Websocket在Android平臺的消息推送上的應用

寫在前面

之前接觸過一個Android小項目，其中有個功能是服務端將文件推送到客戶端，那時候，是通過客戶端的輪詢去詢問是否有文件推送，如果有，則進行文件下載。其實，這種做法不可取，可以用Websocket來實現一個簡單的消息推送系統，用於替代輪詢。

Websocket Server

在這個 server demo中，使用的是GitHub上面的一個開源項目Java-WebSocket

• 在AndroidStudio中添加依賴：

  implementation ‘org.java-websocket:Java-WebSocket:1.3.9’

• 創建Server類：

  public class WsServer extends WebSocketServer {
  
      public WsServer(int port, SSLContext sslContext) {
          super(new InetSocketAddress(port));
          this.setWebSocketFactory(new DefaultSSLWebSocketServerFactory(sslContext));
      }
  
      @Override
      public void onOpen(WebSocket conn, ClientHandshake handshake) {
          //This method sends a message to the new client
          conn.send("Welcome to the server!");
          broadcast("new connection: "
                  + handshake.getResourceDescriptor()); //This method sends a message to all
          // clients connected
          System.out.println(
                  conn.getRemoteSocketAddress().getAddress().getHostAddress() + " entered the room!");
      }
  
      @Override
      public void onClose(WebSocket conn, int code, String reason, boolean remote) {
          broadcast(conn + " has left the room!");
          System.out.println(conn + " has left the room!");
      }
  
      @Override
      public void onMessage(WebSocket conn, String message) {
          broadcast(message);
          System.out.println(conn + ": " + message);
      }
  
      @Override
      public void onMessage(WebSocket conn, ByteBuffer message) {
          broadcast(message.array());
          System.out.println(conn + ": " + message);
      }
  
      @Override
      public void onError(WebSocket conn, Exception ex) {
          ex.printStackTrace();
          if (conn != null) {
              // some errors like port binding failed may not be assignable to a specific websocket
          }
      }
  
      @Override
      public void onStart() {
          System.out.println("Server started!");
          setConnectionLostTimeout(0);
          setConnectionLostTimeout(100);
      }
  }
  

• 開啓Websocket Server

    private void startServer() {
      if (mWsServer != null) {
          try {
              mWsServer.stop();
          } catch (IOException e) {
              e.printStackTrace();
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      }
      mWsServer = new WsServer(8888, makeSSLContext());
      mWsServer.start();
  }
  

WebSocket Client

客戶端的實現，可以直接使用Java-WebSocket庫的Client，可以參考這個項目中的examples，但在本示例中，我是使用OkHttp的Websocket來實現客戶端。

• 創建客戶端項目，並依賴OkHttp

• 創建OKHttpClient

  private void initClient() {
          CustomTrust ct = new CustomTrust();
          client = new OkHttpClient.Builder()
                  .sslSocketFactory(ct.sslSocketFactory, ct.trustManager)
                  .hostnameVerifier(hostnameVerifier)
                  .build();
      }
  

• 連接遠端服務器

   private void connectWs() {
          if (mWebSocket != null) {
              mWebSocket.cancel();
              mListener = null;
          }
          Request.Builder builder = new Request.Builder();
          // wss : support ssl
          // Both server and client can run in a same device, so the ip can be set to localhost
          builder.url("wss://localhost:8888");
          Request request = builder.build();
          mListener = new WsSocketListener();
          mWebSocket = client.newWebSocket(request, mListener);
      }
  

效果展示

源碼下載

源碼下載