blocks|key|1786059|text|Scala不喜欢：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1786060|内部Java类|unordered-list-item|1786061|静态方法和变量(特别是在超类中)|1786062|原始类型|1786063|Java不喜欢：|1786064|Scala对象特性|1786065|闭包|1786066|演员(除了斯嘉丽·约翰逊和阿克卡演员，因为他们有Java+API)|1786067|暗示，尤指表现|1786068|高级类型构造(高级类型、结构类型、抽象类型)|1786069|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$]]|$1|E|3|F|5|D|7|Y|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|Z|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|10|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|11|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|12|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|13|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|14|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|15|8|@]|9|@]|A|$]]|$1|U|3|-4|5|6|7|16|8|@]|9|@]|A|$]]]|V|$]]

Scala doesn't like:

<ul>
<li>inner Java classes </li>
<li>static methods and variables (especially in super classes)</li>
<li>raw types</li>
</ul>

Java doesn't like:

<ul>
<li>Scala objects traits</li>
<li>closures</li>
<li>actors (except Scarlett Johansson and Akka Actors since they have a Java API)</li>
<li>implicits, especially Manifests</li>
<li>advanced type constructs (higher kinded types, structural types, abstract type vars)</li>
</ul>

blocks|key|1786021|text|最初(即迁移的第一阶段)，我想说，您不希望用难以使用的Java+scala构造导出API+(接口/公共方法等)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1786022|实际上，我会将其限制为导出任何特定于scala的内容(同样，我在这里讨论的是迁移的第一阶段)：|1786023|scala库类(函数类型、集合等)|unordered-list-item|1786024|高类泛型类型签名|1786025|默许|1786026|那还剩下什么呢？那么，类的内部(私有方法、字段等)可以转换为使用scala构造和库类。|1786027|如果您有任何API(特别是您打算迁移的面向客户的API)，我将在Scala中从头开始设计它们；最初使用Java后端。然后，我会慢慢地吃掉中间的代码。|1786028|在您强调的要点中，我同意Scala的不变范式和Java的可变范式并不能很好地结合在一起。另外一点，我觉得问题较少。|1786029|范式失配的另一个要点是如何转换您拥有的任何并发代码(即使用java.util.concurrent的代码)。当然，这可以按原样进行转换，但问题是是将基于锁定的并发模型替换为基于演员或STM的并发模型。在任何一种情况下，这也可能是一个完全的重新设计，而不是转换本身。|offset|length|style|BOLD|CODE|1786030|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Actors_%2528concurrency%2529|1|http://en.wikipedia.org/wiki/Software_transactional_memory^0|0|0|0|0|0|0|0|0|L|2|T|K|2G|2|0|2J|3|1|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|18|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|19|8|@]|9|@]|A|$]]|$1|G|3|H|5|F|7|1A|8|@]|9|@]|A|$]]|$1|I|3|J|5|F|7|1B|8|@]|9|@]|A|$]]|$1|K|3|L|5|6|7|1C|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1D|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|1E|8|@]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1F|8|@$S|1G|T|1H|U|V]|$S|1I|T|1J|U|W]]|9|@$S|1K|T|1L|1|1M]|$S|1N|T|1O|1|1P]]|A|$]]|$1|X|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]]|Y|$Z|$5|10|11|12|A|$13|14]]|15|$5|10|11|12|A|$13|16]]]]

Initially (i.e. the first phase of migration), I would say that you don't want to export an API (interface/public method etc) with a hard-to-use-from-Java scala construct.

In practice I would limit this to exporting anything which is scala-specific (again, I'm talking about the first phase of migration here):

<ul>
<li>scala library classes (function types, collections etc)</li>
<li>higher-kinded generic type signatures</li>
<li>implicits</li>
</ul>

So what does that leave? Well, the internals of classes (private methods, fields etc) can be converted to use scala constructs and library classes.

If you have any APIs (especially client-facing APIs which you intend to migrate), I would design them anew from the ground up in Scala; initially using a Java back-end. Then I would slowly eat away at the code inbetween.

Of the points you have highlighted, I would agree that the immutable paradigm of Scala and the mutable paradigm of Java do not mix well. The other points I have found less problematic.

Another main point of paradigm-mismatch is how to convert any concurrent code you have (i.e. that which makes use of <code>java.util.concurrent</code>). This can, of course, be converted as is but the question is whether to replace the concurrency model based around locking with one based around <a href="http://en.wikipedia.org/wiki/Actors_%28concurrency%29" rel="nofollow">actors</a> or <a href="http://en.wikipedia.org/wiki/Software_transactional_memory" rel="nofollow">STM</a>. In either case, this is also likely to be a complete redesign, as opposed to a conversion per se.

blocks|key|587768|text|+is+给出了一些关于主题的见解，这是大卫·科普兰(+David+)的偷偷地将Scala带入您的组织。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|587769|entityMap|0|LINK|mutability|MUTABLE|url|http://days2010.scala-lang.org/node/138/169^0|0|4|Z|F|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@$9|O|A|P|B|C]]|D|@$9|Q|A|R|1|S]]|E|$]]|$1|F|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|G|$H|$5|I|J|K|E|$L|M]]]]

A nice presentation that may give some insights on subject is <a href="http://days2010.scala-lang.org/node/138/169" rel="noreferrer">Sneaking Scala Into Your Organization</a> by David Copeland.

blocks|key|2141247|text|我喜欢使用的一个技巧是，我将使用惯用的Scala属性(vars和vars)定义一个对象，然后添加@BeanProperty注释，然后将其公开为JavaBean属性。这样，每一种语言都可以使用当地的成语。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2141248|@BeanInfo注释也可以用于类似的类级别，但是在这里您必须小心--当使用@BeanInfo时，您另外自定义的任何方法--定义为setXXX或getXXX+--不会通过bean内省公开。这一点很重要，因为如果您还想处理Scala列表和Java列表之间的转换，则必须手动为集合类型编写getter/setter。|2141249|entityMap^0|1C|D|0|0|9|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|P|8|@]|D|@]|E|$]]]|I|$]]

One trick I like to use, I'll define an object using idiomatic Scala properties (vals and vars), then add the <code>@BeanProperty</code> annotation to expose then as JavaBean properties. That way, each language can use the native idioms.

The <code>@BeanInfo</code> annotation can also be used at a class level for a similar purpose, but you have to be careful here - When using @BeanInfo, any methods that you additionally custom-define as setXXX or getXXX won't be exposed via bean introspection. This is important, as you have to manually write getters/setters for collection types if you also want to handle translation between e.g. Scala Lists and Java Lists.

blocks|key|587676|text|我将补充其他人所说的话，因为他们是正确和有意义的。不仅仅是代码，您还需要进行单元测试。这听起来并不难，除非您开始更改可变结构和线程结构，同时仍然试图让一切都像以前一样工作。在转换过程中，在发现迁移过程中可能引入的其他边缘情况时，记住所有边缘情况是非常重要的。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|587677|如果通过直接翻译将单元测试引入到一个良好的Scala测试框架(如ScalaTest+)中，您可能会发现，您正在测试的不是之前测试的那些。在进行迁移时，重要的是要将代码的意图与测试放在一起，而不是允许思想的分裂。|587678|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

I'll add to what others have said since they're correct and meaningful. More than just the code, you'll need to bring over your unit tests. This doesn't sound hard until you start changing mutability and threading constructs while still attempting to have everything work the same way as it did before. During the transition it's very important to keep all the edge cases in mind while discovering additional edge cases you may introduce during the migration.

If you bring your unit tests into a good Scala testing framework like ScalaTest with a direct translation you may find that what you are testing is not what you were testing before. While making your migration it's important that you keep the intent of the code together with the tests instead of allowing fragmentation of thought.

What are the most important points to be aware of, and the workarounds, when gradually migrating an existing Java codebase to Scala? With a (potentially very long) intermediate phase where both languages are in use.

The sort of things I'm thinking about are:

<ul>
<li>different collection hierarchies</li>
<li>Java constructs that Scala can't handle well</li>
<li>Scala constructs that are impractical to use in Java</li>
<li>build tools</li>
<li>compilation order</li>
<li>immutability support in frameworks</li>
<li>etc.</li>
</ul>

Migrating Java to Scala

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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当将现有的Java代码库逐步迁移到Scala时，最需要注意的要点和解决办法是什么？有一个(可能很长的)中间阶段，这两种语言都在使用。我在想的事情是：不同的集合层次Java构造Scala不能很好地处理Java中不实用的Scala构造构建工具编译顺序框架中的不变性支持等。

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