The Socket instance

Table of content

Socket#id
Socket#connected
Lifecycle
Events

Javadoc

Besides emitting and listening to events, the Socket instance has a few attributes that may be of use in your application:

Socket#id

Each new connection is assigned a random 20-characters identifier.

This identifier is synced with the value on the server-side.

Server

io.on("connection", (socket) => {
  console.log(socket.id); // x8WIv7-mJelg7on_ALbx
});

Client

socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        System.out.println(socket.id()); // x8WIv7-mJelg7on_ALbx
    }
});

socket.on(Socket.EVENT_DISCONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        System.out.println(socket.id()); // null
    }
});

Socket#connected

This attribute describes whether the socket is currently connected to the server.

socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        System.out.println(socket.connected()); // true
    }
});

socket.on(Socket.EVENT_DISCONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        System.out.println(socket.connected()); // false
    }
});

Lifecycle

Lifecycle diagram

Events

`Socket.EVENT_CONNECT`

This event is fired by the Socket instance upon connection / reconnection.

socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        // ...
    }
});

Please note that you shouldn’t register event handlers in the connect handler itself, as a new handler will be registered every time the Socket reconnects:

// BAD
socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        socket.on("data", new Emitter.Listener() {
            @Override
            public void call(Object... args) {
                // ...
            }
        });
    }
});

// GOOD
socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        // ...
    }
});

socket.on("data", new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        // ...
    }
});

`Socket.EVENT_CONNECT_ERROR`

This event is fired when the server does not accept the connection (in a middleware function).

You need to manually reconnect. You might need to update the credentials:

socket.on(Socket.EVENT_CONNECT_ERROR, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        options.auth.put("authorization", "bearer 1234");
        socket.connect();
    }
});

`Socket.EVENT_DISCONNECT`

This event is fired upon disconnection.

socket.on(Socket.EVENT_DISCONNECT, new Emitter.Listener() {
    @Override
    public void call(Object... args) {
        System.out.println(socket.id()); // null
    }
});

Here is the list of possible reasons:

Reason	Description
`io server disconnect`	The server has forcefully disconnected the socket with socket.disconnect()
`io client disconnect`	The socket was manually disconnected using `socket.disconnect()`
`ping timeout`	The server did not respond in the `pingTimeout` range
`transport close`	The connection was closed (example: the user has lost connection, or the network was changed from WiFi to 4G)
`transport error`	The connection has encountered an error (example: the server was killed during a HTTP long-polling cycle)

Note: those events, along with disconnecting, newListener and removeListener, are special events that shouldn’t be used in your application:

// BAD, will throw an error
socket.emit("disconnect");