MethodHandle示例在invokeExact调用时抛出WrongMethodTypeException

Question

The example shown in the description of the MethodHandle class throws a WrongMethodTypeException in the invocation of the statement mh.invokeExact("daddy",'d','n') with the following description: (CC)Ljava/lang/String; cannot be called with a different arity as ([Ljava/lang/Object;)Ljava/lang/Object; (CC)Ljava/lang/String; cannot be called with a different arity as ([Ljava/lang/Object;)Ljava/lang/Object; .

The MethodHandle object mh has a symbolic type descriptor corresponding to: (CC)Ljava/lang/String . But when we are invoking mh.invokeExact("daddy",'d','n') , the arguments: d and n are passed as an Object array and then they are not matching with the arguments of the type char .

I know that I can resolve the above problem using the invokeWithArguments instead of the invokeExcat or the invoke , but this example was supposed to work as presented in the description of the MethodHandle of Java 7 API. Besides that, the invokeWithArguments has a performance overhead in relation to invoke / invokeExact .

Answer 1

How are you compiling this?

It sounds suspiciously like a known Eclipse bug to me.

I've just checked with javac and this code:

import java.lang.invoke.*;

public class ScratchMH {    
        private static ScratchMH instance = null;

        public ScratchMH() {
                super();
        }

        private void run() throws Throwable {
                Object x, y; String s; int i;
                MethodType mt; MethodHandle mh;
                MethodHandles.Lookup lookup = MethodHandles.lookup();

                // mt is (char,char)String
                mt = MethodType.methodType(String.class, char.class, char.class);
                mh = lookup.findVirtual(String.class, "replace", mt);
                s = (String) mh.invokeExact("daddy",'d','n');
                // invokeExact(Ljava/lang/String;CC)Ljava/lang/String;

                System.out.println(s);
        }

        public static void main(String[] args) throws Throwable {
                instance = new ScratchMH();
                instance.run();
        }
}

seems to work OK:

ariel-2:src boxcat$ javac scratch/clj/ScratchMH.java 
ariel-2:src boxcat$ java scratch/clj/ScratchMH
nanny
ariel-2:src boxcat$ 

The relevant portion of output from javap seems sane as well:

  35: invokevirtual #8                  // Method java/lang/invoke/MethodHandles$Lookup.findVirtual:(Ljava/lang/Class;Ljava/lang/String;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/MethodHandle;
  38: astore        6
  40: aload         6
  42: ldc           #9                  // String daddy
  44: bipush        100
  46: bipush        110
  48: invokevirtual #10                 // Method java/lang/invoke/MethodHandle.invokeExact:(Ljava/lang/String;CC)Ljava/lang/String;
  51: astore_3      

Answer 2

invokeExact requires exact match between the MH's method type description, and the types of arguments. Since the MH's method type is (cc)string , so what you want to execute the MH, both the first and second argument should be char. Thus, it is like that

``String s = (String)mh.invokeExact('a', 'b')``