Animal Sniffer is missing proper support for @PolymorphicSignature methods (such as MethodHandles.invoke)

[lukehutch]

For the code

AccessibleObject field = /* ... */;

MethodHandle isAccessible =
        MethodHandles.lookup().findVirtual(AccessibleObject.class, "isAccessible",
            MethodType.methodType(boolean.class));

accessible.set((Boolean) isAccessible.invokeExact(field));

I get the following for org.codehaus.mojo.signature:java17:1.0:

[ERROR] Undefined reference: Boolean java.lang.invoke.MethodHandle.invokeExact(java.lang.reflect.AccessibleObject)

However this method is defined in JDK 1.7:

https://docs.oracle.com/javase/7/docs/api/java/lang/invoke/MethodHandle.html#invokeExact(java.lang.Object...)

MethodHandle::invoke is also missing from the java17 signatures (and maybe other things).

[lukehutch]

Hi, pinging on this again... I can't use MethodHandle in my project until this is fixed, so method lookups are running much slower than they should be.

[lukehutch]

I discovered that if I change

accessible.set((Boolean) isAccessible.invokeExact(field));

to

accessible.set((Boolean) isAccessible.invokeExact(new Object[] {field}));

I get

Covariant return type change detected: Object java.lang.invoke.MethodHandle.invoke(Object[]) has 
been changed to Boolean java.lang.invoke.MethodHandle.invoke(Object[])

This explains part of the problem. This is the same issue as experienced with a change in return type of several ByteBuffer methods:

plasma-umass/doppio#497 (comment)

However, Animal Sniffer was not able to detect the covariant return type problem without the cast to (Object), instead giving an error Unidentified reference: Boolean java.lang.invoke.MethodHandle.invokeExact(java.lang.reflect.AccessibleObject) (which made it very difficult to find out what was wrong). This seems to be a bug or shortcoming in Animal Sniffer itself, not the signature files.

There is a second problem though: in fact this is a false positive. invokeExact (and invoke etc.) still returns Object today in JDK 13. I just happened to cast the result to Boolean because the isAccessible method happens to return boolean.

However there is a third problem, due to Animal Sniffer not being able to recognize that a single parameter of type AccessibleObject matches a varargs parameter of type Object.... If I change the line to the following:

accessible.set((Boolean) isAccessible.invokeExact(new Object[] {field}));

then I still get

Undefined reference: Object java.lang.invoke.MethodHandle.invoke(java.lang.reflect.AccessibleObject)

The only way to fix this error completely is to change the code to:

accessible.set((Boolean) (Object) isAccessible.invokeExact(new Object[] {field}));

which is (I assume) less efficient than calling the varargs method the normal way. Why does Animal Sniffer require this new Object[] {...} in order to match the method?

[lukehutch]

Furthermore when trying to call a static method from a MethodHandle, the wrapping of parameters in new Object[] {...} does not even work. I get:

java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle(ParamType1,ParamType2)ReturnType to (Object[])Object

I think the real issue here is that Animal Sniffer is missing explicit support for @PolymorphicSignature methods, and they cannot be supported properly just using type inference or type coercion.

[lukehutch]

Hi, I ran into this again. Can somebody from Mojohaus please take a look at this?