Scala 反射，使用给定的注释查找和实例化所有类

Question

我想在运行时使用反射来查找具有给定注释的所有类，但是我不知道如何在 Scala 中这样做。然后我想获取注释的值并动态实例化每个映射到关联注释值的带注释类的实例。

这是我想要做的:

package problem
import scala.reflect.runtime._

object Program {

  case class Foo (key: String) extends scala.annotation.StaticAnnotation

  case class Bar ()
  @Foo ("x")
  case class Bar0 extends Bar
  @Foo ("y")
  case class Bar1 extends Bar
  @Foo ("z")
  case class Bar2 extends Bar

  def main (args : Array[String]): Unit = {

    // I want to use reflection to build
    // the following dynamically at run time:
    // val whatIWant: Map [String, Bar] =
    //   Map("x" -> Bar0 (), "y" -> Bar1 (), "z" -> Bar2 ())
    // (it's a map of attribute key -> an instance
    // of the type that has that attribute with that key)
    val whatIWant: Map [String, Bar] = ?
  }
}

而且，为了能够更好地解释自己，这里是我将如何在 C# 中解决问题的方法。

using System;
using System.Linq;
using System.Reflection;
using System.Collections.Generic;

namespace scalaproblem
{
    public class FooAttribute : Attribute
    {
        public FooAttribute (String s) { Id = s; }
        public String Id { get; private set; }
    }

    public abstract class Bar {}

    [Foo ("x")]
    public class Bar0: Bar {}

    [Foo ("y")]
    public class Bar1: Bar {}

    [Foo ("z")]
    public class Bar2: Bar {}

    public static class AttributeExtensions
    {
        public static TValue GetAttributeValue<TAttribute, TValue>(this Type type, Func<TAttribute, TValue> valueSelector) 
            where TAttribute : Attribute
        {
            var att = type.GetCustomAttributes (typeof(TAttribute), true).FirstOrDefault() as TAttribute;
            if (att != null)
                return valueSelector(att);
            return default(TValue);
        }
    }

    public static class Program
    {
        public static void Main ()
        {
            var assembly = Assembly.GetExecutingAssembly ();
            Dictionary<String, Bar> whatIWant = assembly
                .GetTypes()
                .Where (t => Attribute.IsDefined (t, typeof(FooAttribute)))
                .ToDictionary (t => t.GetAttributeValue((FooAttribute f) => f.Id), t => Activator.CreateInstance (t) as Bar);

            whatIWant.Keys.ToList().ForEach (k => Console.WriteLine (k + " ~ " + whatIWant [k]));
        }
    }
}

Answer 1

最实用的答案是使用 reflections library扫描具有特定注释的所有类的类路径(或其子集)；然后，您可以使用 Java 反射 API 或 Scala 反射来实例化它们。

(请注意，这不是 100% 可靠的，因为例如类加载器允许是动态的，因此可能有一个类不会出现在扫描中。但实际上，对于“正常”用例(即从普通 jar 文件)它运行良好)