Why is "checkcast MethodHandle" necessary in methodHandleInvokeLinkerMethod?

[Ioi Lam]

So why is this first argument not declared as a MethodHandle?

Thanks

- Ioi


On 4/9/18 1:00 PM, Attila Szegedi wrote:
> invokeExact_MT000_LFL_V is a static method and its first declared argument type is java.lang.Object, not MethodHandle. So aload_0 will load a reference with static type of Object, hence making a checkcast necessary to cast it to a MethodHandle.
>
> HTH,
>    Attila.
>
>> On 2018. Apr 9., at 21:49, Ioi Lam <ioi.lam at oracle.com> wrote:
>>
>> I am looking at the code generated by MethodHandleNatives.linkMethod:
>>
>> (note the output was edited for brevity)
>>
>> $ cat HelloMH.java
>> import java.lang.invoke.*;
>>
>> public class HelloMH {
>>      public static void main(String ...args) throws Throwable {
>>          MethodHandles.Lookup lookup = MethodHandles.lookup();
>>          MethodType mt = MethodType.methodType(void.class, float.class);
>>          MethodHandle mh = lookup.findStatic(HelloMH.class, "callme", mt);
>>          mh.invokeExact(4.0f);
>>      }
>>
>>      private static void callme(float x) {
>>          System.out.println("Hello MH.invoke: " + x);
>>          Thread.dumpStack();
>>      }
>> }
>>
>> $ javac HelloMH.java
>>
>> $ java -Djava.lang.invoke.MethodHandle.DUMP_CLASS_FILES=true \
>>      -XX:+UnlockDiagnosticVMOptions -XX:+ShowHiddenFrames -cp . HelloMH
>> [...]
>> Hello MH.invoke: 4.0
>> java.lang.Exception: Stack trace
>>      at java.base/java.lang.Thread.dumpStack(Thread.java:1434)
>>      at HelloMH.callme(HelloMH.java:13)
>>      at java.base/java.lang.invoke.LambdaForm$DM.invokeStatic_LF_V()
>>      at java.base/java.lang.invoke.LambdaForm$MH.invokeExact_MT_LFL_V()
>>      at HelloMH.main(HelloMH.java:8)
>>
>>
>> $ javap -c 'DUMP_CLASS_FILES/java/lang/invoke/LambdaForm$MH.class'
>> final class java.lang.invoke.LambdaForm$MH {
>>    static void invokeExact_MT000_LFL_V(java.lang.Object, float, java.lang.Object);
>>      Code:
>>         0: aload_0
>>         1: checkcast     #14    // class java/lang/invoke/MethodHandle
>>         4: dup
>>         ...
>>
>>
>> Why is the checkcast necessary here?
>>
>> I thought the verifier would ensure that local#0 must of type MethodHandle.
>>
>> Is this checkcast needed only for reflective invocation of MethodHandle.invokeExact()?
>>
>> Thanks
>> - Ioi
>>
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