RFR: 8296244: Alternate implementation of user-based authorization Subject APIs that doesn’t depend on Security Manager APIs [v3]

[Weijun Wang]

> This code change adds an alternative implementation of user-based authorization `Subject` APIs that doesn't depend on Security Manager APIs. Depending on if the Security Manager is allowed, the methods store the current subject differently. See the spec change in the `Subject.java` file for details. When the Security Manager APIs are finally removed in a future release, this new implementation will be only implementation for these methods.
> 
> One major change in the new implementation is that `Subject.getSubject` always throws an `UnsupportedOperationException` since it has an `AccessControlContext` argument but the current subject is no longer associated with an `AccessControlContext` object.
> 
> Now it's the time to migrate from the `getSubject` and `doAs` methods to `current` and `callAs`. If the user application is simply calling `getSubject(AccessController.getContext())`, then switching to `current()` would work. If the `AccessControlContext` argument is retrieved from an earlier `getContext()` call and the associated subject might be different from that of the current `AccessControlContext`, then instead of storing the previous `AccessControlContext` object and passing it into `getSubject` to get the "previous" subject, the application should store the `current()` return value directly.

Weijun Wang has updated the pull request incrementally with one additional commit since the last revision:

  fix MBeanServerFileAccessController, more test in SM

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17472/files
  - new: https://git.openjdk.org/jdk/pull/17472/files/a16472fb..8f270d09

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17472&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17472&range=01-02

  Stats: 18 lines in 2 files changed: 8 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/17472.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17472/head:pull/17472

PR: https://git.openjdk.org/jdk/pull/17472