Spring Webflux从Netty切换Tomcat嵌入式服务器后报错排查

我通过修改pom.xml将Spring Webflux项目的嵌入式服务器从Netty切换为Tomcat，配置如下：

<?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>2.7.5</version>
    <relativePath/> <!-- lookup parent from repository -->
</parent>
<groupId>com.fer</groupId>
<artifactId>DemoReactive</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>DemoReactive</name>
<description>Demo project for Reactive</description>
<properties>
    <java.version>11</java.version>
</properties>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-webflux</artifactId>
        <exclusions>
            <exclusion>
                <artifactId>spring-boot-starter-reactor-netty</artifactId>
                <groupId>org.springframework.boot</groupId>
            </exclusion>
        </exclusions>
    </dependency>

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

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>io.projectreactor</groupId>
        <artifactId>reactor-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

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

但现在出现错误：java.lang.IllegalStateException: No suitable default ClientHttpConnector found，此前使用Netty时一切正常。

原因说明 #

• Spring Webflux的WebClient等客户端组件默认依赖Reactor Netty作为ClientHttpConnector的实现，用来处理客户端HTTP请求。
• 你排除了spring-boot-starter-reactor-netty依赖后，没有提供其他替代的客户端连接器实现，导致Spring无法找到合适的默认连接器。
• 切换Tomcat只是替换了服务器端的嵌入式容器，但Webflux的客户端部分依然需要对应的HTTP连接器支持，两者是独立的组件。

解决方案 #

方案1：保留Reactor Netty客户端依赖（推荐）

如果只是想替换服务器端为Tomcat，无需完全移除Netty，只需保留客户端依赖即可。修改pom.xml的依赖配置：

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
    <exclusions>
        <!-- 只排除Netty的服务器启动器，保留客户端相关依赖 -->
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-reactor-netty</artifactId>
        </exclusion>
    </exclusions>
</dependency>

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

<!-- 手动添加Reactor Netty客户端依赖 -->
<dependency>
    <groupId>io.projectreactor.netty</groupId>
    <artifactId>reactor-netty-http</artifactId>
</dependency>

方案2：替换为Tomcat客户端连接器

如果想完全移除Netty，可以使用Tomcat的HTTP客户端实现，需要手动添加依赖并配置连接器：

1. 添加Tomcat客户端依赖到pom.xml：

<dependency>
    <groupId>org.apache.tomcat.embed</groupId>
    <artifactId>tomcat-embed-core</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webflux</artifactId>
</dependency>
<!-- 版本需和项目中Tomcat的版本匹配，此处以2.7.5对应的Tomcat 9.0.68为例 -->
<dependency>
    <groupId>org.apache.tomcat</groupId>
    <artifactId>tomcat-httpclient</artifactId>
    <version>9.0.68</version>
</dependency>

2. 配置TomcatClientHttpConnector作为WebClient的连接器：

import org.apache.tomcat.httpclient.TomcatHttpClient;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.reactive.TomcatClientHttpConnector;
import org.springframework.web.reactive.function.client.WebClient;

@Configuration
public class WebClientConfig {
    @Bean
    public WebClient webClient() {
        TomcatHttpClient tomcatHttpClient = TomcatHttpClient.create();
        TomcatClientHttpConnector connector = new TomcatClientHttpConnector(tomcatHttpClient);
        return WebClient.builder()
                .clientConnector(connector)
                .build();
    }
}

内容的提问来源于stack exchange，提问作者Stefano Maglione