml.jackson.core:jackson-databind:2.8.3
|    |    +--- com.fasterxml.jackson.core:jackson-annotations:2.8.0 -> 2.8.3
|    |    \--- com.fasterxml.jackson.core:jackson-core:2.8.3
|    +--- org.springframework:spring-web:4.3.3.RELEASE
|    |    +--- org.springframework:spring-aop:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-beans:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-context:4.3.3.RELEASE (*)
|    |    \--- org.springframework:spring-core:4.3.3.RELEASE
|    +--- org.springframework:spring-webmvc:4.3.3.RELEASE
|    |    +--- org.springframework:spring-aop:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-beans:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-context:4.3.3.RELEASE (*)
|    |    +--- org.springframework:spring-core:4.3.3.RELEASE
|    |    +--- org.springframework:spring-expression:4.3.3.RELEASE (*)
|    |    \--- org.springframework:spring-web:4.3.3.RELEASE (*)
|    \--- org.springframework.boot:spring-boot-starter-tomcat:1.4.1.RELEASE
|         +--- org.apache.tomcat.embed:tomcat-embed-core:8.5.5
|         +--- org.apache.tomcat.embed:tomcat-embed-el:8.5.5
|         \--- org.apache.tomcat.embed:tomcat-embed-websocket:8.5.5
|              \--- org.apache.tomcat.embed:tomcat-embed-core:8.5.5
...

为什么不spring-boot-starter-tomcat排除在外spring-boot-starter-web？