RFR: 8277863: Deprecate sun.misc.Unsafe methods that return offsets

[Alan Bateman]

Deprecate the sun.misc.Unsafe methods that return field offsets. These method are an impediment to possible future changes. Layout may not be fixed in the future, the VM should be allowed to re-layout dynamically based on patterns of usage. We also have the issue of libraries using these methods to get offsets (sometimes of classes with the same layout as JDK classes) so they can directly access the fields of privileged classes. It's untenable for libraries to rely on this going forward.

The java.lang.invoke.VarHandle API (added in Java 9) provides a strongly typed reference to a variable that is a safe and a much better alternative to many cases that use these methods. Deprecating these method provides a gentle nudge in that directory. Once the Panama memory APIs are permanent then we can look at terminally deprecating and removing these methods, along with the accessors.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6700/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6700&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277863
  Stats: 14 lines in 1 file changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6700.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6700/head:pull/6700

PR: https://git.openjdk.java.net/jdk/pull/6700