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 [root@zhoucentos 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
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