为什么spring webflux默认选择jetty然后失败？

我正在尝试运行基于 Spring boot 2.0.0.M7 的应用程序。

以下是我的 build.gradle 中与 spring 相关的依赖项：

compile "org.springframework.boot:spring-boot-starter-webflux:2.0.0.M7"
compile "org.springframework.boot:spring-boot-starter-data-ldap:2.0.0.M7"
compile "org.springframework.boot:spring-boot-starter-actuator:2.0.0.M7"

根据自动配置报告，是否可以告诉为什么 spring 默认情况下要使用 Jetty 作为反应式引擎（实际上，根据 spring 文档，Reactor Netty 是默认引擎）

Spring Boot自动配置报告（摘录）：

ReactiveWebServerConfiguration.ReactorNettyAutoConfiguration:
  Did not match:
     - @ConditionalOnMissingBean (types: org.springframework.boot.web.reactive.server.ReactiveWebServerFactory; SearchStrategy: all) found beans of type'org.springframework.boot.web.reactive.server.ReactiveWebServerFactory'jettyReactiveWebServerFactory (OnBeanCondition)
  Matched:
     - @ConditionalOnClass found required class 'reactor.ipc.netty.http.server.HttpServer'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)

ReactiveWebServerAutoConfiguration#defaultReactiveWebServerCustomizer matched:
      - @ConditionalOnMissingBean (types: org.springframework.boot.autoconfigure.web.reactive.DefaultReactiveWebServerCustomizer; SearchStrategy: all) did not find any beans (OnBeanCondition)

   ReactiveWebServerConfiguration.JettyAutoConfiguration matched:
      - @ConditionalOnClass found required class 'org.eclipse.jetty.server.Server'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
      - @ConditionalOnMissingBean (types: org.springframework.boot.web.reactive.server.ReactiveWebServerFactory; SearchStrategy: all) did not find any beans (OnBeanCondition)

我在应用程序启动期间遇到异常：

[0;39morg.springframework.context.ApplicationContextException: Unable to start reactive web server; nested exception is java.lang.NoClassDefFoundError: org/eclipse/jetty/servlet/ServletHolder
    at org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.onRefresh(ReactiveWebServerApplicationContext.java:64)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:543)
    at org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.refresh(ReactiveWebServerApplicationContext.java:49)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:750)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:386)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:327)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1245)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1233)
    at com.xyz.abc.routing.web.ApiApplication.main(ApiApplication.java:14)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:48)
    at org.springframework.boot.loader.Launcher.launch(Launcher.java:87)
    at org.springframework.boot.loader.Launcher.launch(Launcher.java:50)
    at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:51)
Caused by: java.lang.NoClassDefFoundError: org/eclipse/jetty/servlet/ServletHolder
    at org.springframework.boot.web.embedded.jetty.JettyReactiveWebServerFactory.createJettyServer(JettyReactiveWebServerFactory.java:96)
    at org.springframework.boot.web.embedded.jetty.JettyReactiveWebServerFactory.getWebServer(JettyReactiveWebServerFactory.java:87)
    at org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.createWebServer(ReactiveWebServerApplicationContext.java:87)
    at org.springframework.boot.web.reactive.context.ReactiveWebServerApplicationContext.onRefresh(ReactiveWebServerApplicationContext.java:61)
    ... 16 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.eclipse.jetty.servlet.ServletHolder
    at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at org.springframework.boot.loader.LaunchedURLClassLoader.loadClass(LaunchedURLClassLoader.java:93)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    ... 20 common frames omitted

我怎样才能让Spring使用reactor netty？

正如 Marten 所提到的，检查 Jetty 服务器依赖项的依赖项。 Spring Boot 正在寻找org.eclipse.jetty.server.Serverclass 作为您选择 Jetty 作为服务器的信号。

Try ./gradlew dependencies并查看这种依赖性从何而来。