问题
I have a simple interface and its implementation:
interface Iface {
fun doSomething(s: String)
}
class IfaceImpl : Iface {
override fun doSomething(s: String) {
println("Doing the job, s = $s")
}
}
Also, there are two identical (at least I cannot spot the difference) invocation handlers, one in Java and one in Kotlin:
public class JavaHandler implements InvocationHandler {
private final Iface target;
public JavaHandler(Iface target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Java handler works");
return method.invoke(target, args);
}
}
class KotlinHandler(private val target: Iface) : InvocationHandler {
override fun invoke(proxy: Any?, method: Method?, args: Array<out Any>?): Any {
println("Kotlin proxy works")
return method!!.invoke(target, args)
}
}
They both just output some string and then invoke the method on the target.
Finally, here is the code I run:
fun main(args: Array<String>) {
val target = IfaceImpl()
target.doSomething("one")
val javaProxy = newProxy(JavaHandler(target))
javaProxy.doSomething("two")
val kotlinProxy = newProxy(KotlinHandler(target))
kotlinProxy.doSomething("three")
}
fun newProxy(handler: InvocationHandler): Iface {
return Proxy.newProxyInstance(Iface::class.java.classLoader, arrayOf(Iface::class.java), handler) as Iface
}
It creates two java proxies, using both invocation handlers, and tries to exercise them.
Java handler works fine, but Kotlin handler does not. The output follows:
Doing the job, s = one
Java handler works
Doing the job, s = two
Kotlin proxy works
Exception in thread "main" java.lang.IllegalArgumentException: argument type mismatch
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at KotlinHandler.invoke(KotlinHandler.kt:12)
at com.sun.proxy.$Proxy0.doSomething(Unknown Source)
at TestKt.main(Test.kt:17)
I can see with a debugger that in both cases args consists of 1 element, and it's a java.lang.Integer instance.
An interesting thing is that if the method has 0 parameters, the error message is different:
Exception in thread "main" java.lang.IllegalArgumentException: wrong number of arguments
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
In such a case, null is passed as args parameter (which is allowed by javadocs for parameterless calls).
Do I do something wrong? Or is this a bug?
My build.gradle has the following:
plugins {
id 'org.jetbrains.kotlin.jvm' version '1.2.61'
}
回答1:
UPDATE: In newer versions of Kotlin you can use args.orEmpty() instead of args ?: emptyArray()
TL;DR
You cannot pass args but you need to use *(args ?: emptyArray()) because Method.invoke does not expect an array but a variadic parameter.
See this answer for more information
The way I found the actual issue
I looked at the generated bytecode, for Kotlin I get the following:
override fun invoke(proxy : Any?, method : Method, args : Array<Any>?) : Any?
{
println("Kotlin proxy works")
return method.invoke(target, args)
}
public java.lang.Object invoke(java.lang.Object, java.lang.reflect.Method, java.lang.Object[]);
Code:
0: aload_2
1: ldc #12 // String method
3: invokestatic #18 // Method kotlin/jvm/internal/Intrinsics.checkParameterIsNotNull:(Ljava/lang/Object;Ljava/lang/String;)V
6: ldc #20 // String Kotlin proxy works
8: astore 4
10: getstatic #26 // Field java/lang/System.out:Ljava/io/PrintStream;
13: aload 4
15: invokevirtual #32 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
18: aload_2
19: aload_0
20: getfield #36 // Field target:LIface;
23: iconst_1
24: anewarray #4 // class java/lang/Object
27: dup
28: iconst_0
29: aload_3
30: aastore
31: invokevirtual #41 // Method java/lang/reflect/Method.invoke:(Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;
34: areturn
Now, as you can see, Kotlin does manipulate the args parameter - in fact it creates a new array. Java does not do this (also it skips null-checks):
public Object invoke(Object proxy, Method method, Object args[]) throws Throwable
{
System.out.println("Java handler works");
return method.invoke(target, args);
}
public java.lang.Object invoke(java.lang.Object, java.lang.reflect.Method, java.lang.Object[]) throws java.lang.Throwable;
Code:
0: getstatic #3 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #4 // String Java handler works
5: invokevirtual #5 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: aload_2
9: aload_0
10: getfield #2 // Field target:LIface;
13: aload_3
14: invokevirtual #6 // Method java/lang/reflect/Method.invoke:(Ljava/lang/Object;[Ljava/lang/Object;)Ljava/lang/Object;
17: areturn
Now, let's intercept the actuall array. I created a method in Java code to act as an intermediary:
public static Object invoke0(Iface target, Method method, Object args[]) throws Throwable
{
System.out.println("Invoking method with " + java.util.Arrays.toString(args));
return method.invoke(target, args);
}
Execute that from both Java and Kotlin - and it works.
Now what is the difference? Right, we expect an Object[], but Method.invoke takes an Object....
Change our intermediary to take Object... and we get our error message, along with this output:
Invoking method with [[Ljava.lang.Object;@4b67cf4d]
So obviously, we aren't passing an Object[] but an Object[][], which means type mismatch!
来源：https://stackoverflow.com/questions/52082041/kotlin-argument-type-mismatch-when-passing-array-as-vararg-parameter