| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Deserialize an instance of type `T` from an IO stream of JSON."><meta name="keywords" content="rust, rustlang, rust-lang, from_reader"><title>from_reader in serde_json - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="sidebar-items.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../serde_json/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../serde_json/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In serde_json</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Function <a href="index.html">serde_json</a>::<wbr><a class="fn" href="#">from_reader</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/serde_json/de.rs.html#2516-2522">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><pre class="rust fn"><code>pub fn from_reader<R, T>(rdr: R) -> <a class="type" href="type.Result.html" title="type serde_json::Result">Result</a><T><span class="where fmt-newline">where<br> R: Read,<br> T: <a class="trait" href="../serde/de/trait.DeserializeOwned.html" title="trait serde::de::DeserializeOwned">DeserializeOwned</a>,</span></code></pre></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Deserialize an instance of type <code>T</code> from an IO stream of JSON.</p> |
| <p>The content of the IO stream is deserialized directly from the stream |
| without being buffered in memory by serde_json.</p> |
| <p>When reading from a source against which short reads are not efficient, such |
| as a <a href="https://doc.rust-lang.org/std/fs/struct.File.html"><code>File</code></a>, you will want to apply your own buffering because serde_json |
| will not buffer the input. See <a href="https://doc.rust-lang.org/std/io/struct.BufReader.html"><code>std::io::BufReader</code></a>.</p> |
| <p>It is expected that the input stream ends after the deserialized object. |
| If the stream does not end, such as in the case of a persistent socket connection, |
| this function will not return. It is possible instead to deserialize from a prefix of an input |
| stream without looking for EOF by managing your own <a href="struct.Deserializer.html" title="Deserializer"><code>Deserializer</code></a>.</p> |
| <p>Note that counter to intuition, this function is usually slower than |
| reading a file completely into memory and then applying <a href="./fn.from_str.html"><code>from_str</code></a> |
| or <a href="./fn.from_slice.html"><code>from_slice</code></a> on it. See <a href="https://github.com/serde-rs/json/issues/160">issue #160</a>.</p> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <p>Reading the contents of a file.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::Deserialize; |
| |
| <span class="kw">use </span>std::error::Error; |
| <span class="kw">use </span>std::fs::File; |
| <span class="kw">use </span>std::io::BufReader; |
| <span class="kw">use </span>std::path::Path; |
| |
| <span class="attribute">#[derive(Deserialize, Debug)] |
| </span><span class="kw">struct </span>User { |
| fingerprint: String, |
| location: String, |
| } |
| |
| <span class="kw">fn </span>read_user_from_file<P: AsRef<Path>>(path: P) -> <span class="prelude-ty">Result</span><User, Box<<span class="kw">dyn </span>Error>> { |
| <span class="comment">// Open the file in read-only mode with buffer. |
| </span><span class="kw">let </span>file = File::open(path)<span class="question-mark">?</span>; |
| <span class="kw">let </span>reader = BufReader::new(file); |
| |
| <span class="comment">// Read the JSON contents of the file as an instance of `User`. |
| </span><span class="kw">let </span>u = serde_json::from_reader(reader)<span class="question-mark">?</span>; |
| |
| <span class="comment">// Return the `User`. |
| </span><span class="prelude-val">Ok</span>(u) |
| } |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>u = read_user_from_file(<span class="string">"test.json"</span>).unwrap(); |
| <span class="macro">println!</span>(<span class="string">"{:#?}"</span>, u); |
| }</code></pre></div> |
| <p>Reading from a persistent socket connection.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::Deserialize; |
| |
| <span class="kw">use </span>std::error::Error; |
| <span class="kw">use </span>std::net::{TcpListener, TcpStream}; |
| |
| <span class="attribute">#[derive(Deserialize, Debug)] |
| </span><span class="kw">struct </span>User { |
| fingerprint: String, |
| location: String, |
| } |
| |
| <span class="kw">fn </span>read_user_from_stream(tcp_stream: TcpStream) -> <span class="prelude-ty">Result</span><User, Box<<span class="kw">dyn </span>Error>> { |
| <span class="kw">let </span><span class="kw-2">mut </span>de = serde_json::Deserializer::from_reader(tcp_stream); |
| <span class="kw">let </span>u = User::deserialize(<span class="kw-2">&mut </span>de)<span class="question-mark">?</span>; |
| |
| <span class="prelude-val">Ok</span>(u) |
| } |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>listener = TcpListener::bind(<span class="string">"127.0.0.1:4000"</span>).unwrap(); |
| |
| <span class="kw">for </span>stream <span class="kw">in </span>listener.incoming() { |
| <span class="macro">println!</span>(<span class="string">"{:#?}"</span>, read_user_from_stream(stream.unwrap())); |
| } |
| }</code></pre></div> |
| <h2 id="errors"><a href="#errors">Errors</a></h2> |
| <p>This conversion can fail if the structure of the input does not match the |
| structure expected by <code>T</code>, for example if <code>T</code> is a struct type but the input |
| contains something other than a JSON map. It can also fail if the structure |
| is correct but <code>T</code>’s implementation of <code>Deserialize</code> decides that something |
| is wrong with the data, for example required struct fields are missing from |
| the JSON map or some number is too big to fit in the expected primitive |
| type.</p> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="serde_json" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
