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<h2 id="129-几点忠告"><a class="header" href="#129-几点忠告">12.9. 几点忠告</a></h2>
<p>虽然反射提供的API远多于我们讲到的我们前面的例子主要是给出了一个方向通过反射可以实现哪些功能。反射是一个强大并富有表达力的工具但是它应该被小心地使用原因有三。</p>
<p>第一个原因是基于反射的代码是比较脆弱的。对于每一个会导致编译器报告类型错误的问题在反射中都有与之相对应的误用问题不同的是编译器会在构建时马上报告错误而反射则是在真正运行到的时候才会抛出panic异常可能是写完代码很久之后了而且程序也可能运行了很长的时间。</p>
<p>以前面的readList函数§12.6为例为了从输入读取字符串并填充int类型的变量而调用的reflect.Value.SetString方法可能导致panic异常。绝大多数使用反射的程序都有类似的风险需要非常小心地检查每个reflect.Value的对应值的类型、是否可取地址还有是否可以被修改等。</p>
<p>避免这种因反射而导致的脆弱性的问题的最好方法是将所有的反射相关的使用控制在包的内部如果可能的话避免在包的API中直接暴露reflect.Value类型这样可以限制一些非法输入。如果无法做到这一点在每个有风险的操作前指向额外的类型检查。以标准库中的代码为例当fmt.Printf收到一个非法的操作数时它并不会抛出panic异常而是打印相关的错误信息。程序虽然还有BUG但是会更加容易诊断。</p>
<pre><code class="language-Go">fmt.Printf(&quot;%d %s\n&quot;, &quot;hello&quot;, 42) // &quot;%!d(string=hello) %!s(int=42)&quot;
</code></pre>
<p>反射同样降低了程序的安全性,还影响了自动化重构和分析工具的准确性,因为它们无法识别运行时才能确认的类型信息。</p>
<p>避免使用反射的第二个原因是即使对应类型提供了相同文档但是反射的操作不能做静态类型检查而且大量反射的代码通常难以理解。总是需要小心翼翼地为每个导出的类型和其它接受interface{}或reflect.Value类型参数的函数维护说明文档。</p>
<p>第三个原因,基于反射的代码通常比正常的代码运行速度慢一到两个数量级。对于一个典型的项目,大部分函数的性能和程序的整体性能关系不大,所以当反射能使程序更加清晰的时候可以考虑使用。测试是一个特别适合使用反射的场景,因为每个测试的数据集都很小。但是对于性能关键路径的函数,最好避免使用反射。</p>
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