JDK内存

jdk内存实际是jvm内存，jvm有一个运行时数据区，其实就是对这一部分的大小分配。

运行时数据区通常包括这几个部分：程序计数器(Program Counter Register)、Java栈(VM Stack)、本地方法栈(Native Method Stack)、方法区(Method Area)、堆(Heap)

• Xss：每个线程的stack大小（栈）
• Xmx：JAVA HEAP的最大值、默认为物理内存的1/4
• Xms：JAVA HEAP的初始值，server端最好Xms与Xmx一样
• Xmn：JAVA HEAP young区的大小
• XX:PermSize：设定内存的永久保存区域(JDK7)
• XX:MaxPermSize：设定最大内存的永久保存区域(JDK7)

在JDK1.8中，取消了PermGen，取而代之的是Metaspace，所以PermSize和MaxPermSize参数失效，取而代之的元空间:

　例如下面配置设置元空间的大小和最大大小:(等同于JDK7的永久区)

 -XX:MetaspaceSize=64m -XX:MaxMetaspaceSize=128m

例如:weblogic设置JVM参数:

-Xms2056m -Xmx2056m -XX:MetaspaceSize=512m -XX:MaxMetaspaceSize=512m

查看JVM参数:

第一种:jps+jmap

C:\Users\Administrator>jps
16740 Main
8388 Jps
4248
11564 Server
11884 NetworkServerControl

C:\Users\Administrator>jmap -heap 11564
Attaching to process ID 11564, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.121-b13

using thread-local object allocation.
Parallel GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 2155872256 (2056.0MB)
   NewSize                  = 718274560 (685.0MB)
   MaxNewSize               = 718274560 (685.0MB)
   OldSize                  = 1437597696 (1371.0MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 536870912 (512.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 536870912 (512.0MB)
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 538968064 (514.0MB)
   used     = 132464928 (126.32839965820312MB)
   free     = 406503136 (387.6716003417969MB)
   24.577509661129014% used
From Space:
   capacity = 89653248 (85.5MB)
   used     = 89640016 (85.48738098144531MB)
   free     = 13232 (0.0126190185546875MB)
   99.98524091397113% used
To Space:
   capacity = 89653248 (85.5MB)
   used     = 0 (0.0MB)
   free     = 89653248 (85.5MB)
   0.0% used
PS Old Generation
   capacity = 1437597696 (1371.0MB)
   used     = 75478984 (71.98236846923828MB)
   free     = 1362118712 (1299.0176315307617MB)
   5.250355103518475% used

61705 interned Strings occupying 7127136 bytes.

第二种:用JDK自带的工具 jconsole或者jvisualvm:

原文地址：https://www.cnblogs.com/qlqwjy/p/9668473.html