Java堆默认大小

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PrintFlagsFinal

-XX:+PrintFlagsFinal
打印所有可设置的参数及它们的默认值(从JDK 6 update 21开始才可以用)

示例：16G内存的Centos系统

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$ java -XX:+PrintFlagsFinal -version  | grep -E 'HeapSize|PermSize|ThreadStackSize'
     intx CompilerThreadStackSize                   = 0                                   {pd product}
    uintx ErgoHeapSizeLimit                         = 0                                   {product}
    uintx HeapSizePerGCThread                       = 87241520                            {product}
    uintx InitialHeapSize                          := 264241152                           {product}
    uintx LargePageHeapSizeThreshold                = 134217728                           {product}
    uintx MaxHeapSize                              := 4215275520                          {product}
     intx ThreadStackSize                           = 1024                                {pd product}
     intx VMThreadStackSize                         = 1024                                {pd product}
java version "1.8.0_152"
Java(TM) SE Runtime Environment (build 1.8.0_152-b16)
Java HotSpot(TM) 64-Bit Server VM (build 25.152-b16, mixed mode)

可以看出默认的:
堆最大大小(MaxHeapSize)：4215275520/(1024.0 * 1024 * 1024) = 3.93G
初始堆大小(InitialHeapSiz): 266338304/(1024.0 * 1024) = 254M
HeapSizePerGCThread: 87241520 / (1024.0 * 1024) = 83.20M

PrintCommandLineFlags

-XX:+PrintCommandLineFlags

打印出所有出现在命令行上的可选JVM参数。可用来了解JVM的参数设置，例如堆空间大小、垃圾收集器等。默认情况下，此选项是禁用的，并且不打印标记。

示例：

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$ java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=263453376 -XX:MaxHeapSize=4215254016 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
java version "1.8.0_152"
Java(TM) SE Runtime Environment (build 1.8.0_152-b16)
Java HotSpot(TM) 64-Bit Server VM (build 25.152-b16, mixed mode)