GC needs to know about all references in the program. If the mutator (the program) is running concurrently with the collector, it's difficult (though not impos

GC needs to know about all references in the program. If the mutator (the program) is running concurrently with the collector, it's difficult (though not impossible) for the collector to construct this consistent view. Here is a good introductory talk: http://www.infoq.com/presentations/Understanding-Java-Garbage-Collection http://www.infoq.com/presentations/Understanding-Java-Garbag...