I have problem to implement websocket with tomcat embedded .
this dependencies i used :

    <dependencies>
        <dependency>
            <groupId>org.apache.tomcat.embed</groupId>
            <artifactId>tomcat-embed-websocket</artifactId>
            <version>11.0.4</version>
        </dependency>
        <dependency>
            <groupId>org.apache.tomcat.embed</groupId>
            <artifactId>tomcat-embed-core</artifactId>
            <version>11.0.4</version>
        </dependency>
    </dependencies>

this is my example code :

public class MainClass {
    private static final String contextPath = "";
    private static final String baseDir = "/tmp/tomcat";

    public static void main(String[] args) throws Exception {
        Tomcat tomcat = new Tomcat();
        tomcat.setHostname("0.0.0.0");
        tomcat.setPort(8080);
        tomcat.setBaseDir(baseDir);

        // Add the WebSocket initializer
        Context context = tomcat.addContext(contextPath, baseDir);
        context.addServletContainerInitializer(new WsSci(), null);

        // register servlet
        tomcat.addServlet(contextPath, "sample", new SampleServlet());
        context.addServletMappingDecoded("/sample", "sample");

        // Start the server
        tomcat.start();

        // Retrieve the ServerContainer and register the WebSocket endpoint
        ServerContainer serverContainer = (ServerContainer) context.getServletContext()
                .getAttribute("jakarta.websocket.server.ServerContainer");
        ServerEndpointConfig config = ServerEndpointConfig.Builder.create(SampleWebSocket.class, "/ws").build();
        serverContainer.addEndpoint(config);

        // Add the connector and await
        tomcat.getService().addConnector(tomcat.getConnector());
        tomcat.getServer().await();
    }
} 

this is SampleWebSocket :

public class SampleWebSocket extends Endpoint implements MessageHandler.Whole<String> {
    private Session session;

    @Override
    public void onOpen(Session session, EndpointConfig endpointConfig) {
        this.session = session;
    }

    @Override
    public void onMessage(String s) {
        try {
            System.out.println("Receive : " + s);
            session.getBasicRemote().sendText(LocalDateTime.now() + " Server response");
        } catch (IOException e) {
            throw new RuntimeException(e);
        } 
    }
}

I test that code with linux websocat command :

> websocat ws://localhost:8080/ws
websocat: WebSocketError: WebSocketError: Received unexpected status code (404 Not Found)

I can not found any example code or documentation about that ! How to use annotated classes ?

Project Tree

ws-server-st
├── pom.xml
├── README.txt
└── src
    └── main
        └── java
            └── com
                └── example
                    └── wsserver
                        ├── Main.java
                        └── SampleWebSocket.java

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://maven.apache.org/POM/4.0.0"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>websocket-server-jakarta-api</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>
    <name>tomcat Examples :: Embedded :: WebSocket Server with Jakarta EE API</name>

    <properties>
        <maven.compiler.source>17</maven.compiler.source>
        <maven.compiler.target>17</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <tomcatVersion>11.0.4</tomcatVersion>
    </properties>

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

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

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

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

        <dependency>
            <groupId>jakarta.servlet</groupId>
            <artifactId>jakarta.servlet-api</artifactId>
            <version>6.0.0</version>
            <scope>provided</scope>
        </dependency>

    </dependencies>
    <build>
        <finalName>websocket-server</finalName>
    </build>
</project>

Main.java

package com.example.wsserver;

import org.apache.catalina.startup.Tomcat;
import org.apache.tomcat.websocket.server.WsSci;
import java.io.File;
import java.util.HashSet;
import java.util.List;

public class Main {
    public static void main(String[] args) throws Exception {
        Tomcat tomcat = new Tomcat();
        tomcat.setPort(8080);
        tomcat.getConnector().setProperty("address", "0.0.0.0");

        var context = tomcat.addWebapp("", new File(".").getAbsolutePath());
        context.addServletContainerInitializer(new WsSci(), new HashSet<>(List.of(SampleWebSocket.class)));

        tomcat.start();
        System.out.println("Tomcat startup is completed, listening to port 8080");
        tomcat.getServer().await();
    }
}

SampleWebSocket.java

package com.example.wsserver;

import jakarta.websocket.EndpointConfig;
import jakarta.websocket.OnMessage;
import jakarta.websocket.OnOpen;
import jakarta.websocket.Session;
import jakarta.websocket.server.ServerEndpoint;

import java.io.IOException;
import java.time.LocalDateTime;

@ServerEndpoint("/ws")
public class SampleWebSocket {
    private Session session;

    @OnOpen
    public void onOpen(Session session, EndpointConfig endpointConfig) {
        this.session = session;
    }

    @OnMessage
    public void onMessage(String s) {
        try {
            System.out.println("Receive : " + s);
            session.getBasicRemote().sendText(LocalDateTime.now() + " Server response");
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}

Build And Run

Build

mvn clean package
mvn dependency:copy-dependencies -DoutputDirectory=target/libs

Run

java -cp "target/libs/*:target/websocket-server.jar" com.example.wsserver.Main

Test

websocat

$ ./websocat.x86_64-unknown-linux-musl ws://localhost:8080/ws
Hello
2025-03-04T15:04:11.813977680 Server response
World
2025-03-04T15:04:14.734585249 Server response

Server Output

Receive : Hello

Receive : World

Debug And Check

Check if there is any Java program running, and see if there is any Java program listening to port 8080.

ss -tulnp | grep java

$ ss -tulnp | grep java
tcp   LISTEN 0      100                     *:8080             *:*    users:(("java",pid=47247,fd=13)) 

lsof -i :8080

$ lsof -i :8080
COMMAND   PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
java    47247 demo   13u  IPv6 164794      0t0  TCP *:http-alt (LISTEN)