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javajvm-hotspot

HotSpot LogCompilation output: Should V (Void) match a java.lang.Object return type?

Background: I'm the author of a HotSpot LogCompilation visualiser called JITWatch and I'm refactoring the parsing engine. I have a question on matching LogCompilation method signatures to Class.getDeclaredMethod() signatures, specifically about matching the void return type.

Given the HotSpot LogCompilation output snippet:

<nmethod compile_id='48' compile_kind='c2n' entry='0x00007f01fc3f41c0'
size='328' address='0x00007f01fc3f4090' relocation_offset='296' 
consts_offset='328' insts_offset='304' 
method='java/lang/invoke/MethodHandle linkToStatic 
(Ljava/lang/Object;ILjava/lang/invoke/MemberName;)V' 
bytes='0' count='0' iicount='0' stamp='0.840'/>

Which indicates a compilation (using a native wrapper) of the method linkToStatic on class java.lang.invoke.MethodHandle which takes as parameters

java.lang.Object, int, java.lang.invoke.MemberName and returns void

The only method it could possibly match on java.lang.invoke.MethodHandle has signature:

static native java.lang.Object linkToStatic(java.lang.Object...) throws java.lang.Throwable;

Which has java.lang.Object varargs parameters (matches the LogCompilation parameters) but declares a java.lang.Object return type.

Is it legal to match the void return from the LogCompilation signature with java.lang.Object for the reason that:

Object.class.isAssignableFrom(Void.class); // returns true

I'd be grateful if somebody could explain the mechanics behind the void return type.

Thanks,

Chris

Solution

  • The linkToStatic method is a signature polymorphic method. Their signature varies depending on the call site.

    HotSpot will internally create new Method objects with the signature required by a call site (see SystemDictionary::find_method_handle_intrinsic).

    The method attribute you see for this nmethod is the string representation of such a Method: its signature (including the return type) is specialized to a specific call site.

    So in general it is not ok to match the void return type with java.lang.Object and you probably need a special case to match signature polymorphic methods.