RFR (JDK11) 8137326: Methods for comparing CharSequence, StringBuilder, and StringBuffer

[Xueming Shen]

On 2/22/18, 12:04 PM, Joe Wang wrote:
> Hi Sherman,
>
> Thanks for reviewing the change.
>
> Taking a local copy of the count field, but the boundary check would 
> be almost immediately done against the field itself. Are you worrying 
> about the count field may be out of sync with the byte array? I would 
> think that's unlikely to happen. Whether it's StringBuilder or 
> StringBuffer, it's not advisable/practical to use in multiple threads. 
> In a valid usage, the count is always consistent with the byte array.
>

Hi Joe,

It might not be a "valid usage" but it did happen and when it happens it 
might just crash the
vm without those boundary checks. It's especially true for those 
intrinsics methods with explicit
comments "intrinsic performs no bounds checks". In this case, the 
StringUTF16.getChar() is being
called in new public method StringUTF16.compareTo(byte[], byte[], int, 
int) without appropriate
boundary check. In the "old" code the "index" is guaranteed to be within 
[0, len) in
StringUTF16.compareTo(byte[], byte[]), so it's safe. A real case for 
such scenario can be found in
JDK-8158168 [1], for example.

-Sherman


[1] https://bugs.openjdk.java.net/browse/JDK-8158168