Invoke java.lang.reflect.Method in Scala with Object array

Question

I'm trying to invoke a method via reflection by passing the method's invocation arguments in an array. Params is a List [Any] and carries the values to be passed to the reflected method.

// make absolutely sure it's of the correct type
val args : Array [Object] = params.map (_.asInstanceOf [java.lang.Object]).toArray
System.err.println ("target method: " + method.getName + " :: " + method.getParameterTypes.toList)
System.err.println ("parameters   : " + args.getClass.getCanonicalName + "\n    " + 
    (args map (p => (p.getClass.getCanonicalName, p))).toList)
method.invoke (host, args)

I get the following output, which all looks good. The signature of the method being invoked matches the parameter list perfectly.

target method: echo :: List(class java.lang.String)
parameters   : java.lang.Object[]
    List((java.lang.String,looks good))

Sadly, the invoke throws java.lang.IllegalArgumentException: argument type mismatch

According to the Java docs a varargs argument (as the params to Method.invoke are) can be supplied with an Object[]. I'm stumped!

dhg · Accepted Answer · 2012-04-26 19:35:00Z

6

Vararg methods can be called from scala with the :_* syntax, which tells Scala to unpack the sequence into individual arguments:

method.invoke(host, args: _*)

Here's a complete example:

class A {
  def meth(a: String, b: String) = a + b
}
val host = (new A)
val method = host.getClass.getDeclaredMethods.head
val params: List[Object] = List("a", "b")
val result = method.invoke(host, params: _*)
println(result) // ab

edited Apr 26, 2012 at 19:35

answered Apr 26, 2012 at 19:21

dhg

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1 Comment

F. P. Freely Over a year ago

Outstanding. params : _ * is exactly the piece of arcane syntax I was looking for. Thanks so much!

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