内存溢出及Jvm监控工具

内存泄露与内存溢出

内存溢出 out of memory，是指程序在申请内存时，没有足够的内存空间供其使用，出现out of memory。

内存泄露 memory leak，是指程序在申请内存后，无法释放已申请的内存空间。一次内存泄露危害可以忽略，但内存泄露堆积后果很严重，无论多少内存,迟早会被占光。
memory leak会最终会导致out of memory！

内存溢出分类

堆内存溢出
堆内存中存在大量对象，这些对象都有被引用，当所有对象占用空间达到堆内存的最大值，就会出现内存溢出OutOfMemory:Java heap space
永久代溢出
类的一些信息，如类名、访问修饰符、字段描述、方法描述等，所占空间大于永久代最大值，就会出现OutOfMemoryError:PermGen space

内存泄露检测手段

Jdk/bin目录下有很多检测工具
图形界面（需要更新测试机配置文件）：
Jconsole
Jvisualvm
命令行工具：
Jstat –gcutil pid 1000 100
Jmap –histo pid | head -20
Jmap –heap pid

[root@zhoucentos bin]# jmap -heap 2384   //打印java heap堆内存信息
Attaching to process ID 2384, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 11.0-b15

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize      = 268435456 (256.0MB)
NewSize          = 131072000 (125.0MB)
MaxNewSize       = 131072000 (125.0MB)
OldSize          = 4194304 (4.0MB)
NewRatio         = 12
SurvivorRatio    = 8
PermSize         = 12582912 (12.0MB)
MaxPermSize      = 67108864 (64.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
capacity = 117964800 (112.5MB)
used     = 102007440 (97.28187561035156MB)
free     = 15957360 (15.218124389648438MB)
86.4727783203125% used
Eden Space:
capacity = 104857600 (100.0MB)
used     = 90339496 (86.15445709228516MB)
free     = 14518104 (13.845542907714844MB)
86.15445709228516% used
From Space:
capacity = 13107200 (12.5MB)
used     = 11667944 (11.127418518066406MB)
free     = 1439256 (1.3725814819335938MB)
89.01934814453125% used
To Space:
capacity = 13107200 (12.5MB)
used     = 0 (0.0MB)
free     = 13107200 (12.5MB)
0.0% used
tenured generation:
capacity = 137363456 (131.0MB)
used     = 40348976 (38.47978210449219MB)
free     = 97014480 (92.52021789550781MB)
29.373879469078005% used
Perm Generation:
capacity = 12845056 (12.25MB)
used     = 12656048 (12.069747924804688MB)
free     = 189008 (0.1802520751953125MB)
98.52855448820154% used

配置tomcat jvm参数





编写接口脚本



跑5分钟……

jvisualvm远程连接虚拟机监控



可以看到堆内存到达了顶点，发生了垃圾回收却没有起作用。像这样暴力的，哈哈