How to have jar execute without main manifest attribute?

Question

I have a jar file built from a maven project. In it's pom, I have both spring boot and jetty. The purpose of my project is to implement web sockets on my website. Jetty's implementation of web sockets does not include a main method, just one class so it knows what to do when it gets web socket requests. I then tried java -jar target/myproject-0.0.1-SNAPSHOT.jar, but I got the error 'no main manifest attribute.' I'm not sure why I would need one. Any advice would be greatly appreciated. My pom file:

<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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>myGroupId</groupId>
    <artifactId>myArtifactId</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>myName</name>
    <url>http://maven.apache.org</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>1.8</java.version>
    </properties>
    <repositories>
        <repository>
            <id>repo2_maven_org</id>
            <url>http://repo2.maven.org/maven2</url>
        </repository>
    </repositories>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <!-- Import dependency management from Spring Boot -->
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-dependencies</artifactId>
            <version>1.1.4.RELEASE</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
        <dependency>
            <groupId>org.eclipse.jetty</groupId>
            <artifactId>jetty-server</artifactId>
            <version>9.3.0.v20150612</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.jetty</groupId>
            <artifactId>jetty-servlet</artifactId>
            <version>9.3.0.v20150612</version>
        </dependency>
        <dependency>
            <groupId>org.eclipse.jetty.websocket</groupId>
            <artifactId>javax-websocket-server-impl</artifactId>
            <version>9.3.0.v20150612</version>
        </dependency>
  </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.eclipse.jetty</groupId>
                <artifactId>jetty-maven-plugin</artifactId>
                <version>9.3.0.v20150612</version>
            </plugin>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

My only java code:

import java.io.IOException;
import java.util.ArrayList;

import javax.websocket.CloseReason;
import javax.websocket.OnClose;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;

@ServerEndpoint("/jsr356toUpper")
public class ToUpper356Socket {
    private ArrayList<Session> sessions = new ArrayList<Session>();
    @OnOpen
    public void onOpen(Session session) {
        sessions.add(session);
        System.out.println("WebSocket opened: " + session.getId());
    }
    @OnMessage
    public void onMessage(String txt, Session session) throws IOException {
        System.out.println("Message received: " + txt);
        for (Session ses : sessions){
            ses.getBasicRemote().sendText(txt);
        }
    }

    @OnClose
    public void onClose(CloseReason reason, Session session) {
        sessions.remove(session);
        System.out.println("Closing a WebSocket due to " + reason.getReasonPhrase());

    }
}

Answer 1

you need to provide the mainClass value, from spring-boot-maven plugin

mainClass String 1.0 The name of the main class. If not specified the first compiled class found that contains a 'main' method will be used.

inside you plugin add a configuration

<configuration>
        <mainClass><your main class></mainClass>
    </configuration>

Or

you can simply do something like this, maven properties

<properties>    
    <start-class><your main class></start-class>
</properties>

Answer 2

You need a spring-boot starter class that provides the main method. Something like this. Then the spring-boot-maven-plugin will take care of the rest for you.

@SpringBootApplication
public class MyApp {
    public static void main(String[] args) {
        SpringApplication.run(MyApp.class, args);
    }
}