編碼器和解碼器物件導向的 WebSockets

Created: November-22, 2018

由於編碼器和解碼器，JSR 356 提供了物件導向的通訊模型。

訊息定義

假設在傳送回所有連線的會話之前，伺服器必須轉換所有收到的訊息：

public abstract class AbstractMsg {
    public abstract void transform();
}

我們現在假設伺服器管理兩種訊息型別：基於文字的訊息和基於整數的訊息。

整數訊息將內容自身相乘。

public class IntegerMsg extends AbstractMsg {

    private Integer content;

    public IntegerMsg(int content) {
        this.content = content;
    }

    public Integer getContent() {
        return content;
    }

    public void setContent(Integer content) {
        this.content = content;
    }

    @Override
    public void transform() {
        this.content = this.content * this.content;
    }
}

字串訊息前置一些文字：

public class StringMsg extends AbstractMsg {

    private String content;

    public StringMsg(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    @Override
    public void transform() {
        this.content = "Someone said: " + this.content;
    }
}

編碼器和解碼器

每種訊息型別有一個編碼器，所有訊息都有一個解碼器。當解碼器必須實現 Decoder.XXX<Type> 時，編碼器必須實現 Encoder.XXX<Type> 介面。

編碼非常簡單：從訊息中，encode 方法必須輸出 JSON 格式的 String。以下是 IntegerMsg 的示例。

public class IntegerMsgEncoder implements Encoder.Text<IntegerMsg> {

    @Override
    public String encode(IntegerMsg object) throws EncodeException {
        JsonObjectBuilder builder = Json.createObjectBuilder();
        
        builder.add("content", object.getContent());
        
        JsonObject jsonObject = builder.build();
        return jsonObject.toString();
    }

    @Override
    public void init(EndpointConfig config) {
        System.out.println("IntegerMsgEncoder initializing");
    }

    @Override
    public void destroy() {
        System.out.println("IntegerMsgEncoder closing");
    }
}

StringMsg 類的相似編碼。顯然，編碼器可以通過抽象類進行分解。

public class StringMsgEncoder implements Encoder.Text<StringMsg> {

    @Override
    public String encode(StringMsg object) throws EncodeException {
        JsonObjectBuilder builder = Json.createObjectBuilder();

        builder.add("content", object.getContent());

        JsonObject jsonObject = builder.build();
        return jsonObject.toString();
    }

    @Override
    public void init(EndpointConfig config) {
        System.out.println("StringMsgEncoder initializing");
    }

    @Override
    public void destroy() {
        System.out.println("StringMsgEncoder closing");
    }

}

解碼器分兩步進行：檢查收到的訊息是否符合 willDecode 的例外格式，然後將收到的原始訊息轉換為 decode 的物件：

公共類 MsgDecoder 實現 Decoder.Text {

@Override
public AbstractMsg decode(String s) throws DecodeException {
    // Thanks to willDecode(s), one knows that
    // s is a valid JSON and has the attribute
    // "content"
    JsonObject json = Json.createReader(new StringReader(s)).readObject();
    JsonValue contentValue = json.get("content");

    // to know if it is a IntegerMsg or a StringMsg, 
    // contentValue type has to be checked:
    switch (contentValue.getValueType()) {
        case STRING:
            String stringContent = json.getString("content");
            return new StringMsg(stringContent);
        case NUMBER:
            Integer intContent = json.getInt("content");
            return new IntegerMsg(intContent);
        default:
            return null;
    }

}

@Override
public boolean willDecode(String s) {

    // 1) Incoming message is a valid JSON object
    JsonObject json;
    try {
        json = Json.createReader(new StringReader(s)).readObject();
    }
    catch (JsonParsingException e) {
        // ...manage exception...
        return false;
    }
    catch (JsonException e) {
        // ...manage exception...
        return false;
    }

    // 2) Incoming message has required attributes
    boolean hasContent = json.containsKey("content");

    // ... proceed to additional test ...
    return hasContent;
}

@Override
public void init(EndpointConfig config) {
    System.out.println("Decoding incoming message...");
}

@Override
public void destroy() {
    System.out.println("Incoming message decoding finished");
}

}

ServerEndPoint

Server EndPoint 非常像 *WebSocket 通訊，*有三個主要區別：

ServerEndPoint 註釋具有 encoders 和 decoders 屬性
訊息不會與 sendText 一起傳送，而是與 sendObject 一起傳送

使用 OnError 註釋。如果在 willDecode 期間丟擲了錯誤，它將在此處理並將錯誤資訊傳送回客戶端

@ServerEndpoint（value =“/ webSocketObjectEndPoint”，decoders = {MsgDecoder.class}，encoders = {StringMsgEncoder.class，IntegerMsgEncoder.class}）public class ServerEndPoint {

@OnOpen
public void onOpen(Session session) {
    System.out.println("A session has joined");
}

@OnClose
public void onClose(Session session) {
    System.out.println("A session has left");
}

@OnMessage
public void onMessage(Session session, AbstractMsg message) {
    if (message instanceof IntegerMsg) {
        System.out.println("IntegerMsg received!");
    } else if (message instanceof StringMsg) {
        System.out.println("StringMsg received!");
    }

    message.transform();
    sendMessageToAllParties(session, message);
}

@OnError
public void onError(Session session, Throwable throwable) {
    session.getAsyncRemote().sendText(throwable.getLocalizedMessage());
}

private void sendMessageToAllParties(Session session, AbstractMsg message) {
    session.getOpenSessions().forEach(s -> {
        s.getAsyncRemote().sendObject(message);
    });
}

}

由於我非常詳細，這裡有一個基本的 JavaScript 客戶端，適合那些想要擁有視覺化示例的人。請注意，這是一個類似聊天的示例：所有關聯方都會收到答案。

<!DOCTYPE html>
<html>
    <head>
        <title>Websocket-object</title>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">        
        <!-- start of BAD PRACTICE! all style and script must go into a
             dedicated CSS / JavaScript file-->
        <style>
            body{
                background: dimgray;
            }

            .container{
                width: 100%;
                display: flex;
            }

            .left-side{
                width: 30%;
                padding: 2%;
                box-sizing:  border-box;
                margin: auto;
                margin-top: 0;
                background: antiquewhite;
            }
            .left-side table{
                width: 100%;
                border: 1px solid black;
                margin: 5px;
            }
            .left-side table td{
                padding: 2px;
                width: 50%;
            }
            .left-side table input{
                width: 100%;
                box-sizing: border-box;
            }

            .right-side{
                width: 70%;
                background: floralwhite;
            }
        </style>

        <script>
            var ws = null;
            window.onload = function () {
                // replace the 'websocket-object' with the
                // context root of your web application.
                ws = new WebSocket("ws://localhost:8080/websocket-object/webSocketObjectEndPoint");
                ws.onopen = onOpen;
                ws.onclose = onClose;
                ws.onmessage = onMessage;
            };

            function onOpen() {
                printText("", "connected to server");
            }

            function onClose() {
                printText("", "disconnected from server");
            }

            function onMessage(event) {
                var msg = JSON.parse(event.data);
                printText("server", JSON.stringify(msg.content));
            }

            function sendNumberMessage() {
                var content = new Number(document.getElementById("inputNumber").value);
                var json = {content: content};
                ws.send(JSON.stringify(json));
                printText("client", JSON.stringify(json));
            }

            function sendTextMessage() {
                var content = document.getElementById("inputText").value;
                var json = {content: content};
                ws.send(JSON.stringify(json));
                printText("client", JSON.stringify(json));
            }

            function printText(sender, text) {
                var table = document.getElementById("outputTable");
                var row = table.insertRow(1);
                var cell1 = row.insertCell(0);
                var cell2 = row.insertCell(1);
                var cell3 = row.insertCell(2);

                switch (sender) {
                    case "client":
                        row.style.color = "orange";
                        break;
                    case "server":
                        row.style.color = "green";
                        break;
                    default:
                        row.style.color = "powderblue";
                }
                cell1.innerHTML = new Date().toISOString();
                cell2.innerHTML = sender;
                cell3.innerHTML = text;
            }
        </script>

        <!-- end of bad practice -->
    </head>
    <body>

        <div class="container">
            <div class="left-side">
                <table>
                    <tr>
                        <td>Enter a text</td>
                        <td><input id="inputText" type="text" /></td>
                    </tr>
                    <tr>
                        <td>Send as text</td>
                        <td><input type="submit" value="Send" onclick="sendTextMessage();"/></td>
                    </tr>
                </table>

                <table>
                    <tr>
                        <td>Enter a number</td>
                        <td><input id="inputNumber" type="number" /></td>
                    </tr>
                    <tr>
                        <td>Send as number</td>
                        <td><input type="submit" value="Send" onclick="sendNumberMessage();"/></td>
                    </tr>
                </table>
            </div>
            <div class="right-side">
                <table id="outputTable">
                    <tr>
                        <th>Date</th>
                        <th>Sender</th>
                        <th>Message</th>
                    </tr>
                </table>
            </div>
        </div>
    </body>
</html>

程式碼已完成，並在 Payara 4.1 下進行了測試。示例是純標準（沒有外部庫/框架）