Java Concurrency (&c)

I got asked about this twice in one day, and I didn't know the answer, so I sat down and puzzled it out a bit. A SoftReference is a reference that the garbage collector can decide to clear if it is the only reference left to an object, and the GC decides (through some undefined process) that there is enough memory pressure to warrant clearing it. SoftReferences are generally used to implement memory-sensitive caches. A mistake many people make is to confuse it with a WeakReference , which is a reference that, if it is the only reference left to the object, the garbage collector will aggressively clear. I got asked how Hotspot's GC actually decides whether to clear SoftReferences. Twice. On the same day. (Hotspot is the Sun / OpenJDK JVM). The answer was that I had no idea, so I had to go look it up. Here is what I found out; I hope it is a useful reference for someone (pun intended). First, there is a global clock variable that is set with the current time (in millis)...