Java垃圾回收时避免内存碎片的方法

Compacting Collectors

Compacting collectors slide live objects over free memory space toward one end of the heap. In the process the other end of the heap becomes one large contiguous free area. All references to the moved objects are updated to refer to the new location.

Copying Collectors

A common copying collector algorithm is called "stop and copy." In this scheme, the heap is divided into two regions. Only one of the two regions is used at any time. Objects are allocated from one of the regions until all the space in that region
has been exhausted. At that point program execution is stopped and the heap is traversed. Live objects are copied to the other region as they are encountered by the traversal. When the stop and copy procedure is finished, program execution resumes. Memory
will be
allocated from the new heap region until it too runs out of space. At that point the program will once again be stopped. The heap will be traversed and live objects will be copied back to the original region. The cost associated with this approach
is that twice as much memory is needed for a given amount of heap space because only half of the available memory is used at any time.

Generational Collectors

Generational collectors address this inefficiency by grouping objects by age and garbage collecting younger objects more often than older objects. In this approach, the heap is divided into two or more sub-heaps, each of which serves one "generation"
of objects. The youngest generation is garbage collected most often. As most objects are short-lived, only a small percentage of young objects are likely to survive their first collection. Once an object has survived a few garbage collections as a member of
the youngest generation, the object is promoted to the next generation: it is moved to another sub-heap.