Error during WebSocket handshake: Unexpected response code: 404 when trying to connect with JavaScript

问题

Below is the code for socket.

package com.shankar.rest;

import java.io.IOException;

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

/** 
 * @ServerEndpoint gives the relative name for the end point
 * This will be accessed via ws://localhost:8080/com.shankar.rest/echo
 * Where "localhost" is the address of the host,
 * "com.shankar.rest" is the name of the package
 * and "echo" is the address to access this class from the server
 */
@ServerEndpoint("/echo") 
public class EchoServer {
    /**
     * @OnOpen allows us to intercept the creation of a new session.
     * The session class allows us to send data to the user.
     * In the method onOpen, we'll let the user know that the handshake was 
     * successful.
     */
    @OnOpen
    public void onOpen(Session session){
        System.out.println(session.getId() + " has opened a connection"); 
        try {
            session.getBasicRemote().sendText("Connection Established");
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * When a user sends a message to the server, this method will intercept the message
     * and allow us to react to it. For now the message is read as a String.
     */
    @OnMessage
    public void onMessage(String message, Session session){
        System.out.println("Message from " + session.getId() + ": " + message);
        try {
            session.getBasicRemote().sendText(message);
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

    /**
     * The user closes the connection.
     * 
     * Note: you can't send messages to the client from this method
     */
    @OnClose
    public void onClose(Session session){
        System.out.println("Session " +session.getId()+" has ended");
    }
}

Am not sure why it throwing error when connect with JavaScript.

<!DOCTYPE html>

<html>
<head>
<title>Echo Chamber</title>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width">
</head>
<body>

    <div>
        <input type="text" id="messageinput" />
    </div>
    <div>
        <button type="button" onclick="openSocket();">Open</button>
        <button type="button" onclick="send();">Send</button>
        <button type="button" onclick="closeSocket();">Close</button>
    </div>
    <!-- Server responses get written here -->
    <div id="messages"></div>

    <!-- Script to utilise the WebSocket -->
    <script type="text/javascript">

            var webSocket;
            var messages = document.getElementById("messages");


            function openSocket(){
                // Ensures only one connection is open at a time
                if(webSocket !== undefined && webSocket.readyState !== WebSocket.CLOSED){
                   writeResponse("WebSocket is already opened.");
                    return;
                }
                // Create a new instance of the websocket
                webSocket = new WebSocket("ws://localhost:8080/com.shankar.rest/echo");

                /**
                 * Binds functions to the listeners for the websocket.
                 */
                webSocket.onopen = function(event){
                    // For reasons I can't determine, onopen gets called twice
                    // and the first time event.data is undefined.
                    // Leave a comment if you know the answer.
                    if(event.data === undefined)
                        return;

                    writeResponse(event.data);
                };

                webSocket.onmessage = function(event){
                    writeResponse(event.data);
                };

                webSocket.onclose = function(event){
                    writeResponse("Connection closed");
                };
            }

            /**
             * Sends the value of the text input to the server
             */
            function send(){
                var text = document.getElementById("messageinput").value;
                webSocket.send(text);
            }

            function closeSocket(){
                webSocket.close();
            }

            function writeResponse(text){
                messages.innerHTML += "<br/>" + text;
            }

        </script>

</body>
</html>

When I try to open connection from JavaScript, it throws the following error:

WebSocket connection to 'ws://localhost:8080/com.shankar.rest/echo' failed: Error during WebSocket handshake: Unexpected response code: 404

Here is my web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>Demo</display-name>
  <welcome-file-list>
    <welcome-file>NewFile.html</welcome-file>
  </welcome-file-list>

  <servlet>
      <servlet-name>SimulationAPI</servlet-name>
      <servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>
      <init-param>
           <param-name>jersey.config.server.provider.packages</param-name>
           <param-value>com.shankar.rest</param-value>
      </init-param>
      <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>SimulationAPI</servlet-name>
        <url-pattern>/rest/*</url-pattern>
    </servlet-mapping>

</web-app>

来源：https://stackoverflow.com/questions/38752467/error-during-websocket-handshake-unexpected-response-code-404-when-trying-to-c

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javascript

java

sockets

websocket

Socket.IO