mirror of
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synced 2026-01-17 12:47:14 +08:00
rebuild
This commit is contained in:
chai2010
219
ch4/ch4-05.html
219
ch4/ch4-05.html
@@ -5,7 +5,7 @@
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||||
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||||
<meta charset="UTF-8">
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||||
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
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||||
<title>JSON | Go编程语言</title>
|
||||
<title>JSON | Go语言圣经</title>
|
||||
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
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<meta name="description" content="">
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<meta name="generator" content="GitBook 2.5.2">
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||||
@@ -48,7 +48,7 @@
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||||
<body>
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||||
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||||
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||||
<div class="book" data-level="4.5" data-chapter-title="JSON" data-filepath="ch4/ch4-05.md" data-basepath=".." data-revision="Mon Dec 28 2015 16:03:52 GMT+0800 (中国标准时间)">
|
||||
<div class="book" data-level="4.5" data-chapter-title="JSON" data-filepath="ch4/ch4-05.md" data-basepath=".." data-revision="Thu Dec 31 2015 16:18:40 GMT+0800 (中国标准时间)">
|
||||
|
||||
|
||||
<div class="book-summary">
|
||||
@@ -575,7 +575,7 @@
|
||||
|
||||
<b>4.2.</b>
|
||||
|
||||
切片
|
||||
Slice
|
||||
</a>
|
||||
|
||||
|
||||
@@ -590,7 +590,7 @@
|
||||
|
||||
<b>4.3.</b>
|
||||
|
||||
字典
|
||||
Map
|
||||
</a>
|
||||
|
||||
|
||||
@@ -2013,7 +2013,7 @@
|
||||
<!-- Title -->
|
||||
<h1>
|
||||
<i class="fa fa-circle-o-notch fa-spin"></i>
|
||||
<a href="../" >Go编程语言</a>
|
||||
<a href="../" >Go语言圣经</a>
|
||||
</h1>
|
||||
</div>
|
||||
|
||||
@@ -2024,7 +2024,214 @@
|
||||
<section class="normal" id="section-">
|
||||
|
||||
<h2 id="45-json">4.5. JSON</h2>
|
||||
<p>TODO</p>
|
||||
<p>JavaScript對象表示法（JSON）是一種用於發送和接收結構化信息的標準協議。JSON併不是唯一標準協議。 XML（§7.14）、ASN.1和Google的Protocol Buffers都是類似的協議，併且有各自的特色，但是由於簡潔性、可讀性和流行程度等原因，JSON是應用最廣泛的一個。</p>
|
||||
<p>Go語言對於這些標準格式的編碼和解碼都有良好的支持，由標準庫中的encoding/json、encoding/xml、encoding/asn1等包提供（譯註：Protocol Buffers的支持由 github.com/golang/protobuf 包提供），併且這類包都有着相似的API接口。本節，我們將對重要的encoding/json包的用法做個概述。</p>
|
||||
<p>JSON是對JavaScript中各種值——字符串、數字、布爾值和對象——Unicode本文編碼。它可以用有效可讀的方式表示第三章的基礎數據類型和本章的數組、slice、結構體和map等聚合數據類型。</p>
|
||||
<p>基本的JSON類型有數字（十進製或科學記數法）、布爾值（true或false）、字符串，其中字符串是以雙引號包含的Unicode字符序列，支持和Go語言類似的反斜槓轉義特性，不過JSON使用的是\Uhhhh轉義數字來表示一個UTF-16編碼，而不是Go語言的rune類型。</p>
|
||||
<p>這些基礎類型可以通過JSON的數組和對象類型進行遞歸組合。一個JSON數組是一個有序的值序列，寫在一個方括號中併以逗號分隔；一個JSON數組可以用於編碼Go語言的數組和slice。一個JSON對象是一個字符串到值的映射，寫成以繫列的name:value對形式，用花括號包含併以逗號分隔；JSON的對象類型可以用於編碼Go語言的map類型（key類型是字符串）和結構體。例如：</p>
|
||||
<pre><code>boolean true
|
||||
number -273.15
|
||||
string "She said \"Hello, BF\""
|
||||
array ["gold", "silver", "bronze"]
|
||||
object {"year": 1980,
|
||||
"event": "archery",
|
||||
"medals": ["gold", "silver", "bronze"]}
|
||||
</code></pre><p>考慮一個應用程序，該程序負責收集各種電影評論併提供反饋功能。它的Movie數據類型和一個典型的表示電影的值列表如下所示。（其中結構體聲明中，Year和Color成員後面的字符串面值是結構體成員Tag；我們稍後會解釋它的作用。）</p>
|
||||
<pre><code class="lang-Go">gopl.io/ch4/movie
|
||||
|
||||
<span class="hljs-keyword">type</span> Movie <span class="hljs-keyword">struct</span> {
|
||||
Title <span class="hljs-typename">string</span>
|
||||
Year <span class="hljs-typename">int</span> <span class="hljs-string">`json:"released"`</span>
|
||||
Color <span class="hljs-typename">bool</span> <span class="hljs-string">`json:"color,omitempty"`</span>
|
||||
Actors []<span class="hljs-typename">string</span>
|
||||
}
|
||||
|
||||
<span class="hljs-keyword">var</span> movies = []Movie{
|
||||
{Title: <span class="hljs-string">"Casablanca"</span>, Year: <span class="hljs-number">1942</span>, Color: <span class="hljs-constant">false</span>,
|
||||
Actors: []<span class="hljs-typename">string</span>{<span class="hljs-string">"Humphrey Bogart"</span>, <span class="hljs-string">"Ingrid Bergman"</span>}},
|
||||
{Title: <span class="hljs-string">"Cool Hand Luke"</span>, Year: <span class="hljs-number">1967</span>, Color: <span class="hljs-constant">true</span>,
|
||||
Actors: []<span class="hljs-typename">string</span>{<span class="hljs-string">"Paul Newman"</span>}},
|
||||
{Title: <span class="hljs-string">"Bullitt"</span>, Year: <span class="hljs-number">1968</span>, Color: <span class="hljs-constant">true</span>,
|
||||
Actors: []<span class="hljs-typename">string</span>{<span class="hljs-string">"Steve McQueen"</span>, <span class="hljs-string">"Jacqueline Bisset"</span>}},
|
||||
<span class="hljs-comment">// ...</span>
|
||||
}
|
||||
</code></pre>
|
||||
<p>這樣的數據結構特别適合JSON格式，併且在兩種之間相互轉換也很容易。將一個Go語言中類似movies的結構體slice轉爲JSON的過程叫編組（marshaling）。編組通過調用json.Marshal函數完成：</p>
|
||||
<pre><code class="lang-Go">data, err := json.Marshal(movies)
|
||||
<span class="hljs-keyword">if</span> err != <span class="hljs-constant">nil</span> {
|
||||
log.Fatalf(<span class="hljs-string">"JSON marshaling failed: %s"</span>, err)
|
||||
}
|
||||
fmt.Printf(<span class="hljs-string">"%s\n"</span>, data)
|
||||
</code></pre>
|
||||
<p>Marshal函數生成一個字節slice，包含很長的字符串，併且沒有空白縮進；我們將它摺行以便於顯示：</p>
|
||||
<pre><code>[{"Title":"Casablanca","released":1942,"Actors":["Humphrey Bogart","Ingr
|
||||
id Bergman"]},{"Title":"Cool Hand Luke","released":1967,"color":true,"Ac
|
||||
tors":["Paul Newman"]},{"Title":"Bullitt","released":1968,"color":true,"
|
||||
Actors":["Steve McQueen","Jacqueline Bisset"]}]
|
||||
</code></pre><p>這種緊湊的表示形式雖然包含了全部的信息，但是很難閲讀。爲了生成便於閲讀的格式，另一個json.MarshalIndent函數將産生整齊縮進的輸出。有兩個額外的字符串參數用於表示每一行輸出的前綴和每一個層級的縮進：</p>
|
||||
<pre><code class="lang-Go">data, err := json.MarshalIndent(movies, <span class="hljs-string">""</span>, <span class="hljs-string">" "</span>)
|
||||
<span class="hljs-keyword">if</span> err != <span class="hljs-constant">nil</span> {
|
||||
log.Fatalf(<span class="hljs-string">"JSON marshaling failed: %s"</span>, err)
|
||||
}
|
||||
fmt.Printf(<span class="hljs-string">"%s\n"</span>, data)
|
||||
</code></pre>
|
||||
<p>上面的代碼將産生這樣的輸出：</p>
|
||||
<pre><code class="lang-Json">[
|
||||
{
|
||||
<span class="hljs-string">"Title"</span>: <span class="hljs-string">"Casablanca"</span>,
|
||||
<span class="hljs-string">"released"</span>: <span class="hljs-number">1942</span>,
|
||||
<span class="hljs-string">"Actors"</span>: [
|
||||
<span class="hljs-string">"Humphrey Bogart"</span>,
|
||||
<span class="hljs-string">"Ingrid Bergman"</span>
|
||||
]
|
||||
},
|
||||
{
|
||||
<span class="hljs-string">"Title"</span>: <span class="hljs-string">"Cool Hand Luke"</span>,
|
||||
<span class="hljs-string">"released"</span>: <span class="hljs-number">1967</span>,
|
||||
<span class="hljs-string">"color"</span>: <span class="hljs-literal">true</span>,
|
||||
<span class="hljs-string">"Actors"</span>: [
|
||||
<span class="hljs-string">"Paul Newman"</span>
|
||||
]
|
||||
},
|
||||
{
|
||||
<span class="hljs-string">"Title"</span>: <span class="hljs-string">"Bullitt"</span>,
|
||||
<span class="hljs-string">"released"</span>: <span class="hljs-number">1968</span>,
|
||||
<span class="hljs-string">"color"</span>: <span class="hljs-literal">true</span>,
|
||||
<span class="hljs-string">"Actors"</span>: [
|
||||
<span class="hljs-string">"Steve McQueen"</span>,
|
||||
<span class="hljs-string">"Jacqueline Bisset"</span>
|
||||
]
|
||||
}
|
||||
]
|
||||
</code></pre>
|
||||
<p>在編碼時，默認使用Go語言結構體的成員名字作爲JSON的對象（通過reflect反射技術，我們將在12.6節討論）。隻有導出的結構體成員才會被編碼，這也就是我們爲什麽選擇用大寫字母開頭的成員名稱。</p>
|
||||
<p>細心的讀者可能已經註意到，其中Year名字的成員在編碼後變成了released，還有Color長遠編碼後變成了小寫字母開頭的color。這是因爲構體成員Tag所導致的。一個構體成員Tag是和在編譯階段關聯到該成員的元信息字符串：</p>
|
||||
<pre><code>Year int `json:"released"`
|
||||
Color bool `json:"color,omitempty"`
|
||||
</code></pre><p>結構體的成員Tag可以是任意的字符串面值，但是通常是一繫列用空格分隔的key:"value"鍵值對序列；因爲值中含義雙引號字符，因此成員Tag一般用原生字符串面值的形式書寫。json開頭鍵對應的值用於控製encoding/json包的編碼和解碼的行爲，併且encoding/...下面其它的包也遵循這個約定。成員Tag中json對應值的第一部分用於指定JSON對象的名字，比如將Go語言中的TotalCount成員對應到JSON中的total_count對象。Color成員的Tag還帶了一個額外的omitempty選項，表示當Go語言結構體成員爲空或零值時不生成JSON對象（這里false爲零值）。果然，Casablanca是一個黑白電影，併沒有輸出Color成員。</p>
|
||||
<p>編碼的逆操作是解碼，對應將JSON數據解碼爲GO語言的數據結構，Go語言中一般叫unmarshaling，通過json.Unmarshal函數完成。下面的代碼將JSON格式的電影數據解碼爲一個結構體slice，結構體中隻有Title成員。通過定義合適的Go語言數據結構，我們可以選擇性地解碼JSON中感興趣的成員。當Unmarshal返迴，slice將被隻含有Title信息值填充，其它JSON成員將被忽略。</p>
|
||||
<pre><code class="lang-Go"><span class="hljs-keyword">var</span> titles []<span class="hljs-keyword">struct</span>{ Title <span class="hljs-typename">string</span> }
|
||||
<span class="hljs-keyword">if</span> err := json.Unmarshal(data, &titles); err != <span class="hljs-constant">nil</span> {
|
||||
log.Fatalf(<span class="hljs-string">"JSON unmarshaling failed: %s"</span>, err)
|
||||
}
|
||||
fmt.Println(titles) <span class="hljs-comment">// "[{Casablanca} {Cool Hand Luke} {Bullitt}]"</span>
|
||||
</code></pre>
|
||||
<p>許多web服務都提供JSON接口，通過HTTP接口發送JSON格式請求併返迴JSON格式的信息。爲了説明這一點，我們通過Github的issue査詢服務。首先，我們要定義合適的類型和常量：</p>
|
||||
<pre><code class="lang-Go">gopl.io/ch4/github
|
||||
<span class="hljs-comment">// Package github provides a Go API for the GitHub issue tracker.</span>
|
||||
<span class="hljs-comment">// See https://developer.github.com/v3/search/#search-issues.</span>
|
||||
<span class="hljs-keyword">package</span> github
|
||||
|
||||
<span class="hljs-keyword">import</span> <span class="hljs-string">"time"</span>
|
||||
|
||||
<span class="hljs-keyword">const</span> IssuesURL = <span class="hljs-string">"https://api.github.com/search/issues"</span>
|
||||
|
||||
<span class="hljs-keyword">type</span> IssuesSearchResult <span class="hljs-keyword">struct</span> {
|
||||
TotalCount <span class="hljs-typename">int</span> <span class="hljs-string">`json:"total_count"`</span>
|
||||
Items []*Issue
|
||||
}
|
||||
|
||||
<span class="hljs-keyword">type</span> Issue <span class="hljs-keyword">struct</span> {
|
||||
Number <span class="hljs-typename">int</span>
|
||||
HTMLURL <span class="hljs-typename">string</span> <span class="hljs-string">`json:"html_url"`</span>
|
||||
Title <span class="hljs-typename">string</span>
|
||||
State <span class="hljs-typename">string</span>
|
||||
User *User
|
||||
CreatedAt time.Time <span class="hljs-string">`json:"created_at"`</span>
|
||||
Body <span class="hljs-typename">string</span> <span class="hljs-comment">// in Markdown format</span>
|
||||
}
|
||||
|
||||
<span class="hljs-keyword">type</span> User <span class="hljs-keyword">struct</span> {
|
||||
Login <span class="hljs-typename">string</span>
|
||||
HTMLURL <span class="hljs-typename">string</span> <span class="hljs-string">`json:"html_url"`</span>
|
||||
}
|
||||
</code></pre>
|
||||
<p>和前面一樣，卽使對應的JSON對象名是小寫字母，每個結構體的成員名也是聲明爲大小字母開頭的。因爲有些JSON成員名字和Go結構體成員名字併不相同，因此需要Go語言結構體成員Tag來指定對應的JSON名字。同樣，在解碼的時候也需要做同樣的處理，GitHub服務返迴的信息比我們定義的要多很多。</p>
|
||||
<p>SearchIssues函數發出一個HTTP請求，然後解碼返迴的JSON格式的結果。因爲用戶提供的査詢條件可能包含類似<code>?</code>和<code>&</code>之類的特殊字符，爲了避免對URL造成衝突，我們用url.QueryEscape來對査詢中的特殊字符進行轉義操作。</p>
|
||||
<pre><code class="lang-Go">gopl.io/ch4/github
|
||||
<span class="hljs-keyword">package</span> github
|
||||
|
||||
<span class="hljs-keyword">import</span> (
|
||||
<span class="hljs-string">"encoding/json"</span>
|
||||
<span class="hljs-string">"fmt"</span>
|
||||
<span class="hljs-string">"net/http"</span>
|
||||
<span class="hljs-string">"net/url"</span>
|
||||
<span class="hljs-string">"strings"</span>
|
||||
)
|
||||
|
||||
<span class="hljs-comment">// SearchIssues queries the GitHub issue tracker.</span>
|
||||
<span class="hljs-keyword">func</span> SearchIssues(terms []<span class="hljs-typename">string</span>) (*IssuesSearchResult, error) {
|
||||
q := url.QueryEscape(strings.Join(terms, <span class="hljs-string">" "</span>))
|
||||
resp, err := http.Get(IssuesURL + <span class="hljs-string">"?q="</span> + q)
|
||||
<span class="hljs-keyword">if</span> err != <span class="hljs-constant">nil</span> {
|
||||
<span class="hljs-keyword">return</span> <span class="hljs-constant">nil</span>, err
|
||||
}
|
||||
|
||||
<span class="hljs-comment">// We must close resp.Body on all execution paths.</span>
|
||||
<span class="hljs-comment">// (Chapter 5 presents 'defer', which makes this simpler.)</span>
|
||||
<span class="hljs-keyword">if</span> resp.StatusCode != http.StatusOK {
|
||||
resp.Body.Close()
|
||||
<span class="hljs-keyword">return</span> <span class="hljs-constant">nil</span>, fmt.Errorf(<span class="hljs-string">"search query failed: %s"</span>, resp.Status)
|
||||
}
|
||||
|
||||
<span class="hljs-keyword">var</span> result IssuesSearchResult
|
||||
<span class="hljs-keyword">if</span> err := json.NewDecoder(resp.Body).Decode(&result); err != <span class="hljs-constant">nil</span> {
|
||||
resp.Body.Close()
|
||||
<span class="hljs-keyword">return</span> <span class="hljs-constant">nil</span>, err
|
||||
}
|
||||
resp.Body.Close()
|
||||
<span class="hljs-keyword">return</span> &result, <span class="hljs-constant">nil</span>
|
||||
}
|
||||
</code></pre>
|
||||
<p>在早些的例子中，我們使用了json.Unmarshal函數來將JSON格式的字符串解碼爲字節slice。但是這個例子中，我們使用了基於流式的解碼器json.Decoder，它可以從一個流解碼JSON數據，盡管這不是必鬚的。如您所料，還有一個針對輸出流的json.Encoder編碼對象。</p>
|
||||
<p>我們調用Decode方法來填充變量。這里有多種方法可以格式化結構。下面是最簡單的一種，以一個固定寬度打印每個issue，但是在下一節我們將看到如果利用模闆來輸出複雜的格式。</p>
|
||||
<pre><code class="lang-Go">gopl.io/ch4/issues
|
||||
|
||||
<span class="hljs-comment">// Issues prints a table of GitHub issues matching the search terms.</span>
|
||||
<span class="hljs-keyword">package</span> main
|
||||
|
||||
<span class="hljs-keyword">import</span> (
|
||||
<span class="hljs-string">"fmt"</span>
|
||||
<span class="hljs-string">"log"</span>
|
||||
<span class="hljs-string">"os"</span>
|
||||
|
||||
<span class="hljs-string">"gopl.io/ch4/github"</span>
|
||||
)
|
||||
|
||||
<span class="hljs-keyword">func</span> main() {
|
||||
result, err := github.SearchIssues(os.Args[<span class="hljs-number">1</span>:])
|
||||
<span class="hljs-keyword">if</span> err != <span class="hljs-constant">nil</span> {
|
||||
log.Fatal(err)
|
||||
}
|
||||
fmt.Printf(<span class="hljs-string">"%d issues:\n"</span>, result.TotalCount)
|
||||
<span class="hljs-keyword">for</span> _, item := <span class="hljs-keyword">range</span> result.Items {
|
||||
fmt.Printf(<span class="hljs-string">"#%-5d %9.9s %.55s\n"</span>,
|
||||
item.Number, item.User.Login, item.Title)
|
||||
}
|
||||
}
|
||||
</code></pre>
|
||||
<p>通過命令行參數指定檢索條件。下面的命令是査詢Go語言項目中和JSON解碼相關的問題，還有査詢返迴的結果：</p>
|
||||
<pre><code>$ go build gopl.io/ch4/issues
|
||||
$ ./issues repo:golang/go is:open json decoder
|
||||
13 issues:
|
||||
#5680 eaigner encoding/json: set key converter on en/decoder
|
||||
#6050 gopherbot encoding/json: provide tokenizer
|
||||
#8658 gopherbot encoding/json: use bufio
|
||||
#8462 kortschak encoding/json: UnmarshalText confuses json.Unmarshal
|
||||
#5901 rsc encoding/json: allow override type marshaling
|
||||
#9812 klauspost encoding/json: string tag not symmetric
|
||||
#7872 extempora encoding/json: Encoder internally buffers full output
|
||||
#9650 cespare encoding/json: Decoding gives errPhase when unmarshalin
|
||||
#6716 gopherbot encoding/json: include field name in unmarshal error me
|
||||
#6901 lukescott encoding/json, encoding/xml: option to treat unknown fi
|
||||
#6384 joeshaw encoding/json: encode precise floating point integers u
|
||||
#6647 btracey x/tools/cmd/godoc: display type kind of each named type
|
||||
#4237 gjemiller encoding/base64: URLEncoding padding is optional
|
||||
</code></pre><p>GitHub的Web服務接口 <a href="https://developer.github.com/v3/" target="_blank">https://developer.github.com/v3/</a> 包含了更多的特性。</p>
|
||||
<p><strong>練習 4.10：</strong> 脩改issues程序，根據問題的時間進行分類，比如不到一個月的、不到一年的、超過一年。</p>
|
||||
<p><strong>練習 4.11：</strong> 編寫一個工具，允許用戶在命令行創建、讀取、更新和刪除GitHub上的issue，當必要的時候自動打開用戶默認的編輯器用於輸入文本信息。</p>
|
||||
<p><strong>練習 4.12：</strong> 流行的web漫畵服務xkcd也提供了JSON接口。例如，一個 <a href="https://xkcd.com/571/info.0.json" target="_blank">https://xkcd.com/571/info.0.json</a> 請求將返迴一個很多人喜愛的571編號的詳細描述。下載每個鏈接（隻下載一次）然後創建一個離線索引。編寫一個xkcd工具，使用這些離線索引，打印和命令行輸入的檢索詞相匹配的漫畵的URL。</p>
|
||||
<p><strong>練習 4.13：</strong> 使用開放電影數據庫的JSON服務接口，允許你檢索和下載 <a href="https://omdbapi.com/" target="_blank">https://omdbapi.com/</a> 上電影的名字和對應的海報圖像。編寫一個poster工具，通過命令行輸入的電影名字，下載對應的海報。</p>
|
||||
|
||||
|
||||
</section>
|
||||
|
||||
Reference in New Issue
Block a user