Scala - 动态对象/类加载

Scala - Dynamic object/class loading

提问by

In Java, I load external class (in .jar file) by this way:

在 Java 中，我通过这种方式加载外部类（在 .jar 文件中）：

ClassLoader classLoader = new URLClassLoader(new URL[] {
  new File("module.jar").toURI().toURL()});
Class clazz = classLoader.loadClass("my.class.name");
Object instance = clazz.newInstance();

//check and cast to an interface, then use it
if (instance instanceof MyInterface)
  ...

And it works fine.

它工作正常。

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Now I want to do the same thing in Scala. I have a traitnamed Module(Module.scala):

现在我想在 Scala 中做同样的事情。我有一个trait名为Module( Module.scala):

trait Module {
  def name: String
}

object Module {
  lazy val ModuleClassName = "my.module.ExModule"
}

I write a module extending Module, then compile it to module.jar:

我编写了一个扩展模块Module，然后将其编译为module.jar：

package my.module

import Module

object ExModule extends Module {}

Then I load it by this code:

然后我通过这个代码加载它：

var classLoader = new URLClassLoader(Array[URL](
  new File("module.jar").toURI.toURL))
var clazz = classLoader.loadClass(Module.ModuleClassName)

It works fine. But if I create new instance, I get this exception:

它工作正常。但是，如果我创建新实例，则会出现此异常：

java.lang.InstantiationException: my.module.ExModule

If I test it:

如果我测试它：

clazz.isInstanceOf[Module]

-> always return false.

-> 总是返回false。

So could you help me on this problem?

那么你能帮我解决这个问题吗？

Edited

已编辑

I guess it is because ExModuleis an object(not class). But when I change it to class, and classLoader.loadClass(...)raises a java.lang.NoClassDefFoundError. I guess it is because ExModuleis extended from a trait.

我想这是因为ExModule是object（不是class）。但是当我将其更改为class, 并classLoader.loadClass(...)引发java.lang.NoClassDefFoundError. 我想这是因为它是ExModule从trait.

I'm confused. Could anyone please help me?

我糊涂了。有人可以帮我吗？

Edited

已编辑

clazz.isInstanceOf[Class[Module]]//or Class[Byte], or Class[_]...

returns true.

返回true。

采纳答案by

Oops... I got the answer.

哎呀...我得到了答案。

Learn from:

借鉴：

• https://stackoverflow.com/a/8868537/942821(thanks Neil again).

• How do I call a Scala Object method using reflection?

• https://stackoverflow.com/a/8868537/942821（再次感谢尼尔）。

• 如何使用反射调用 Scala 对象方法？

====================

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I guess this way is temporary before Scala team provides right way to load an object/class/trait... from external jar file. Or because I can't find out the right way. But currently this helps me out of the problem.

我想这种方式在 Scala 团队提供object/class/trait从外部 jar 文件加载... 的正确方法之前是暂时的。或者因为我找不到正确的方法。但目前这可以帮助我解决问题。

var classLoader = new java.net.URLClassLoader(
  Array(new File("module.jar").toURI.toURL),
  /*
   * need to specify parent, so we have all class instances
   * in current context
   */
  this.getClass.getClassLoader)

/*
 * please note that the suffix "$" is for Scala "object",
 * it's a trick
 */
var clazzExModule = classLoader.loadClass(Module.ModuleClassName + "$")

/*
 * currently, I don't know how to check if clazzExModule is instance of
 * Class[Module], because clazzExModule.isInstanceOf[Class[_]] always
 * returns true,
 * so I use try/catch
 */
try {
  //"MODULE$" is a trick, and I'm not sure about "get(null)"
  var module = clazzExModule.getField("MODULE$").get(null).asInstanceOf[Module]
} catch {
  case e: java.lang.ClassCastException =>
    printf(" - %s is not Module\n", clazzExModule)
}

That's all :-)

就这样 ：-）

Edited

已编辑

I'd better design ExModuleas a class. After loading it from jar file, I can check it as like as:

我最好把设计ExModule当成一个班级。从 jar 文件加载后，我可以像这样检查它：

var clazz = classLoader.loadClass(Module.ModuleClassName)
if (classOf[Module].isAssignableFrom(clazz))
  ...

Note:

笔记：

You can not do it the opposite way:

你不能以相反的方式做到这一点：

if (clazz.isAssignableFrom(classOf[Module]))

because Moduleis a trait/object, isAssignableFrom()will not work in this case.

因为Module是trait/ object，isAssignableFrom()在这种情况下不起作用。

回答by Chris Shain

Have you seen this article? http://kneissl.eu/Members/martin/blog/reflection-from-scala-heaven-and-hell

你看过这篇文章吗？ http://kneissl.eu/Members/martin/blog/reflection-from-scala-heaven-and-hell

Got it from Need to load a scala class at runtime from a jar and initialize it

从需要在运行时从 jar 加载 scala 类并初始化它

Also (and maybe more relevant): http://coffeetimecode.wordpress.com/2010/07/24/remote-and-dynamic-class-loading-in-scala/

另外（也许更相关）：http: //coffeetimecode.wordpress.com/2010/07/24/remote-and-dynamic-class-loading-in-scala/