Why is Kotlin throw IllegalArgumentException when using Proxy

Question

This is the Kotlin equivalent of Java code using InvocationHandler:

override fun invoke(proxy: Any?, method: Method?, args: Array         


        

Answer 1

When you pass args into method!!.invoke(obj, args) in Kotlin, it is actually a single argument of array type, and by default it is not decomposed into its elements as separate arguments.

To achieve that behavior instead, use the spread operator: *args

val ret = method!!.invoke(obj, *args)

With this syntax, args will be passed in the same way as in Java varargs. For example, these lines of code are equivalent:

someVarargsFunction("a", "b", "c", "d", "e")
someVarargsFunction("a", "b", *arrayOf("c", "d"), "e")

Note: if a method doesn't have any parameters, args will be null, and spreading it in Kotlin would result into a NullPointerException. As a workaround, use *(args ?: arrayOfNulls<Any>(0)), and in the described corner case the right part is chosen and spread into zero arguments.

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My example proxy implementation:

interface SomeInterface {
    fun f(a: Int, b: Int): Int
}

val obj = object : SomeInterface {
    override fun f(a: Int, b: Int) = a + b
}

val a = Proxy.newProxyInstance(
        SomeInterface::class.java.classLoader,
        arrayOf(SomeInterface::class.java)) { proxy, method, args ->
    println("Before; args: " + args?.contentToString())
    val ret = method!!.invoke(obj, *(args ?: arrayOfNulls<Any>(0)))
    println("After; result: $ret")
    ret
} as SomeInterface

println(a.f(1, 2))

And the output is:

Before; args: [1, 2]
After; result: 3
3