Dart ServerSocket

last modified April 4, 2025

The ServerSocket class in Dart provides TCP server functionality. It listens for incoming client connections on a specified port and IP address.

ServerSocket is part of Dart’s dart:io library for server-side programming. It enables building network servers that handle multiple clients.

Basic Definition

ServerSocket represents a listening TCP socket that accepts incoming connections. Each connection creates a Socket object.

Key features include asynchronous operation, address binding, and connection management. It’s the foundation for network servers in Dart applications.

Basic Echo Server

This example creates a simple echo server that sends back received messages.

main.dart

import ‘dart:io’;

void main() async { var server = await ServerSocket.bind(‘127.0.0.1’, 4040); print(‘Echo server listening on ${server.address}:${server.port}’);

await for (var socket in server) { socket.listen( (data) { print(‘Received: ${String.fromCharCodes(data)}’); socket.add(data); // Echo back }, onDone: () => print(‘Client disconnected’), ); } }

We bind to localhost port 4040 and process each client connection asynchronously. The server echoes back any received data and logs client disconnections.

$ dart main.dart Echo server listening on 127.0.0.1:4040 Received: Hello Client disconnected

Handling Multiple Clients

This example demonstrates a server handling multiple concurrent clients.

main.dart

import ‘dart:io’;

void main() async { var server = await ServerSocket.bind(‘0.0.0.0’, 4041); print(‘Multi-client server running on port ${server.port}’);

var clientCount = 0;

await for (var socket in server) { clientCount++; print(‘Client $clientCount connected from ${socket.remoteAddress}’);

socket.listen(
  (data) => handleClientData(socket, data),
  onDone: () => print('Client $clientCount disconnected'),
);

} }

void handleClientData(Socket socket, List<int> data) { var message = String.fromCharCodes(data).trim(); print(‘Received: $message’); socket.write(‘Echo: $message\n’); }

The server tracks connected clients and processes their messages independently. Each client connection is handled in its own asynchronous execution context.

$ dart main.dart Multi-client server running on port 4041 Client 1 connected from 127.0.0.1:54321 Received: Test message Client 1 disconnected

Broadcast Server

This example shows a server that broadcasts messages to all connected clients.

main.dart

import ‘dart:io’; import ‘dart:async’;

void main() async { var server = await ServerSocket.bind(‘127.0.0.1’, 4042); print(‘Broadcast server started on port ${server.port}’);

var clients = <Socket>[];

server.listen((socket) { clients.add(socket); print(‘New client connected (${clients.length} total)’);

socket.listen(
  (data) => broadcast(clients, data),
  onDone: () {
    clients.remove(socket);
    print('Client disconnected (${clients.length} remaining)');
  },
);

});

// Broadcast periodic messages Timer.periodic(Duration(seconds: 5), (_) { if (clients.isNotEmpty) { broadcast(clients, ‘Server time: ${DateTime.now()}\n’.codeUnits); } }); }

void broadcast(List<Socket> clients, List<int> message) { for (var client in clients) { client.add(message); } }

The server maintains a list of connected clients and broadcasts messages to all. It also sends periodic time updates and manages client disconnections.

$ dart main.dart Broadcast server started on port 4042 New client connected (1 total) Client disconnected (0 remaining)

Secure Server with SSL

This example demonstrates creating a secure server with SSL/TLS encryption.

main.dart

import ‘dart:io’;

void main() async { var context = SecurityContext() ..useCertificateChain(‘certificate.pem’) ..usePrivateKey(‘key.pem’);

var server = await SecureServerSocket.bind( ‘0.0.0.0’, 4043, context, );

print(‘Secure server listening on port ${server.port}’);

await for (var socket in server) { socket.listen( (data) { var message = String.fromCharCodes(data); print(‘Secure message: $message’); socket.write(‘SECURE: $message’); }, onDone: () => print(‘Secure client disconnected’), ); } }

We create a secure context with certificate files and bind a secure socket. The server handles encrypted communications while maintaining the same API.

$ dart main.dart Secure server listening on port 4043 Secure message: Test Secure client disconnected

Connection Limits and Timeouts

This example shows how to implement connection limits and timeouts.

main.dart

import ‘dart:io’; import ‘dart:async’;

void main() async { var server = await ServerSocket.bind(‘127.0.0.1’, 4044); print(‘Managed server started with connection limits’);

var activeConnections = 0; const maxConnections = 3;

await for (var socket in server) { if (activeConnections >= maxConnections) { socket.write(‘Server busy. Try again later.\n’); socket.close(); continue; }

activeConnections++;
print('New connection ($activeConnections/$maxConnections)');

// Set timeout
var timer = Timer(Duration(seconds: 30), () {
  socket.write('Connection timeout\n');
  socket.close();
});

socket.listen(
  (data) {
    timer.cancel(); // Reset timeout on activity
    timer = Timer(Duration(seconds: 30), () => socket.close());
    socket.add(data);
  },
  onDone: () {
    timer.cancel();
    activeConnections--;
    print('Connection closed ($activeConnections/$maxConnections)');
  },
);

} }

The server enforces a maximum connection limit and implements idle timeouts. Each connection gets 30 seconds of inactivity before being automatically closed.

$ dart main.dart Managed server started with connection limits New connection (1/3) Connection closed (0/3)

Best Practices

• Error Handling: Always handle connection and I/O errors

• Resource Management: Close sockets properly when done

• Concurrency: Consider isolates for CPU-intensive tasks

• Security: Use SSL/TLS for sensitive communications

• Logging: Implement proper logging for debugging

Source

Dart ServerSocket Documentation

This tutorial covered Dart’s ServerSocket class with practical examples showing basic servers, secure connections, and advanced connection management.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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