Understanding dart:io – Files, Sockets, HTTP, Processes, and Streams in Dart

The dart:io library provides APIs for non‑web Dart applications to perform file, socket, HTTP, and other I/O operations, making it essential for server‑side scripts, command‑line tools, and Flutter mobile apps.

Importing the library is straightforward: import 'dart:io'; File and directory operations are performed using classes such as File, Directory, and Link. For example, renaming a file:

File myFile = new File('我的文件.txt');
myFile.rename('新的文件名.txt').then((_) => print('文件被重命名'));

The abstract class FileSystemEntity is the superclass of these types and offers static methods like isDirectory, isFile, and exists to query file system information:

FileSystemEntity.isDirectory(myPath).then((isDir) {
  if (isDir) {
    print('$myPath 是目录');
  } else {
    print('$myPath 不是目录');
  }
});

Networking is supported via HttpServer and HttpClient. A simple HTTP server bound to IPv6 port 80 can be created as follows:

import 'dart:io';

main() {
  HttpServer.bind(InternetAddress.anyIPv6, 80).then((server) {
    server.listen((HttpRequest request) {
      request.response.write('Hello, world!');
      request.response.close();
    });
  });
}

Clients can use HttpClient to send requests:

HttpClient client = new HttpClient();
client.getUrl(Uri.parse("http://www.example.com/"))
    .then((HttpClientRequest request) => request.close())
    .then((HttpClientResponse response) {
      // Process response
    });

Process management is handled by the Process class. Starting a process and streaming its output:

Process.start('ls', ['-R', 'web']).then((process) {
  stdout.addStream(process.stdout);
  stderr.addStream(process.stderr);
  process.exitCode.then(print);
});

Running a command and obtaining the result:

Process.run('ls', ['-l']).then((ProcessResult results) {
  print(results.stdout);
});

WebSocket support allows full‑duplex communication. A server can upgrade an HTTP request to a WebSocket:

runZoned(() async {
  var server = await HttpServer.bind('127.0.0.1', 4040);
  server.listen((HttpRequest req) async {
    if (req.uri.path == '/ws') {
      var socket = await WebSocketTransformer.upgrade(req);
      socket.listen(handleMsg);
    }
  });
}, onError: (e) => print('An error occurred.'));

Clients connect with:

var socket = await WebSocket.connect('ws://127.0.0.1:4040/ws');
socket.add('Hello, World!');

ServerSocket and Socket provide TCP communication. A server listening on a port:

ServerSocket.bind('127.0.0.1', 4041).then((serverSocket) {
  serverSocket.listen((socket) {
    socket.transform(utf8.decoder).listen(print);
  });
});

A client connecting to the server:

Socket.connect('127.0.0.1', 4041).then((socket) {
  socket.write('Hello, World!');
});

The library also exposes standard streams stdout, stderr, and stdin. Writing to stdout: stdout.writeln('Hello, World!'); Writing multiple values to stderr:

stderr.writeAll(['That ', 'is ', 'an ', 'error.', '
']);

Reading a line synchronously from stdin: String inputText = stdin.readLineSync(); Overall, dart:io equips Dart developers with a comprehensive set of tools for file system access, network communication, process control, and standard I/O, making it a cornerstone for backend and Flutter mobile development.