Core Java 面试题与答案
Question: What comes to mind when you hear about a young generation in Java?Answer: Garbage collection.In the J2SE platform version 1.4.1 two new garbage collectors were introduced to make a total of four garbage collectors from which to choose. Beginning with the J2SE platform, version 1.2, the virtual machine incorporated a number of different garbage collection algorithms that are combined using generational collection. While naive garbage collection examines every live object in the heap, generational collection exploits several empirically observed properties of most applications to avoid extra work. The default collector in HotSpot has two generations: the young generation and the tenured generation. Most allocations are done in the young generation. The young generation is optimized for objects that have a short lifetime relative to the interval between collections. Objects that survive several collections in the young generation are moved to the tenured generation. The young generation is typically smaller and is collected more often. The tenured generation is typically larger and collected less often. The young generation collector is a copying collector. The young generation is divided into 3 spaces: eden-space, to-space, and from-space. Allocations are done from eden-space and from-space. When those are full a young generation is collection is done. The expectation is that most of the objects are garbage and any surviving objects can be copied to to-space. If there are more surviving objects than can fit into to-space, the remaining objects are copied into the tenured generation. There is an option to collect the young generation in parallel. The tenured generation is collected with a mark-sweep-compact collection. There is an option to collect the tenured generation concurrently. |
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