Java Reflection in Scala

Question

I am trying to get a simple java reflection program working in Scala, and seem to be missing something ...

scala> val cl = new URLClassLoader(Array(new File("Hi.jar").toURI.toURL), getClass.getClassLoader)
cl: java.net.URLClassLoader = java.net.URLClassLoader@3c7b137a

scala> val c = cl.loadClass("Hi")
c: Class[_] = class Hi

scala> val m = c.getMethod("run")
m: java.lang.reflect.Method = public void Hi.run()

scala> m.invoke()
<console>:21: error: not enough arguments for method invoke: (x$1: Any, x$2: Object*)Object.
Unspecified value parameters x$1, x$2.
       m.invoke()
               ^

What am I missing, as the prior line has indicated -

public void Hi.run()

What exactly is it expecting for the two arguments?

Answer 1

Scala is telling you exactly what your problem is: invoke needs 1+ parameters!

See the java doc :

invoke(Object obj, Object... args)

Invokes the underlying method represented by this Method object, on the specified object with the specified parameters.

So, you have to provide at least one argument - a reference to the object (or class) you want to call that method on! As Hi.run() seems to be static, you would want to use your c as only argument to your call.

The following arguments would be the actual parameters that your "reflected" method expects. In your case, no further arguments.

Long story short: you better keep the excellent tutorials from Oracle on reflection close to your scala console while experimenting. If you try to learn "reflection" by trial&error; I guarantee you: a lot of frustrating trials with many strange errors. Really: the reflection API is not very forgiving when you don't know what you are doing; even the slightest mistakes can lead to very unexpected results.

Answer 2

There is nothing specific to Scala there. Method.invoke requires the at least one argument being the instance on which it's applied (or null for a static method).

In Scala, you can use structural typing for such simple case.