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<hr>
<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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