List<Integer> list;

and adjusted your the comment accordingly – I think the angle brackets were stripped out as “invalid HTML” .

Indeed, running the code prints

Type: interface java.util.List
GenericType: java.util.List
class java.lang.Integer

as you mention. This also works using Class c = Foo.class.

Thanks for that, very useful. But it does require you to have access to the field declaration of list, i.e. compile-time information.

What I meant – and could have expressed more clearly – when I wrote “there is no way to get the Agent from” is that you cannot access the type information if given only the list object.

Execute this code and see that it will print Intger in the end.

public class FieldSpy {
  public List<Integer> list;

  public static void main(String... args) {
    try {
      Class c = Class.forName("FieldSpy");
      Field f = c.getField("list");
      System.out.format("Type: %s%n", f.getType());
      System.out.format("GenericType: %s%n", f.getGenericType());

      Type type = f.getGenericType();
      if (type instanceof ParameterizedType) {
          System.out.println(((ParameterizedType)type).getActualTypeArguments()[0]);
      }
    } catch (ClassNotFoundException x) {
      x.printStackTrace();
    } catch (NoSuchFieldException x) {
      x.printStackTrace();
    }
  }
}

Where is the SetUtils.java that is imported?