RFR: JDK-8205549 JDK-8205698 Support of flattened values in Unsafe

[Frederic Parain]

> On Jun 29, 2018, at 17:15, mandy chung <mandy.chung at oracle.com> wrote:
> 
> 
> 
> On 6/29/18 1:36 PM, Frederic Parain wrote:
>> I’d prefer asserts then just let the VM crash unpredictably.
>> But we’re speaking about Unsafe, do whatever you want
>> for getObject()/putObject().
>> However, put*() methods should (must?) enforce value types immutability
>> (this includes primitive fields too). I haven’t seen this in Mandy’s changeset
>> (but I might I missed it).
> 
> This is not enforced (as John explains) in Unsafe::putXXX methods as they are unsafe access.  VarHandle and core reflection have an explicit guard to disallow writing to fields of a value type [1][2].  Final is final for value types.  The caller to putValueXXX does require the
> new value to be non-null (see X-VarHandle.java.template).

Thank you for the explanation.

I just want to clarify the consequences of unrestricted Unsafe::putXXX on values:
a single call can corrupt many values and have persistent effects on future values.

Fred


> 
> The check you put in in your patch is to throw NPE if it's writing
> null to a flat field of value type.  That is not done in the library
> and I will create a JBS issue and fix that separately.
> 
> Mandy
> [1] http://hg.openjdk.java.net/valhalla/valhalla/file/tip/src/java.base/share/classes/java/lang/invoke/MethodHandles.java#l2419
> [2] http://hg.openjdk.java.net/valhalla/valhalla/file/tip/src/java.base/share/classes/java/lang/reflect/Field.java#l168