查看运行中的Java其配置的堆大小

一、背景

有题目中的需求，也不是空穴来风；前一阵给公司搭建了一个持续集成服务器，Jenkins。最近发现，运行一段时间后，就变慢了。

随便一个操作，cpu就飙高了。然后就思考会不会是内存不够用，频繁GC导致的呢？

Jenkins是个war包，我直接放在tomcat运行的。所以，我需要查看下，当前运行的这个tomcat的堆大小。

二、步骤

[root@pas jenkins-apache-tomcat-8.5.28]# jmap -heap 90170
Attaching to process ID 90170, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.161-b12

using thread-local object allocation.
Parallel GC with 8 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 0
   MaxHeapFreeRatio         = 100
   MaxHeapSize              = 4160749568 (3968.0MB)  -----------堆的最大容量
   NewSize                  = 86507520 (82.5MB)
   MaxNewSize               = 1386741760 (1322.5MB) 
   OldSize                  = 173539328 (165.5MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 1306001408 (1245.5MB)
   used     = 1123777200 (1071.7174530029297MB)
   free     = 182224208 (173.7825469970703MB)
   86.04716603797107% used
From Space:
   capacity = 37748736 (36.0MB)
   used     = 17255448 (16.456077575683594MB)
   free     = 20493288 (19.543922424316406MB)
   45.711326599121094% used
To Space:
   capacity = 37224448 (35.5MB)
   used     = 0 (0.0MB)
   free     = 37224448 (35.5MB)
   0.0% used
PS Old Generation
   capacity = 212336640 (202.5MB)
   used     = 90248224 (86.06741333007812MB)
   free     = 122088416 (116.43258666992188MB)
   42.50242633584105% used

44622 interned Strings occupying 4789016 bytes.

具体的字段意思，我查阅了oracle的官方资料，如下，但也没有看到具体解释，只能慢慢探索了：

https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr014.html#BABJIIIH

可以看到，上面的tomcat的最大堆容量还是很大的，当然，我们自己没有配置过，应该是JVM根据机器的物理内存大小，自动计算的。