Java for Web学习笔记（四六）：WebSocket（3）Java Server

Maven相关库

<dependency>
<groupId>javax.websocket</groupId>
<artifactId>javax.websocket-api</artifactId>
<version>1.1</version>
<scope>provided</scope>
</dependency>

注意，这里是provided，不是compiled，已经集成在JavaEE 7中。上面是支持WebSocket Server和Client的，如果我们只需要Client，可以使用：

<dependency>
<groupId>javax.websocket</groupId>
<artifactId>javax.websocket-client-api</artifactId>
<version>1.1</version>
<scope>provided</scope>
</dependency>

一个简单的WebSocket Server

承接之前WebSocket Client的小例子，server在收到WebSocket请求后发出Hello消息，然后采用异步方式，每隔1秒发送一个消息，连发三次，然后关闭连接。

//【1】进行websocket server endpoint的标注，给出匹配的url。当收到一个webSocket请求时，会创建一个实例，这点和Servlet不一样，需要注意。
@ServerEndpoint("/test/{id}")
public class TestServer {
//【2】server和client握手后，同样提供了@OnOpen, @OnClose, @OnError和@OnMessage触发发放，这些方法可以通过@PathParam()来获取path的信息，如本例中的id。我们可以通过javax.websocket.Session来发送信息和关闭连接

//【2.1】可选的Session参数，可选的EndpointConfig参数，以及path信息
@OnOpen
public void onOpen(Session session, @PathParam("id") String id){
System.out.println("onOpen : (" + id + ")");
try {
session.getBasicRemote().sendText("Hello, " + id);

new Thread(new Runnable() {
public void run() {
for(int i = 0 ; i < 3 ; i ++){
try {
Thread.sleep(1000);
session.getBasicRemote().sendText("count " + i);
} catch (Exception e) {
}
}
try {
session.close();
} catch (IOException ignoreE) {
}
}
}).start();
} catch (IOException e) {
e.printStackTrace();
}
}

//【2.2】消息的参数会比较复杂
//    1、String给出text消息，如本例
//    2、String以及boolean表示chunks，true表示为最后一个chunks。
//    3、One Java primitive or primitive wrapper to receive an entire text message converted to that type
//    4、One java.io.Reader to receive a text message as a blocking stream
//    5、One byte[] or one java.nio.ByteBuffer to receive an entire binary message
//    6、One byte[] or one ByteBuffer, plus one boolean to receive a binary message in chunks
//    7、One java.io.InputStream to receive a binary message as a blocking stream
//    8、One PongMessage for custom handling of heartbeat responses
//    9、Any Java object if the endpoint has a Decoder.Text, Decoder.Binary, Decoder .TextStream, or Decoder.BinaryStream registered to convert that type. The message type of text or binary must match the registered decoder.
@OnMessage
public void onMessage(Session session, String message, @PathParam("id") String id){
System.out.println("onMessage : (" + id + ") " + message);
}

//【2.3】可选的Session参数，可选的CloseReason参数，以及path信息
@OnClose
public void onClose(Session session, @PathParam("id") String id){
System.out.println("onClose : (" + id + ")");
}

//【2.4】@OnError提供可选的Session参数，必选的Throwable，以及path信息，本例无处理
}

更为复杂一点的处理

两个client之间的通信

server可以作为agent之类的，实现两个或者多个client之间的通信，方式很简单，将session关联即可。

@ServerEndpoint("/ticTacToe/{gameId}/{username}")
public class GameServer {
private static class Game{
public long gameId;
public Session player1;
public Session player2;
public TicTacToeGame ticTacToeGame;
}

//我们通过gameId可以获得两个关联的client的session。
private static Map<Long, Game> games = new Hashtable<>();

... 略 ...
}

使用JSON作为Message

Gson是一个选择，我们也可以使用可能更为传统的方式：

<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-core</artifactId>
<version>2.8.7</version>
<scope>compile</scope>
</dependency>

<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-annotations</artifactId>
<version>2.8.7</version>
<scope>compile</scope>
</dependency>

<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.8.7</version>
<scope>compile</scope>
</dependency>

<dependency>
<groupId>com.fasterxml.jackson.datatype</groupId>
<artifactId>jackson-datatype-jsr310</artifactId>
<version>2.8.7</version>
<scope>compile</scope>
</dependency>

@ServerEndpoint("/ticTacToe/{gameId}/{username}")
public class GameServer {
... 见上面代码片段，略 ...

//（1）消息的基础格式
public static abstract class Message{
private final String action;
public Message(String action){
this.action = action;
}
public String getAction() {
return action;
}
}

//（2）具体的消息类型
public static class GameStartedMessage extends Message{
private final TicTacToeGame game;

public GameStartedMessage(TicTacToeGame game) {
super("gameStarted");
this.game = game;
}

public TicTacToeGame getGame() {
return game;
}
}

... 其他的消息类型，略 ...
//【1】定义json封装
private static ObjectMapper mapper = new ObjectMapper();

//【2】发送JSON消息
private void sendJsonMessage(Session session, Game game, Message message){
try {
session.getBasicRemote().sendText(GameServer.mapper.writeValueAsString(message));
} catch (Exception e) {
handleException(e,game);
}
}

//【3】解析JSON消息
@OnMessage
public void onMessage(Session session, String message, @PathParam("gameId") long gameId){
Game game = GameServer.games.get(gameId);
... ...
//解析JSON
MoveMessage move = GameServer.mapper.readValue(message, MoveMessage.class);
... ...
//发送JSON
this.sendJsonMessage((isPlayer1 ? game.player2 : game.player1), game,
new OpponentMadeMoveMessage(move));
}
}

从session中获取信息

在jsp文件中，我们用javascript写到

server = new WebSocket('ws://' + window.location.host +
'<c:url value="/game/${gameId}/${username}">'
+'<c:param name="action" value="${action}" />'
+'</c:url>');

我们要读出握手的HTTP请求的相关信息，例如action参数的值

//在WebSocket server中，如果我们需要获取action的信息
//getRequestParameterMap：Return the request parameters associated with the request this session was opened under.
List<String> actions = session.getRequestParameterMap().get("action");
if(actions != null && actions.size() == 1){
String action = actions.get(0);
... ...
}

相关链接：
我的Professional Java for Web Applications相关文章