內存泄露與內存溢出
內存溢出 out of memory，是指程序在申請內存時，沒有足夠的內存空間供其使用，出現out of memory。
內存泄露 memory leak，是指程序在申請內存后，無法釋放已申請的內存空間。一次內存泄露危害可以忽略，但內存泄露堆積后果很嚴重，無論多少內存,遲早會被占光。
memory leak會最終會導致out of memory！
內存溢出分類
堆內存溢出
堆內存中存在大量對象，這些對象都有被引用，當所有對象占用空間達到堆內存的最大值，就會出現內存溢出OutOfMemory:Java heap space
永久代溢出
類的一些信息，如類名、訪問修飾符、字段描述、方法描述等，所占空間大於永久代最大值，就會出現OutOfMemoryError:PermGen space
內存泄露檢測手段
Jdk/bin目錄下有很多檢測工具
圖形界面（需要更新測試機配置文件）：
Jconsole
Jvisualvm
命令行工具：
Jstat –gcutil pid 1000 100
Jmap –histo pid | head -20
Jmap –heap pid
1 [root@zhoucentos bin]# jmap -heap 2384 //打印java heap堆內存信息 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參數
編寫接口腳本
跑5分鍾……
jvisualvm遠程連接虛擬機監控
可以看到堆內存到達了頂點，發生了垃圾回收卻沒有起作用。像這樣暴力的，哈哈