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-b04using 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 GenerationEden Space:   capacity = 1682440192 (1604.5MB)   used     = 1513288224 (1443.1841125488281MB)   free     = 169151968 (161.31588745117188MB)   89.94603381419932% usedFrom Space:   capacity = 29884416 (28.5MB)   used     = 26140048 (24.929092407226562MB)   free     = 3744368 (3.5709075927734375MB)   87.47049967447917% usedTo Space:   capacity = 33030144 (31.5MB)   used     = 0 (0.0MB)   free     = 33030144 (31.5MB)   0.0% usedPS Old Generation   capacity = 350748672 (334.5MB)   used     = 26275344 (25.058120727539062MB)   free     = 324473328 (309.44187927246094MB)   7.491216958905549% usedPS 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-b04using 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 GenerationEden Space:   capacity = 319815680 (305.0MB)   used     = 147022504 (140.21158599853516MB)   free     = 172793176 (164.78841400146484MB)   45.971011802798415% usedFrom Space:   capacity = 18874368 (18.0MB)   used     = 11712952 (11.170341491699219MB)   free     = 7161416 (6.829658508300781MB)   62.05745273166232% usedTo Space:   capacity = 18874368 (18.0MB)   used     = 0 (0.0MB)   free     = 18874368 (18.0MB)   0.0% usedPS Old Generation   capacity = 716177408 (683.0MB)   used     = 65918920 (62.86518096923828MB)   free     = 650258488 (620.1348190307617MB)   9.204272469873834% usedPS 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。