Document IAE for Unsafe offset methods

[liach]

Unsafe throws IAE for misusing static vs instance fields, and it's revealed that AtomicXxxFieldUpdaters are using this mechanism to reject static fields. This is not a good practice, but we can at least document this so we don't accidentally introduce problems.

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[ExE-Boss]

Maybe these checks could be done in Java like:

// implicit null check:
if ((f.getModifiers() & ACC_STATIC) != 0) {
	throw new IllegalArgumentException("Field is static");
}

and for the staticField(Offset/Base) methods:

// implicit null check:
if ((f.getModifiers() & ACC_STATIC) == 0) {
	throw new IllegalArgumentException("Field is not static");
}

[liach]

Don't think anyone is willing to change code here...

[liach]

That said, the only places that depend on this IAE behavior is the AtomicXxxFieldUpdater classes. All other sites pass trusted fields into these methods, of course besides the sun.misc.Unsafe users which definitely need this IAE. Still I personally prefer adding these checks to atomic field updaters instead of here to reduce risky dependencies.

[ExE-Boss]

I noticed that the native implementation of Unsafe​::array⁠(BaseOffset​/IndexScale) refers to the non‑existent class java.lang.InvalidClassException instead of java.lang.IllegalArgumentException (which results in java.lang.NoClassDefFoundError being thrown):

jdk/src/hotspot/share/prims/unsafe.cpp

Lines 587 to 594 in 1ca008f

	static void getBaseAndScale(int& base, int& scale, jclass clazz, TRAPS) {
	assert(clazz != nullptr, "clazz must not be null");
	oop mirror = JNIHandles::resolve_non_null(clazz);
	Klass* k = java_lang_Class::as_Klass(mirror);
	if (k == nullptr || !k->is_array_klass()) {
	THROW(vmSymbols::java_lang_InvalidClassException());