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

[Brian Burkhalter]

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 `slice()` 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.

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Commit messages:
 - 4833719: (bf) Views of MappedByteBuffers are not MappedByteBuffers, and cannot be forced

Changes: https://git.openjdk.java.net/jdk/pull/2902/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2902&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-4833719
  Stats: 217 lines in 3 files changed: 204 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2902.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2902/head:pull/2902

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