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<!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 I/O stream of JSON."><title>from_reader in serde_json::de - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="serde_json" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../../static.files/storage-e2aeef58.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-044be391.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]--><rustdoc-topbar><h2><a href="#">from_reader</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../serde_json/index.html">serde_<wbr>json</a><span class="version">1.0.149</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">from_<wbr>reader</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#example" title="Example">Example</a></li><li><a href="#errors" title="Errors">Errors</a></li></ul></section><div id="rustdoc-modnav"><h2><a href="index.html">In serde_<wbr>json::<wbr>de</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="../index.html">serde_json</a>::<wbr><a href="index.html">de</a></div><h1>Function <span class="fn">from_<wbr>reader</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../src/serde_json/de.rs.html#2614-2620">Source</a> </span></div><pre class="rust item-decl"><code>pub fn from_reader<R, T>(rdr: R) -> <a class="type" href="../type.Result.html" title="type serde_json::Result">Result</a><T><div class="where">where
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R: <a class="trait" href="https://doc.rust-lang.org/1.93.1/std/io/trait.Read.html" title="trait std::io::Read">Read</a>,
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T: <a class="trait" href="../../serde_core/de/trait.DeserializeOwned.html" title="trait serde_core::de::DeserializeOwned">DeserializeOwned</a>,</div></code></pre><details class="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 I/O stream of JSON.</p>
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<p>The content of the I/O stream is deserialized directly from the stream
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without being buffered in memory by serde_json.</p>
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<p>When reading from a source against which short reads are not efficient, such
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as a <a href="https://doc.rust-lang.org/1.93.1/std/fs/struct.File.html" title="struct std::fs::File"><code>File</code></a>, you will want to apply your own buffering because serde_json
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will not buffer the input. See <a href="https://doc.rust-lang.org/1.93.1/std/io/buffered/bufreader/struct.BufReader.html" title="struct std::io::buffered::bufreader::BufReader"><code>std::io::BufReader</code></a>.</p>
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<p>It is expected that the input stream ends after the deserialized object.
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If the stream does not end, such as in the case of a persistent socket connection,
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this function will not return. It is possible instead to deserialize from a prefix of an input
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stream without looking for EOF by managing your own <a href="../struct.Deserializer.html" title="struct serde_json::Deserializer"><code>Deserializer</code></a>.</p>
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<p>Note that counter to intuition, this function is usually slower than
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reading a file completely into memory and then applying <a href="../fn.from_str.html" title="fn serde_json::from_str"><code>from_str</code></a>
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or <a href="../fn.from_slice.html" title="fn serde_json::from_slice"><code>from_slice</code></a> on it. See <a href="https://github.com/serde-rs/json/issues/160">issue #160</a>.</p>
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<h2 id="example"><a class="doc-anchor" href="#example">§</a>Example</h2>
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<p>Reading the contents of a file.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::Deserialize;
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<span class="kw">use </span>std::error::Error;
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<span class="kw">use </span>std::fs::File;
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<span class="kw">use </span>std::io::BufReader;
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<span class="kw">use </span>std::path::Path;
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<span class="attr">#[derive(Deserialize, Debug)]
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</span><span class="kw">struct </span>User {
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fingerprint: String,
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location: String,
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}
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<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>> {
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<span class="comment">// Open the file in read-only mode with buffer.
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</span><span class="kw">let </span>file = File::open(path)<span class="question-mark">?</span>;
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<span class="kw">let </span>reader = BufReader::new(file);
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<span class="comment">// Read the JSON contents of the file as an instance of `User`.
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</span><span class="kw">let </span>u = serde_json::from_reader(reader)<span class="question-mark">?</span>;
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<span class="comment">// Return the `User`.
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</span><span class="prelude-val">Ok</span>(u)
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>u = read_user_from_file(<span class="string">"test.json"</span>).unwrap();
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<span class="macro">println!</span>(<span class="string">"{:#?}"</span>, u);
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}</code></pre></div>
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<p>Reading from a persistent socket connection.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>serde::Deserialize;
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<span class="kw">use </span>std::error::Error;
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<span class="kw">use </span>std::io::BufReader;
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<span class="kw">use </span>std::net::{TcpListener, TcpStream};
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<span class="attr">#[derive(Deserialize, Debug)]
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</span><span class="kw">struct </span>User {
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fingerprint: String,
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location: String,
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}
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<span class="kw">fn </span>read_user_from_stream(stream: <span class="kw-2">&mut </span>BufReader<TcpStream>) -> <span class="prelude-ty">Result</span><User, Box<<span class="kw">dyn </span>Error>> {
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<span class="kw">let </span><span class="kw-2">mut </span>de = serde_json::Deserializer::from_reader(stream);
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<span class="kw">let </span>u = User::deserialize(<span class="kw-2">&mut </span>de)<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(u)
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>listener = TcpListener::bind(<span class="string">"127.0.0.1:4000"</span>).unwrap();
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<span class="kw">for </span>tcp_stream <span class="kw">in </span>listener.incoming() {
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<span class="kw">let </span><span class="kw-2">mut </span>buffered = BufReader::new(tcp_stream.unwrap());
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<span class="macro">println!</span>(<span class="string">"{:#?}"</span>, read_user_from_stream(<span class="kw-2">&mut </span>buffered));
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}
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}</code></pre></div><h2 id="errors"><a class="doc-anchor" href="#errors">§</a>Errors</h2>
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<p>This conversion can fail if the structure of the input does not match the
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structure expected by <code>T</code>, for example if <code>T</code> is a struct type but the input
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contains something other than a JSON map. It can also fail if the structure
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is correct but <code>T</code>’s implementation of <code>Deserialize</code> decides that something
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is wrong with the data, for example required struct fields are missing from
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the JSON map or some number is too big to fit in the expected primitive
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type.</p>
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</div></details></section></div></main></body></html> |