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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