RFR: 8260621: Avoid memory leak in ImageBufferCache [v2]

[Bo Zhang]

> Previously, `ImageBufferCache` contains a ThreadLocal field which holds
> strong reference to `ImageBufferCache$BufferReference.class`. When loaded
> from `jrt-fs.jar`, this will keep `JrtFileSystemProvider$JrtFsLoader`
> in memory forever and never being GCed.
> 
> The fix replace the old `ImageBufferCache$BufferReference` class with
> `WeakReference<ByteBuffer>`, which is verified by provided test.

Bo Zhang has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  8260621: Avoid memory leak in ImageBufferCache
  
  Previously, `ImageBufferCache` contains a ThreadLocal field which holds
  strong reference to `ImageBufferCache$BufferReference.class`. When loaded
  from `jrt-fs.jar`, this will keep `JrtFileSystemProvider$JrtFsLoader`
  in memory forever and never being GCed.
  
  The fix replace the old `ImageBufferCache$BufferReference` class with
  `WeakReference<ByteBuffer>`, which is verified by provided test.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2307/files
  - new: https://git.openjdk.java.net/jdk/pull/2307/files/8d61d77d..27b55790

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2307&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2307&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2307.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2307/head:pull/2307

PR: https://git.openjdk.java.net/jdk/pull/2307