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<!DOCTYPE html>
<!--
| Generated by Apache Maven Doxia Site Renderer 1.9.2 from src/site/markdown/socket_instance.md at 2025-02-18
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<title>socket.io-client – The Socket instance</title>
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<div class="pull-left"><div id="bannerLeft"><h2>Socket.IO Java client</h2>
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<li id="publishDate">Last Published: 2025-02-18<span class="divider">|</span>
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<li id="projectVersion">Version: 2.1.2</li>
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<li><a href="listening_to_events.html" title="Listening to events"><span class="none"></span>Listening to events</a></li>
<li class="active"><a href="#"><span class="none"></span>The Socket instance</a></li>
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<h1>The Socket instance</h1>
<p><b>Table of content</b></p>
<ul>
<li><a href="#Socket.23id">Socket#id</a></li>
<li><a href="#Socket.23connected">Socket#connected</a></li>
<li><a href="#Lifecycle">Lifecycle</a></li>
<li><a href="#Events">Events</a>
<ul>
<li><a href="#Socket.EVENT_CONNECT">Socket.EVENT_CONNECT</a></li>
<li><a href="#Socket.EVENT_CONNECT_ERROR">Socket.EVENT_CONNECT_ERROR</a></li>
<li><a href="#Socket.EVENT_DISCONNECT">Socket.EVENT_DISCONNECT</a></li></ul></li></ul>
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<li><a href="apidocs/index.html?io/socket/client/Socket.html">Javadoc</a></li>
</ul>
<p>Besides <a href="emitting_events.html">emitting</a> and <a href="listening_to_events.html">listening to</a> events, the Socket instance has a few attributes that may be of use in your application:</p><section>
<h2><a name="Socket.23id"></a>Socket#id</h2>
<p>Each new connection is assigned a random 20-characters identifier.</p>
<p>This identifier is synced with the value on the server-side.</p>
<p><i>Server</i></p>
<div class="source">
<div class="source"><pre class="prettyprint">io.on("connection", (socket) => {
console.log(socket.id); // x8WIv7-mJelg7on_ALbx
});
</pre></div></div>
<p><i>Client</i></p>
<div class="source">
<div class="source"><pre class="prettyprint">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
}
});
</pre></div></div>
</section><section>
<h2><a name="Socket.23connected"></a>Socket#connected</h2>
<p>This attribute describes whether the socket is currently connected to the server.</p>
<div class="source">
<div class="source"><pre class="prettyprint">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
}
});
</pre></div></div>
</section><section>
<h2><a name="Lifecycle"></a>Lifecycle</h2>
<p><img src="images/client_socket_events.png" alt="Lifecycle diagram" style="max-width: 800px" /></p></section><section>
<h2><a name="Events"></a>Events</h2><section>
<h3><a name="Socket.EVENT_CONNECT"></a><code>Socket.EVENT_CONNECT</code></h3>
<p>This event is fired by the Socket instance upon connection / reconnection.</p>
<div class="source">
<div class="source"><pre class="prettyprint">socket.on(Socket.EVENT_CONNECT, new Emitter.Listener() {
@Override
public void call(Object... args) {
// ...
}
});
</pre></div></div>
<p>Please note that you shouldn’t register event handlers in the <code>connect</code> handler itself, as a new handler will be registered every time the Socket reconnects:</p>
<div class="source">
<div class="source"><pre class="prettyprint">// 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) {
// ...
}
});
</pre></div></div>
</section><section>
<h3><a name="Socket.EVENT_CONNECT_ERROR"></a><code>Socket.EVENT_CONNECT_ERROR</code></h3>
<p>This event is fired when the server does not accept the connection (in a <a class="externalLink" href="https://socket.io/docs/v4/middlewares/#sending-credentials">middleware function</a>).</p>
<p>You need to manually reconnect. You might need to update the credentials:</p>
<div class="source">
<div class="source"><pre class="prettyprint">socket.on(Socket.EVENT_CONNECT_ERROR, new Emitter.Listener() {
@Override
public void call(Object... args) {
options.auth.put("authorization", "bearer 1234");
socket.connect();
}
});
</pre></div></div>
</section><section>
<h3><a name="Socket.EVENT_DISCONNECT"></a><code>Socket.EVENT_DISCONNECT</code></h3>
<p>This event is fired upon disconnection.</p>
<div class="source">
<div class="source"><pre class="prettyprint">socket.on(Socket.EVENT_DISCONNECT, new Emitter.Listener() {
@Override
public void call(Object... args) {
System.out.println(socket.id()); // null
}
});
</pre></div></div>
<p>Here is the list of possible reasons:</p>
<table border="0" class="table table-striped">
<thead>
<tr class="a">
<th>Reason </th>
<th> Description</th></tr>
</thead><tbody>
<tr class="b">
<td><code>io server disconnect</code> </td>
<td> The server has forcefully disconnected the socket with <a class="externalLink" href="https://socket.io/docs/v4/server-api/#socketdisconnectclose">socket.disconnect()</a></td></tr>
<tr class="a">
<td><code>io client disconnect</code> </td>
<td> The socket was manually disconnected using <code>socket.disconnect()</code></td></tr>
<tr class="b">
<td><code>ping timeout</code> </td>
<td> The server did not respond in the <code>pingTimeout</code> range</td></tr>
<tr class="a">
<td><code>transport close</code> </td>
<td> The connection was closed (example: the user has lost connection, or the network was changed from WiFi to 4G)</td></tr>
<tr class="b">
<td><code>transport error</code> </td>
<td> The connection has encountered an error (example: the server was killed during a HTTP long-polling cycle)</td></tr>
</tbody>
</table>
<p>Note: those events, along with <code>disconnecting</code>, <code>newListener</code> and <code>removeListener</code>, are special events that shouldn’t be used in your application:</p>
<div class="source">
<div class="source"><pre class="prettyprint">// BAD, will throw an error
socket.emit("disconnect");
</pre></div></div></section></section>
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