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....@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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