RFR: 4833719: (bf) Views of MappedByteBuffers are not MappedByteBuffers, and cannot be forced

[Alan Bateman]

On Tue, 9 Mar 2021 22:20:52 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

> Please consider this proposal to add covariant overrides to `MappedByteBuffer` for the methods `compact()`, `duplicate()`, `slice()`, and `slice(int,int)`.
> 
> The methods in question are added as abstract specifications in `MappedByteBuffer` and their implementations in `Direct-X-Buffer.java.template`. In `MappedByteBuffer` the `isSync()` method is changed to have package access, and a final package scope method `FileDescriptor fileDescriptor()` is added to return the associated file descriptor. Specification verbiage is added to the new covariant overrides, and the specification of `force()` is enhanced slightly. (The `unmapper()` method offers an alternative way to obtain the file descriptor and sync mode without the need for package access `fileDescriptor()` and `isSync()` methods.)
> 
> In `Direct-X-Buffer.java.template` the constructor for duplicates and slices is modified to accept parameters for the file descriptor and sync mode for byte buffers. The uses of this constructor are correspondingly modified.
> 
> A test is added to exercise the new methods. Verifying that `force()` is actually doing anything is not verified by this test but was checked manually. The change passes all other existing tests in tiers 1-3.
> 
> Other methods for which it might be worth adding covariant overrides are the `get()` and `put()` methods which return a buffer, and, less interesting, the `put$Type$()` methods.

src/java.base/share/classes/java/nio/MappedByteBuffer.java line 396:

> 394:      */
> 395:     @Override
> 396:     public abstract MappedByteBuffer slice(int index, int length);

The updates to make use covariant returns is good but I'm not sure about the proposal "API note"s. I should drop "API note" because it's really spec text. Also I think it would be simpler to drop the "A similar consideration ..." to avoid confusing the reader.

-------------

PR: https://git.openjdk.java.net/jdk/pull/2902