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| <h1>Apache Flink 代码样式与质量指南 — Scala</h1> |
| |
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| 格式指南 |
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| <div class="page-toc"> |
| <ul id="markdown-toc"> |
| <li><a href="#scala-" id="markdown-toc-scala-">Scala 语言特性</a> <ul> |
| <li><a href="#scala" id="markdown-toc-scala">在哪儿使用（和不使用） Scala</a></li> |
| <li><a href="#api-" id="markdown-toc-api-">API 等价</a></li> |
| <li><a href="#section" id="markdown-toc-section">语言特性</a></li> |
| <li><a href="#section-1" id="markdown-toc-section-1">编码格式</a></li> |
| </ul> |
| </li> |
| </ul> |
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| </div> |
| |
| <h2 id="scala-">Scala 语言特性</h2> |
| |
| <h3 id="scala">在哪儿使用（和不使用） Scala</h3> |
| |
| <p><strong>对于 Scala 的 API 或者纯 Scala libraries，我们会选择使用 Scala。</strong></p> |
| |
| <p><strong>在 core API 和 运行时的组件中，我们不使用 Scala。我们的目标是从这些组件中删除现有的 Scala 使用(代码和依赖项)。</strong></p> |
| |
| <p>⇒ 这并不是因为我们不喜欢 Scala，而是考虑到“用正确的工具做正确的事”的结果（见下文）。</p> |
| |
| <p>对于 API，我们使用 Java 开发基础内容，并在上层使用 Scala。</p> |
| |
| <ul> |
| <li>这在传统上为 Java 和 Scala 提供了最佳的互通性</li> |
| <li>这意味着要致力于保持 Scala API 的更新</li> |
| </ul> |
| |
| <p>为什么我们不在 Core API 和 Runtime 中使用 Scala ？</p> |
| |
| <ul> |
| <li>过去的经验显示， Scala 在功能上的变化太快了。对于 Flink 社区来说，每次 Scala 版本升级都是一个比较棘手的处理过程。</li> |
| <li>Scala 并不总能很好地与 Java 的类交互，例如 Scala 的可见性范围的工作方式不同，而且常常向 Java 消费者公开的内容比预期的要多。</li> |
| <li>由于使用 Scala ，所以 Flink 的 artifact/dependency 管理增加了一层额外的复杂性。 |
| <ul> |
| <li>我们希望通过接口抽象，同时也在运行时保留像 Akka 这样依赖 Scala 的库，然后将它们加载到单独的类加载器中，以保护它们并避免版本冲突。</li> |
| </ul> |
| </li> |
| <li>Scala 让懂 Scala 的程序员很容易编写代码，而对于不太懂 Scala 的程序员来说，这些代码很难理解。对于一个拥有不同经验水平的广大社区的开源项目来说，这尤其棘手。解决这个问题意味着大量限制 Scala 特性集，这首先就违背了使用 Scala 的很多目的。</li> |
| </ul> |
| |
| <h3 id="api-">API 等价</h3> |
| |
| <p>保持 Java API 和 Scala API 在功能和代码质量方面的同步。</p> |
| |
| <p>Scala API 也应该涵盖 Java API 的所有特性。</p> |
| |
| <p>Scala API 应该有一个“完整性测试”，就如下面 DataStream API 的示例中的一样： <a href="https://github.com/apache/flink/blob/master/flink-streaming-scala/src/test/scala/org/apache/flink/streaming/api/scala/StreamingScalaAPICompletenessTest.scala">https://github.com/apache/flink/blob/master/flink-streaming-scala/src/test/scala/org/apache/flink/streaming/api/scala/StreamingScalaAPICompletenessTest.scala</a></p> |
| |
| <h3 id="section">语言特性</h3> |
| |
| <ul> |
| <li><strong>避免 Scala 隐式转换。</strong> |
| <ul> |
| <li>Scala 的隐式转换应该只用于面向用户的 API 改进，例如 Table API 表达式或类型信息提取。</li> |
| <li>不要把它们用于内部 “magic”。</li> |
| </ul> |
| </li> |
| <li><strong>为类成员添加显式类型。</strong> |
| <ul> |
| <li> |
| <p>对于类字段和方法返回类型，不要依赖隐式类型推断:</p> |
| |
| <p><strong>不要这样：</strong> |
| <code> |
| var expressions = new java.util.ArrayList[String]() |
| </code></p> |
| |
| <p><strong>要这样：</strong> |
| <code> |
| var expressions: java.util.List[String] = new java.util.ArrayList[]() |
| </code></p> |
| </li> |
| <li> |
| <p>堆栈上局部变量的类型推断是可以的。</p> |
| </li> |
| </ul> |
| </li> |
| <li><strong>用严格的可见性。</strong> |
| <ul> |
| <li>避免使用 Scala 的包私有特性(如 private[flink])，而是使用常规 private/protected 替代。</li> |
| <li>请注意：在 Java 中，<code>private[flink]</code> 和 <code>protected</code> 的成员是公开的。</li> |
| <li>请注意：在 Flink 提供的示例中， <code>private[flink]</code> 仍然会暴露所有成员。</li> |
| </ul> |
| </li> |
| </ul> |
| |
| <h3 id="section-1">编码格式</h3> |
| |
| <p><strong>使用换行来构造你的代码。</strong></p> |
| |
| <ul> |
| <li>Scala 的函数性质允许长的转换链（ <code>x.map().map().foreach()</code> ）。</li> |
| <li>为了强制让实现者构造其代码，因此将行长度限制为 100 个字符以内。</li> |
| <li>为了更好的可维护性，每次转换使用一行。</li> |
| </ul> |
| |
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