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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface.html">前言</a></li><li class="chapter-item expanded "><a href="../ch1/ch1.html"><strong aria-hidden="true">1.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch1/ch1-01.html"><strong aria-hidden="true">1.1.</strong> Hello, World</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-02.html"><strong aria-hidden="true">1.2.</strong> 命令行参数</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-03.html"><strong aria-hidden="true">1.3.</strong> 查找重复的行</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-04.html"><strong aria-hidden="true">1.4.</strong> GIF动画</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-05.html"><strong aria-hidden="true">1.5.</strong> 获取URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-06.html"><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-07.html"><strong aria-hidden="true">1.7.</strong> Web服务</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-08.html"><strong aria-hidden="true">1.8.</strong> 本章要点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch2/ch2-01.html"><strong aria-hidden="true">2.1.</strong> 命名</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-02.html"><strong aria-hidden="true">2.2.</strong> 声明</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-03.html"><strong aria-hidden="true">2.3.</strong> 变量</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-04.html"><strong aria-hidden="true">2.4.</strong> 赋值</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-05.html"><strong aria-hidden="true">2.5.</strong> 类型</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-06.html"><strong aria-hidden="true">2.6.</strong> 包和文件</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-07.html"><strong aria-hidden="true">2.7.</strong> 作用域</a></li></ol></li><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch3/ch3-01.html"><strong aria-hidden="true">3.1.</strong> 整型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-02.html"><strong aria-hidden="true">3.2.</strong> 浮点数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-03.html"><strong aria-hidden="true">3.3.</strong> 复数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-04.html"><strong aria-hidden="true">3.4.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-05.html"><strong aria-hidden="true">3.5.</strong> 字符串</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-06.html"><strong aria-hidden="true">3.6.</strong> 常量</a></li></ol></li><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch4/ch4-01.html"><strong aria-hidden="true">4.1.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-02.html"><strong aria-hidden="true">4.2.</strong> Slice</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-03.html"><strong aria-hidden="true">4.3.</strong> Map</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-04.html"><strong aria-hidden="true">4.4.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-05.html"><strong aria-hidden="true">4.5.</strong> JSON</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-06.html"><strong aria-hidd
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<!-- 头部 -->
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<ul dir="auto">
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<li><em>KusonStack一站式可编程配置技术栈: <a href="https://github.com/KusionStack/kusion">https://github.com/KusionStack/kusion</a></em></li>
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<li><em>凹语言™: <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
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<h2 id="105-包的匿名导入"><a class="header" href="#105-包的匿名导入">10.5. 包的匿名导入</a></h2>
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<p>如果只是导入一个包而并不使用导入的包将会导致一个编译错误。但是有时候我们只是想利用导入包而产生的副作用:它会计算包级变量的初始化表达式和执行导入包的init初始化函数(§2.6.2)。这时候我们需要抑制“unused import”编译错误,我们可以用下划线<code>_</code>来重命名导入的包。像往常一样,下划线<code>_</code>为空白标识符,并不能被访问。</p>
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<pre><code class="language-Go">import _ "image/png" // register PNG decoder
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</code></pre>
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<p>这个被称为包的匿名导入。它通常是用来实现一个编译时机制,然后通过在main主程序入口选择性地导入附加的包。首先,让我们看看如何使用该特性,然后再看看它是如何工作的。</p>
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<p>标准库的image图像包包含了一个<code>Decode</code>函数,用于从<code>io.Reader</code>接口读取数据并解码图像,它调用底层注册的图像解码器来完成任务,然后返回image.Image类型的图像。使用<code>image.Decode</code>很容易编写一个图像格式的转换工具,读取一种格式的图像,然后编码为另一种图像格式:</p>
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<p><u><i>gopl.io/ch10/jpeg</i></u></p>
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<pre><code class="language-Go">// The jpeg command reads a PNG image from the standard input
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// and writes it as a JPEG image to the standard output.
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package main
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import (
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"fmt"
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"image"
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"image/jpeg"
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_ "image/png" // register PNG decoder
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"io"
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"os"
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)
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func main() {
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if err := toJPEG(os.Stdin, os.Stdout); err != nil {
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fmt.Fprintf(os.Stderr, "jpeg: %v\n", err)
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os.Exit(1)
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}
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}
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func toJPEG(in io.Reader, out io.Writer) error {
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img, kind, err := image.Decode(in)
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if err != nil {
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return err
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}
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fmt.Fprintln(os.Stderr, "Input format =", kind)
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return jpeg.Encode(out, img, &jpeg.Options{Quality: 95})
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}
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</code></pre>
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<p>如果我们将<code>gopl.io/ch3/mandelbrot</code>(§3.3)的输出导入到这个程序的标准输入,它将解码输入的PNG格式图像,然后转换为JPEG格式的图像输出(图3.3)。</p>
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<pre><code>$ go build gopl.io/ch3/mandelbrot
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$ go build gopl.io/ch10/jpeg
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$ ./mandelbrot | ./jpeg >mandelbrot.jpg
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Input format = png
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</code></pre>
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<p>要注意image/png包的匿名导入语句。如果没有这一行语句,程序依然可以编译和运行,但是它将不能正确识别和解码PNG格式的图像:</p>
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<pre><code>$ go build gopl.io/ch10/jpeg
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$ ./mandelbrot | ./jpeg >mandelbrot.jpg
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jpeg: image: unknown format
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</code></pre>
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<p>下面的代码演示了它的工作机制。标准库还提供了GIF、PNG和JPEG等格式图像的解码器,用户也可以提供自己的解码器,但是为了保持程序体积较小,很多解码器并没有被全部包含,除非是明确需要支持的格式。image.Decode函数在解码时会依次查询支持的格式列表。每个格式驱动列表的每个入口指定了四件事情:格式的名称;一个用于描述这种图像数据开头部分模式的字符串,用于解码器检测识别;一个Decode函数用于完成解码图像工作;一个DecodeConfig函数用于解码图像的大小和颜色空间的信息。每个驱动入口是通过调用image.RegisterFormat函数注册,一般是在每个格式包的init初始化函数中调用,例如image/png包是这样注册的:</p>
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<pre><code class="language-Go">package png // image/png
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func Decode(r io.Reader) (image.Image, error)
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func DecodeConfig(r io.Reader) (image.Config, error)
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func init() {
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const pngHeader = "\x89PNG\r\n\x1a\n"
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image.RegisterFormat("png", pngHeader, Decode, DecodeConfig)
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}
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</code></pre>
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<p>最终的效果是,主程序只需要匿名导入特定图像驱动包就可以用image.Decode解码对应格式的图像了。</p>
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<p>数据库包database/sql也是采用了类似的技术,让用户可以根据自己需要选择导入必要的数据库驱动。例如:</p>
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<pre><code class="language-Go">import (
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"database/sql"
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_ "github.com/lib/pq" // enable support for Postgres
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_ "github.com/go-sql-driver/mysql" // enable support for MySQL
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)
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db, err = sql.Open("postgres", dbname) // OK
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db, err = sql.Open("mysql", dbname) // OK
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db, err = sql.Open("sqlite3", dbname) // returns error: unknown driver "sqlite3"
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</code></pre>
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<p><strong>练习 10.1:</strong> 扩展jpeg程序,以支持任意图像格式之间的相互转换,使用image.Decode检测支持的格式类型,然后通过flag命令行标志参数选择输出的格式。</p>
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<p><strong>练习 10.2:</strong> 设计一个通用的压缩文件读取框架,用来读取ZIP(archive/zip)和POSIX tar(archive/tar)格式压缩的文档。使用类似上面的注册技术来扩展支持不同的压缩格式,然后根据需要通过匿名导入选择导入要支持的压缩格式的驱动包。</p>
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