RFR: 8277072: ObjectStreamClass caches keep ClassLoaders alive [v2]

[Roman Kennke]

> The caches in ObjectStreamClass basically map WeakReference<Class> to SoftReference<ObjectStreamClass>, where the ObjectStreamClass also references the same Class. That means that the cache entry, and thus the class and its class-loader, will not get reclaimed, unless the GC determines that memory pressure is very high.
> 
> However, this seems bogus, because that unnecessarily keeps ClassLoaders and all its classes alive much longer than necessary: as soon as a ClassLoader (and all its classes) become unreachable, there is no point in retaining the stuff in OSC's caches.
> 
> The proposed change is to use WeakReference instead of SoftReference for the values in caches.
> 
> Testing:
>  - [x] tier1
>  - [x] tier2
>  - [x] tier3
>  - [ ] tier4

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Use ForceGC instead of System.gc()
 - Convert test to testng

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6375/files
  - new: https://git.openjdk.java.net/jdk/pull/6375/files/430f54e5..6f099c9c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6375&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6375&range=00-01

  Stats: 21 lines in 1 file changed: 8 ins; 7 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6375.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6375/head:pull/6375

PR: https://git.openjdk.java.net/jdk/pull/6375