如何在 Scala REPL 中重新加载类或包? [英] How to reload a class or package in Scala REPL?

问题描述

我几乎总是打开一两个 Scala REPL 会话，这使得对 Java 或 Scala 类进行快速测试变得非常容易.但是如果我更改一个类并重新编译它，REPL 会继续加载旧的.有没有办法让它重新加载类，而不必重新启动 REPL?

I almost always have a Scala REPL session or two open, which makes it very easy to give Java or Scala classes a quick test. But if I change a class and recompile it, the REPL continues with the old one loaded. Is there a way to get it to reload the class, rather than having to restart the REPL?

举一个具体的例子，假设我们有文件 Test.scala:

Just to give a concrete example, suppose we have the file Test.scala:

object Test { def hello = "Hello World" }

我们编译它并启动 REPL:

We compile it and start the REPL:

~/pkg/scala-2.8.0.Beta1-prerelease$ bin/scala
Welcome to Scala version 2.8.0.Beta1-prerelease
(Java HotSpot(TM) Server VM, Java 1.6.0_16).
Type in expressions to have them evaluated.
Type :help for more information.

scala> Test.hello
res0: java.lang.String = Hello World

然后我们将源文件改为

object Test {
  def hello = "Hello World"
  def goodbye = "Goodbye, Cruel World"
}

但我们不能使用它:

scala> Test.goodbye
<console>:5: error: value goodbye is not a member of object Test
       Test.goodbye
            ^

scala> import Test;
<console>:1: error: '.' expected but ';' found.
       import Test;

推荐答案

类重新加载不是一个容易的问题.事实上，这是 JVM 非常困难的事情.不过，您确实有几个选择:

Class reloading is not an easy problem. In fact, it's something that the JVM makes very difficult. You do have a couple options though:

• 在调试模式下启动 Scala REPL.JVM 调试器具有一些在方法级别工作的内置重新加载.它不会帮助您处理您提供的案例，但它会处理一些简单的事情，例如更改方法实现.
• 使用 JRebel(http://www.zeroturnaround.com/jrebel).JRebel 基本上是一个超级充电的类重新加载解决方案虚拟机.它可以处理成员添加/删除、新的/删除的类、定义更改等.它不能处理的唯一一件事是类层次结构的更改(添加一个超级接口，用于例子).它不是免费工具，但它们确实提供了仅限于 Scala 编译单元的补充许可.

• Start the Scala REPL in debug mode. The JVM debugger has some built-in reloading which works on the method level. It won't help you with the case you gave, but it would handle something simple like changing a method implementation.
• Use JRebel (http://www.zeroturnaround.com/jrebel). JRebel is basically a super-charged class reloading solution for the JVM. It can handle member addition/removal, new/removed classes, definition changes, etc. Just about the only thing it can't handle is changes in class hierarchy (adding a super-interface, for example). It's not a free tool, but they do offer a complementary license which is limited to Scala compilation units.

不幸的是，这两者都受到 Scala REPL 实现细节的限制.我使用 JRebel，它通常可以解决问题，但仍然存在 REPL 不会反映重新加载的类的情况.不过，总比没有好.

Unfortunately, both of these are limited by the Scala REPL's implementation details. I use JRebel, and it usually does the trick, but there are still cases where the REPL will not reflect the reloaded class(es). Still, it's better than nothing.

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