一次线上问题排查：java.lang.OutOfmemoryError: PermGen Space

背景

用基于Spring Boot（环境是java7）的系统做了一个报表导出的功能，第一次部署的时候用的默认JVM参数，但是测试人员在点击了一次导出之后，其他功能就疑似挂掉了，过了一会后才能正常使用；再点击导出，又出现这个问题，对这个问题的排查如下。

jmap -heap pid查看堆内存使用情况

Attaching to process ID 6082, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.65-b04

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

Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize      = 8417968128 (8028.0MB)
NewSize          = 1310720 (1.25MB)
MaxNewSize       = 17592186044415 MB
OldSize          = 5439488 (5.1875MB)
NewRatio         = 2
SurvivorRatio    = 8
PermSize         = 21757952 (20.75MB)
MaxPermSize      = 85983232 (82.0MB)//没加启动参数perm就这么大一点
G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
capacity = 1682440192 (1604.5MB)
used     = 1513288224 (1443.1841125488281MB)
free     = 169151968 (161.31588745117188MB)
89.94603381419932% used
From Space:
capacity = 29884416 (28.5MB)
used     = 26140048 (24.929092407226562MB)
free     = 3744368 (3.5709075927734375MB)
87.47049967447917% used
To Space:
capacity = 33030144 (31.5MB)
used     = 0 (0.0MB)
free     = 33030144 (31.5MB)
0.0% used
PS Old Generation
capacity = 350748672 (334.5MB)
used     = 26275344 (25.058120727539062MB)
free     = 324473328 (309.44187927246094MB)
7.491216958905549% used
PS Perm Generation
capacity = 76546048 (73.0MB)
used     = 76144792 (72.6173324584961MB)
free     = 401256 (0.38266754150390625MB)
99.47579788835081% used  //好吧，这里知道为啥了吧

32763 interned Strings occupying 3822560 bytes.

原因已经很清楚了，默认的永久代几乎被占满了，而我们知道，永久带在被占满的情况下会引发Full GC，而在Full GC的时候，会导致JVM暂停工作，要等到GC完成的时候才能工作。进一步查看了代码，发现在导出报表的代码中，涉及到大部分的类方法和常量的初始化，而刚好初始化完成之后，超过了永久代的大小，好了，问题就在这里。

解决

启动时设置JVM参数：

# nohup java -Xms1024m -Xmx1024m -Xss1024K -XX:PermSize=128m -XX:MaxPermSize=256m -jar spring-boot-1.0.0.jar >/dev/null 2>&1 &

启动之后

jmap -heap pid

：

Attaching to process ID 7992, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.65-b04

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

Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize      = 1073741824 (1024.0MB)
NewSize          = 1310720 (1.25MB)
MaxNewSize       = 17592186044415 MB
OldSize          = 5439488 (5.1875MB)
NewRatio         = 2
SurvivorRatio    = 8
PermSize         = 134217728 (128.0MB)
MaxPermSize      = 268435456 (256.0MB)
G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
capacity = 319815680 (305.0MB)
used     = 147022504 (140.21158599853516MB)
free     = 172793176 (164.78841400146484MB)
45.971011802798415% used
From Space:
capacity = 18874368 (18.0MB)
used     = 11712952 (11.170341491699219MB)
free     = 7161416 (6.829658508300781MB)
62.05745273166232% used
To Space:
capacity = 18874368 (18.0MB)
used     = 0 (0.0MB)
free     = 18874368 (18.0MB)
0.0% used
PS Old Generation
capacity = 716177408 (683.0MB)
used     = 65918920 (62.86518096923828MB)
free     = 650258488 (620.1348190307617MB)
9.204272469873834% used
PS Perm Generation
capacity = 134217728 (128.0MB)
used     = 97287360 (92.78045654296875MB)
free     = 36930368 (35.21954345703125MB)
72.48473167419434% used  //

好了，再怎么导出都没有问题了。