查看 JVM 参数的默认值

查看初始默认值：-XX:+PrintFlagsInitial

HuandeMacBook-Air:~ huanliu$ java -XX:+PrintFlagsInitial
[Global flags]
    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}
    uintx AdaptiveSizeThroughPutPolicy              = 0                                   {product}
    uintx AdaptiveTimeWeight                        = 25                                  {product}
     bool AdjustConcurrency                         = false                               {product}
     bool AggressiveOpts                            = false                               {product}
     intx AliasLevel                                = 3                                   {C2 product}
     bool AlignVector                               = true                                {C2 product}
     intx AllocateInstancePrefetchLines             = 1                                   {product}
     intx AllocatePrefetchDistance                  = -1                                  {product}
...

查看修改更新：-XX:+PrintFlagsFinal

HuandeMacBook-Air:~ huanliu$ java -XX:+PrintFlagsFinal
[Global flags]
...
    uintx HeapFirstMaximumCompactionCount           = 3                                   {product}
    uintx HeapMaximumCompactionInterval             = 20                                  {product}
    uintx HeapSizePerGCThread                       = 87241520                            {product}
     bool IgnoreEmptyClassPaths                     = false                               {product}
     bool IgnoreUnrecognizedVMOptions               = false                               {product}
    uintx IncreaseFirstTierCompileThresholdAt       = 50                                  {product}
     bool IncrementalInline                         = true                                {C2 product}
    uintx InitialBootClassLoaderMetaspaceSize       = 4194304                             {product}
    uintx InitialCodeCacheSize                      = 2555904                             {pd product}
    uintx InitialHeapSize                          := 134217728                           {product}
    uintx InitialRAMFraction                        = 64                                  {product}
    uintx InitialSurvivorRatio                      = 8                                   {product}
    uintx InitialTenuringThreshold                  = 7                                   {product}
    uintx InitiatingHeapOccupancyPercent            = 45                                  {product}
     bool Inline                                    = true                                {product}
    ccstr InlineDataFile                            =                                     {product}
     intx InlineSmallCode                           = 2000                                {pd product}
     bool InlineSynchronizedMethods                 = true                                {C1 product}
     bool InsertMemBarAfterArraycopy                = true                                {C2 product}
     intx InteriorEntryAlignment                    = 16                                  {C2 pd product}
...

　　= 与 := 的区别是，一个是默认，一个是人为改变或者 jvm 加载时自动改变的参数

打印命令行参数(可以看默认垃圾回收器)：-XX:+PrintCommandLineFlags

HuandeMacBook-Air:~ huanliu$ java -XX:+PrintCommandLineFlags
-XX:InitialHeapSize=134217728 -XX:MaxHeapSize=2147483648 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC

附：JVM学习目录