Java笔记4--JVM&GC

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垃圾回收算法

1.引用计数法

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2.复制算法 (新生代)

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复制活的到空的(复活)

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复制之后有交换,谁空谁是To (To 0 --->往占用0的位置挪)
优点:没有产生内存碎片, 缺点:大对象复制耗时(所以用于新生代)

3.标记清除法 (老年代)

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4.标记整理法 (老年代)

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GC Roots 一定是从Roots开始的

可以作为GC Root的对象

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盘点家底查看 JVM 默认值

查看初始默认值：-XX:+PrintFlagsInitial

ginseng@ginseng-pc:~/Project/demo$ java -XX:+PrintFlagsInitial
[Global flags]
    intx ActiveProcessorCount                      = -1                                  {product}
    uintx AdaptiveSizeDecrementScaleFactor          = 4                                   {product}
    uintx AdaptiveSizeMajorGCDecayTimeScale         = 10                                  {product}
    uintx AdaptiveSizePausePolicy                   = 0                                   {product}
    uintx AdaptiveSizePolicyCollectionCostMargin    = 50                                  {product}
    uintx AdaptiveSizePolicyInitializingSteps       = 20                                  {product}
    uintx AdaptiveSizePolicyOutputInterval          = 0                                   {product}
    uintx AdaptiveSizePolicyWeight                  = 10                                  {product}
   ...

查看修改更新：-XX:+PrintFlagsFinal

bool UsePSAdaptiveSurvivorSizePolicy           = true                                {product}
bool UseParNewGC                               = false                               {product}
bool UseParallelGC                            := true                                {product}
bool UseParallelOldGC                          = true                                {product}
bool UsePerfData                               = true                                {product}
bool UsePopCountInstruction                    = true                                {product}
bool UseRDPCForConstantTableBase               = false                               {C2 product}