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使用myeclipse启动tomcat 报java heap space ，PermGen space 错误，分别为 heap内存不足，PermGen内存不足
需加大 tomcat启动项参数 Xmx 和 XX:MaxPermSize
PermGen是指内存的永久保存区域，它用于存放class和 method 对象，以及String 对象
（sun原文：permanent generation is the area of the heap where class and method objects are stored. If an application loads a very large number of classes, then the size of the permanent generation might need to be increased using the -XX:MaxPermSize option.
）
PermGen又是一个特殊内存区域：Classloader 加载的东东是不能回收的，它们放在PermGen中
（tomcat原文：Why does the memory usage increase when I redeploy a web application? Because the Classloader (and the Class objects it loaded) cannot be recycled. They are stored in the permanent heap generation by the JVM, and when you redepoy a new class loader is created, which loads another copy of all these classes. This can cause OufOfMemoryErrors eventually.）
回到我的问题来，打开%java_home%bin\jvisualvm.exe (jdk6以上有)
查看tomcat的内存情况，点击tomcat标签下monitor , 当used heap = max heap used PermGen = max PermGen 时tomcat还在启动中，一会就报错；
由于此前已经将myeclipse中server->tomcat->tomcatx.x->jdk的参数设置成-Xms64m -Xmx512m -XX:MaxPermSize=80m
但是monitor 画面中max heap为 256M
点击overview标签,发现jvm参数如下：
-Xms64m
-Xmx512m
-XX:MaxPermSize=80m
-Xms64m
-Xmx256m
显然tomcat使用了最后的设置-Xms64m -Xmx256m；这个是在myeclipse的属性-》java->installed jres 中的vm启动参数里，双击默认启用的jre，把参数去掉或者设置成-Xms64m -Xmx512m
再回到server->tomcat->tomcatx.x->jdk中把-XX:MaxPermSize=80m修改为-XX:MaxPermSize=128m
保存后再启动tomcat，ok
另外，使用jvisualvm持续监测tomcat内存情况发现heap区域的内存使用会在一个高峰后稳定降低，内存被回收了；
而PermGen区域的内存则是不断增加到达一个峰值后就不再增加，但之后此区的内存没有被回收，验证了上面的说法；
多说两句：java的动态加载衍生出诸多框架，在空间上也暴露出问题，反射在时间上也存在效率问题：下面是组测试数据：
Java version 1.6.0_13
Java HotSpot(TM) Client VM
11.3-b02
Sun Microsystems Inc.
Direct access using member field:
47 125 47 46 46
average time = 66 ms.
Reference access to member field:
109 109 110 94 109
average time = 106 ms.
Reflection access to member field:
13094 12984 13063 13062 13094
average time = 13051 ms.
Java version 1.6.0_13
Java HotSpot(TM) Client VM
11.3-b02
Sun Microsystems Inc.
Direct call using member field:
47 31 109 109 31
average time = 70 ms.
Direct call using passed value:
16 16 16 31 15
average time = 20 ms.
Call to object using member field:
46 47 47 47 32
average time = 43 ms.
Call to object using passed value:
15 16 31 16 16
average time = 20 ms.
Reflection call using member field:
812 782 844 844 844
average time = 829 ms.
Reflection call using passed value:
938 953 954 1031 953
average time = 973 ms.
Java version 1.6.0_13
Java HotSpot(TM) Client VM
11.3-b02
Sun Microsystems Inc.
Direct Object creation:
62 47 78 32 93
average time = 63 ms.
Reflection Object creation:
125 94 94 109 187
average time = 121 ms.
Direct byte[8] creation:
125 187 94 172 94
average time = 137 ms.
Reflection byte[8] creation:
266 171 187 188 219
average time = 191 ms.
Direct byte[64] creation:
250 172 156 125 203
average time = 164 ms.
Reflection byte[64] creation:
281 219 203 203 219
average time = 211 ms.
华丽的上层需要坚实的底层基础
http://wiki.apache.org/tomcat/FAQ/Deployment
http://download.oracle.com/javase/7/docs/webnotes/tsg/TSG-VM/html/memleaks.html#gbyuu
java heap space， PermGen space 错误 使用jvisualvm监测设置合理值
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原文地址：http://www.cnblogs.com/tonyliult/p/4520201.html
