# The Socket instance

**Table of content**

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- [Javadoc](apidocs/index.html?io/socket/client/Socket.html)

Besides [emitting](emitting_events.html) and [listening to](listening_to_events.html) 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*

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

*Client*

```java
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.

```java
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

<img src="images/client_socket_events.png" alt="Lifecycle diagram" style="max-width: 800px" />

## Events

### `Socket.EVENT_CONNECT`

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

```java
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:

```java
// 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](https://socket.io/docs/v4/middlewares/#sending-credentials)).

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

```java
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.

```java
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()](https://socket.io/docs/v4/server-api/#socketdisconnectclose)
`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:

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