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

 1 [[email protected] bin]# jmap -heap 2384   //打印java heap堆内存信息
 2 Attaching to process ID 2384, please wait...
 3 Debugger attached successfully.
 4 Client compiler detected.
 5 JVM version is 11.0-b15
 6
 7 using thread-local object allocation.
 8 Mark Sweep Compact GC
 9
10 Heap Configuration:
11    MinHeapFreeRatio = 40
12    MaxHeapFreeRatio = 70
13    MaxHeapSize      = 268435456 (256.0MB)
14    NewSize          = 131072000 (125.0MB)
15    MaxNewSize       = 131072000 (125.0MB)
16    OldSize          = 4194304 (4.0MB)
17    NewRatio         = 12
18    SurvivorRatio    = 8
19    PermSize         = 12582912 (12.0MB)
20    MaxPermSize      = 67108864 (64.0MB)
21
22 Heap Usage:
23 New Generation (Eden + 1 Survivor Space):
24    capacity = 117964800 (112.5MB)
25    used     = 102007440 (97.28187561035156MB)
26    free     = 15957360 (15.218124389648438MB)
27    86.4727783203125% used
28 Eden Space:
29    capacity = 104857600 (100.0MB)
30    used     = 90339496 (86.15445709228516MB)
31    free     = 14518104 (13.845542907714844MB)
32    86.15445709228516% used
33 From Space:
34    capacity = 13107200 (12.5MB)
35    used     = 11667944 (11.127418518066406MB)
36    free     = 1439256 (1.3725814819335938MB)
37    89.01934814453125% used
38 To Space:
39    capacity = 13107200 (12.5MB)
40    used     = 0 (0.0MB)
41    free     = 13107200 (12.5MB)
42    0.0% used
43 tenured generation:
44    capacity = 137363456 (131.0MB)
45    used     = 40348976 (38.47978210449219MB)
46    free     = 97014480 (92.52021789550781MB)
47    29.373879469078005% used
48 Perm Generation:
49    capacity = 12845056 (12.25MB)
50    used     = 12656048 (12.069747924804688MB)
51    free     = 189008 (0.1802520751953125MB)
52    98.52855448820154% used

配置tomcat jvm参数