嵌入式Tomcat 8无web.xml时@WebServlet注解失效无法访问资源

根因 #

嵌入式Tomcat默认未开启Servlet3.0+注解扫描，且未将项目编译后的类路径纳入资源扫描范围，导致@WebServlet注解无法被识别，同时资源配置不全导致jsp访问失败。 #

第一步：调整pom.xml依赖 #

统一Tomcat组件版本，避免依赖冲突，同时调整servlet-api作用域：

<properties>
    <tomcat.version>8.5.23</tomcat.version>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <java.version>14</java.version>
</properties>

<dependencies>
    <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>
    <dependency>
        <groupId>org.eclipse.jdt.core.compiler</groupId>
        <artifactId>ecj</artifactId>
        <version>4.6.1</version>
    </dependency>
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-logging-juli</artifactId>
        <version>${tomcat.version}</version>
    </dependency>

    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>4.0.1</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

第二步：修改App.java启动配置 #

新增注解扫描支持、添加编译类路径到Tomcat资源目录：

import org.apache.catalina.LifecycleException;
import org.apache.catalina.StandardServer;
import org.apache.catalina.WebResourceRoot;
import org.apache.catalina.core.StandardContext;
import org.apache.catalina.startup.Tomcat;
import org.apache.catalina.webresources.DirResourceSet;
import org.apache.catalina.webresources.StandardRoot;
import org.apache.catalina.core.AprLifecycleListener;
import java.io.File;

public class App {

    public static void main(String[] args) throws LifecycleException {

        int port = 8085;
        Tomcat tomcat = new Tomcat();
        tomcat.setPort(port);

        StandardServer server = (StandardServer) tomcat.getServer();
        AprLifecycleListener listener = new AprLifecycleListener();
        server.addLifecycleListener(listener);

        String docBase = new File("src/main/webapp").getAbsolutePath();
        StandardContext context = (StandardContext) tomcat.addWebapp("/Servlet_Basic", docBase);
        // 关闭metadata-complete，开启注解扫描
        context.setMetadataComplete(false);
        // 明确Servlet版本为3.1，支持@WebServlet注解
        context.getServletContext().setEffectiveMajorVersion(3);
        context.getServletContext().setEffectiveMinorVersion(1);
        
        // 添加编译后的类路径到Tomcat资源目录，让Tomcat能扫描到自定义Servlet
        File classesDir = new File("target/classes");
        WebResourceRoot resources = new StandardRoot(context);
        resources.addPreResources(new DirResourceSet(resources, "/WEB-INF/classes",
                classesDir.getAbsolutePath(), "/"));
        context.setResources(resources);

        tomcat.start();
        System.out.println("Tomcat server started at port " + port);
        tomcat.getServer().await();
    }
}

第三步：运行验证 #

1. 启动项目前先执行mvn compile，确保编译后的class文件生成在target/classes目录下
2. 启动App类，访问以下地址即可正常响应：
   • http://localhost:8085/Servlet_Basic/index.jsp ：返回Hello World页面
   • http://localhost:8085/Servlet_Basic/hello ：返回Servlet定义的页面内容

内容的提问来源于stack exchange，提问作者kverma28