运行廖雪峰MVC示例报错：Main类初始化失败，找不到WebResourceRoot

运行嵌入式Tomcat的Main类时，抛出如下错误：

Error: can't initialize the main class: com.itranswarp.learnjava.Main Cause:java.lang.NoClassDefFoundError:org/apache/catalina/WebResourceRoot

当前配置与代码 #

pom.xml 核心内容 #

<groupId>com.itranswarp.learnjava</groupId>
<artifactId>web-mvc</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>war</packaging>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>
    <java.version>17</java.version>

    <jackson.version>2.12.7.1</jackson.version>
    <pebble.version>3.1.6</pebble.version>
    <tomcat.version>10.1.6</tomcat.version>
</properties>

<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>${jackson.version}</version>
    </dependency>
    <dependency>
        <groupId>io.pebbletemplates</groupId>
        <artifactId>pebble</artifactId>
        <version>${pebble.version}</version>
    </dependency>

    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-core</artifactId>
        <version>${tomcat.version}</version>
        <scope>provided</scope>
    </dependency>
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-jasper</artifactId>
        <version>${tomcat.version}</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

Main类代码 #

package com.itranswarp.learnjava;

import org.apache.catalina.Context;
import org.apache.catalina.WebResourceRoot;
import org.apache.catalina.startup.Tomcat;
import org.apache.catalina.webresources.DirResourceSet;
import org.apache.catalina.webresources.StandardRoot;
import java.io.File;

public class Main {

    public static void main(String[] args) throws Exception {
        Tomcat tomcat = new Tomcat();
        tomcat.setPort(Integer.getInteger("port", 8080));
        tomcat.getConnector();
        Context ctx = tomcat.addWebapp("", new File("src/main/webapp").getAbsolutePath());
        WebResourceRoot resources = new StandardRoot(ctx);
        resources.addPreResources(new DirResourceSet(resources, "/WEB-INF/classes", new File("target/classes").getAbsolutePath(), "/"));
        ctx.setResources(resources);
        tomcat.start();
        tomcat.getServer().await();
    }
}

已尝试操作：更新pom.xml中的Tomcat版本及插件、切换使用JDK17，问题未解决。

解决方案 #

核心原因 #

Tomcat嵌入依赖的<scope>provided</scope>导致运行时无法获取相关类。provided scope表示依赖仅在编译、测试阶段可用，运行时会假设由外部容器提供这些类，但这里是直接运行Main类启动嵌入式Tomcat，需要将这些依赖包含在运行环境中。

修复步骤 #

1. 修改pom.xml中的依赖scope：
   移除tomcat-embed-core和tomcat-embed-jasper的<scope>provided</scope>配置，或改为<scope>compile</scope>，修改后依赖块如下：

   <dependency>
       <groupId>org.apache.tomcat.embed</groupId>
       <artifactId>tomcat-embed-core</artifactId>
       <version>${tomcat.version}</version>
   </dependency>
   <dependency>
       <groupId>org.apache.tomcat.embed</groupId>
       <artifactId>tomcat-embed-jasper</artifactId>
       <version>${tomcat.version}</version>
   </dependency>
   

2. 重新编译项目：
   执行Maven命令清理并编译：

   mvn clean compile
   

3. 重新运行Main类：
   再次启动Main类，即可正常加载嵌入式Tomcat所需的类。

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