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site</a></li></ul></li></ul></div></div><main id="page-content"><div id="site-toc--side" class="site-toc"><header class="site-toc__title">Contents</header><ul class="section-nav"><li class="toc-entry nav-item"><a class="nav-link" href="#files-and-directories">Files and directories</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#reading-a-file-as-text">Reading a file as text</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#reading-a-file-as-binary">Reading a file as binary</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#handling-errors">Handling errors</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#streaming-file-contents">Streaming file contents</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#writing-file-contents">Writing file contents</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#listing-files-in-a-directory">Listing files in a directory</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#other-common-functionality">Other common functionality</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#http-clients-and-servers">HTTP clients and servers</a><ul class="nav"><li class="toc-entry nav-item"><a class="nav-link" href="#http-server">HTTP server</a></li><li class="toc-entry nav-item"><a class="nav-link" href="#http-client">HTTP client</a></li></ul></li><li class="toc-entry nav-item"><a class="nav-link" href="#more-information">More information</a></li></ul></div><article><div class="content"><nav id="subnav"><ul><li class="previous"><a href="/libraries/dart-convert">&lang;  dart:convert</a></li><li class="next"><a href="/libraries/dart-html">dart:html  &rang;</a></li></ul></nav><div id="site-content-title"><h1>dart:io</h1></div><div id="site-toc--inline" class="site-toc toc-collapsible toc-collapsed"><header class="site-toc__title">Contents keyboard_arrow_down keyboard_arrow_up</header><ul class="section-nav"><li class="toc-entry"><a href="#files-and-directories">Files and directories</a><ul><li class="toc-entry"><a href="#reading-a-file-as-text">Reading a file as text</a></li><li class="toc-entry"><a href="#reading-a-file-as-binary">Reading a file as binary</a></li><li class="toc-entry"><a href="#handling-errors">Handling errors</a></li><li class="toc-entry"><a href="#streaming-file-contents">Streaming file contents</a></li><li class="toc-entry"><a href="#writing-file-contents">Writing file contents</a></li><li class="toc-entry"><a href="#listing-files-in-a-directory">Listing files in a directory</a></li><li class="toc-entry"><a href="#other-common-functionality">Other common functionality</a></li></ul></li><li class="toc-entry"><a href="#http-clients-and-servers">HTTP clients and servers</a><ul><li class="toc-entry"><a href="#http-server">HTTP server</a></li><li class="toc-entry"><a href="#http-client">HTTP client</a></li></ul></li><li class="toc-entry"><a href="#more-information">More information</a></li></ul>more_horiz</div> <?code-excerpt plaster="none"?> The <a href="https://api.dart.dev/dart-io/dart-io-library.html">dart:io</a> library provides APIs to deal with files, directories, processes, sockets, WebSockets, and HTTP clients and servers.<aside class="alert alert-warning"><div class="alert-header">error Important</div><div class="alert-content">Only non-web <a href="https://flutter.dev">Flutter apps,</a> command-line scripts, and servers can import and use <code>dart:io</code>, not web apps.</div></aside>In general, the dart:io library implements and promotes an asynchronous API. Synchronous methods can easily block an application, making it difficult to scale. Therefore, most operations return results via Future or Stream objects, a pattern common with modern server platforms such as Node.js.The few synchronous methods in the dart:io library are clearly marked with a Sync suffix on the method name. Synchronous methods aren't covered here.To use the dart:io library you must import it: <?code-excerpt "misc/test/library_tour/io_test.dart (import)"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>import 'dart:io';</code></pre></div></div><div class="header-wrapper"><h2 id="files-and-directories">Files and directories</h2><a class="heading-link" href="#files-and-directories" aria-label="Link to 'Files and directories' section">#</a></div>The I/O library enables command-line apps to read and write files and browse directories. You have two choices for reading the contents of a file: all at once, or streaming. Reading a file all at once requires enough memory to store all the contents of the file. If the file is very large or you want to process it while reading it, you should use a Stream, as described in <a href="#streaming-file-contents">Streaming file contents</a>.<div class="header-wrapper"><h3 id="reading-a-file-as-text">Reading a file as text</h3><a class="heading-link" href="#reading-a-file-as-text" aria-label="Link to 'Reading a file as text' section">#</a></div>When reading a text file encoded using UTF-8, you can read the entire file contents with <code>readAsString()</code>. When the individual lines are important, you can use <code>readAsLines()</code>. In both cases, a Future object is returned that provides the contents of the file as one or more strings. <?code-excerpt "misc/test/library_tour/io_test.dart (read-as-string)" replace="/\btest_data\///g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { var config = File('config.txt'); // Put the whole file in a single string. var stringContents = await config.readAsString(); print('The file is ${stringContents.length} characters long.'); // Put each line of the file into its own string. var lines = await config.readAsLines(); print('The file is ${lines.length} lines long.'); }</code></pre></div></div><div class="header-wrapper"><h3 id="reading-a-file-as-binary">Reading a file as binary</h3><a class="heading-link" href="#reading-a-file-as-binary" aria-label="Link to 'Reading a file as binary' section">#</a></div>The following code reads an entire file as bytes into a list of ints. The call to <code>readAsBytes()</code> returns a Future, which provides the result when it's available. <?code-excerpt "misc/test/library_tour/io_test.dart (read-as-bytes)" replace="/\btest_data\///g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { var config = File('config.txt'); var contents = await config.readAsBytes(); print('The file is ${contents.length} bytes long.'); }</code></pre></div></div><div class="header-wrapper"><h3 id="handling-errors">Handling errors</h3><a class="heading-link" href="#handling-errors" aria-label="Link to 'Handling errors' section">#</a></div>To capture errors so they don't result in uncaught exceptions, you can register a <code>catchError</code> handler on the Future, or (in an <code>async</code> function) use try-catch: <?code-excerpt "misc/test/library_tour/io_test.dart (try-catch)" replace="/does-not-exist/config/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { var config = File('config.txt'); try { var contents = await config.readAsString(); print(contents); } catch (e) { print(e); } }</code></pre></div></div><div class="header-wrapper"><h3 id="streaming-file-contents">Streaming file contents</h3><a class="heading-link" href="#streaming-file-contents" aria-label="Link to 'Streaming file contents' section">#</a></div>Use a Stream to read a file, a little at a time. You can use either the <a href="/libraries/dart-async#stream">Stream API</a> or <code>await for</code>, part of Dart's <a href="/language/async">asynchrony support.</a> <?code-excerpt "misc/test/library_tour/io_test.dart (read-from-stream)" replace="/_?test_\w*\/?//g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>import 'dart:io'; import 'dart:convert'; void main() async { var config = File('config.txt'); Stream<List<int>> inputStream = config.openRead(); var lines = utf8.decoder.bind(inputStream).transform(const LineSplitter()); try { await for (final line in lines) { print('Got ${line.length} characters from stream'); } print('file is now closed'); } catch (e) { print(e); } }</code></pre></div></div><div class="header-wrapper"><h3 id="writing-file-contents">Writing file contents</h3><a class="heading-link" href="#writing-file-contents" aria-label="Link to 'Writing file contents' section">#</a></div>You can use an <a href="https://api.dart.dev/dart-io/IOSink-class.html">IOSink</a> to write data to a file. Use the File <code>openWrite()</code> method to get an IOSink that you can write to. The default mode, <code>FileMode.write</code>, completely overwrites existing data in the file. <?code-excerpt "misc/test/library_tour/io_test.dart (write-file)" replace="/\btest_data\///g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>var logFile = File('log.txt'); var sink = logFile.openWrite(); sink.write('FILE ACCESSED ${DateTime.now()}\n'); await sink.flush(); await sink.close();</code></pre></div></div>To add to the end of the file, use the optional <code>mode</code> parameter to specify <code>FileMode.append</code>: <?code-excerpt "misc/test/library_tour/io_test.dart (append)" replace="/_?test_\w*\/?//g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>var sink = logFile.openWrite(mode: FileMode.append);</code></pre></div></div>To write binary data, use <code>add(List<int> data)</code>.<div class="header-wrapper"><h3 id="listing-files-in-a-directory">Listing files in a directory</h3><a class="heading-link" href="#listing-files-in-a-directory" aria-label="Link to 'Listing files in a directory' section">#</a></div>Finding all files and subdirectories for a directory is an asynchronous operation. The <code>list()</code> method returns a Stream that emits an object when a file or directory is encountered. <?code-excerpt "misc/test/library_tour/io_test.dart (list-dir)" replace="/\btest_data\b/tmp/g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { var dir = Directory('tmp'); try { var dirList = dir.list(); await for (final FileSystemEntity f in dirList) { if (f is File) { print('Found file ${f.path}'); } else if (f is Directory) { print('Found dir ${f.path}'); } } } catch (e) { print(e.toString()); } }</code></pre></div></div><div class="header-wrapper"><h3 id="other-common-functionality">Other common functionality</h3><a class="heading-link" href="#other-common-functionality" aria-label="Link to 'Other common functionality' section">#</a></div>The File and Directory classes contain other functionality, including but not limited to:<ul><li>Creating a file or directory: <code>create()</code> in File and Directory</li><li>Deleting a file or directory: <code>delete()</code> in File and Directory</li><li>Getting the length of a file: <code>length()</code> in File</li><li>Getting random access to a file: <code>open()</code> in File</li></ul>Refer to the API docs for <a href="https://api.dart.dev/dart-io/File-class.html">File</a> and <a href="https://api.dart.dev/dart-io/Directory-class.html">Directory</a> for a full list of methods.<div class="header-wrapper"><h2 id="http-clients-and-servers">HTTP clients and servers</h2><a class="heading-link" href="#http-clients-and-servers" aria-label="Link to 'HTTP clients and servers' section">#</a></div>The dart:io library provides classes that command-line apps can use for accessing HTTP resources, as well as running HTTP servers.<div class="header-wrapper"><h3 id="http-server">HTTP server</h3><a class="heading-link" href="#http-server" aria-label="Link to 'HTTP server' section">#</a></div>The <a href="https://api.dart.dev/dart-io/HttpServer-class.html">HttpServer</a> class provides the low-level functionality for building web servers. You can match request handlers, set headers, stream data, and more.The following sample web server returns simple text information. This server listens on port 8888 and address 127.0.0.1 (localhost), responding to requests for the path <code>/dart</code>. For any other path, the response is status code 404 (page not found). <?code-excerpt "misc/lib/library_tour/io/http_server.dart (process-requests)" replace="/Future<\w+\W/void/g; /\b_//g"?> <div class="code-block-wrapper language-dart"><div class="code-block-body">dart<pre class="shiki dash-light" tabindex="0"><code>void main() async { final requests = await HttpServer.bind('localhost', 8888); await for (final request in requests) { processRequest(request); } } void processRequest(HttpRequest request) { print('Got request for ${request.uri.path}'); final response = request.response; if (request.uri.path == '/dart') { response ..headers.contentType = ContentType( 'text', 'plain', ) ..write('Hello from the server'); } else { response.statusCode = HttpStatus.notFound; } response.close(); }</code></pre></div></div><div class="header-wrapper"><h3 id="http-client">HTTP client</h3><a class="heading-link" href="#http-client" aria-label="Link to 'HTTP client' section">#</a></div>You should avoid directly using <code>dart:io</code> to make HTTP requests. The <a href="https://api.dart.dev/dart-io/HttpClient-class.html">HttpClient</a> class in <code>dart:io</code> is platform-dependent and tied to a single implementation. Instead, use a higher-level library like <a href="https://pub.dev/packages/http"><code>package:http</code></a>.The <a href="/tutorials/server/fetch-data">Fetch data from the internet</a> tutorial explains how to make HTTP requests using <code>package:http</code>.<div class="header-wrapper"><h2 id="more-information">More information</h2><a class="heading-link" href="#more-information" aria-label="Link to 'More information' section">#</a></div>This page showed how to use the major features of the <a href="https://api.dart.dev/dart-io/dart-io-library.html">dart:io</a> library. Besides the APIs discussed in this section, the dart:io library also provides APIs for <a href="https://api.dart.dev/dart-io/Process-class.html">processes,</a> <a href="https://api.dart.dev/dart-io/Socket-class.html">sockets,</a> and <a href="https://api.dart.dev/dart-io/WebSocket-class.html">web sockets.</a> For more information about server-side and command-line app development, see the <a href="/server">server-side Dart overview.</a><nav id="subnav"><ul><li class="previous"><a href="/libraries/dart-convert">&lang;  dart:convert</a></li><li class="next"><a href="/libraries/dart-html">dart:html  &rang;</a></li></ul></nav>Unless stated otherwise, the documentation on this site reflects Dart 3.5.4. 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