OutOfMemoryError: PermGen space

Recently, I found a problem with our system in which it ran out of PermGen space under heavy usage. The exception from the logs is:

java.lang.OutOfMemoryError: PermGen space
        at java.lang.String.intern(Native Method)

PermGen Space
The memory a jvm uses is split up into three "generations": young (eden), tenured, and permanent. PermGen refers to the permanent generation which holds meta-data describing user classes and "interned" strings. If you have a large code base and intern lots of strings, more permgen space is used.

Interning Strings
You can intern strings using String.intern() which, from the javadocs:

Returns a canonical representation for the string object.
A pool of strings, initially empty, is maintained privately by the class String.
When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

Interned strings are stored in permgen space, so if you interned all your strings, you would eventually run out of memory. In our application, I found that we were interning string representations of timestamps which wasn't necessary! You should only intern strings which are used frequently.

Increasing PermGen Space
The maximum size is typically 64MB. You can double it by starting your JVM with the following system properties:
-XX:MaxPermSize=128m -XX:PermSize=128m

Using JConsole to Monitor Memory
An easy way to see how much memory your JVM is using is to use jconsole. First, you need to enable monitoring on your JVM by starting it up with the following system properties:
-Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.port=<port>
Next start jconsole and point it at this JVM using the host and port (or pid). Select the Memory tab and then choose the Memory Pool "PS Perm Gen" chart. You should now see the usage of your PermGen space as shown in the screenshot below:

XP Uptime Record

My computer has been up for 151 days!

This is the XP machine I use every day at work and even log on from home, after work and on weekends. I'm surprised to see that it hasn't needed a reboot for so long.

Sadly, there's a powerdown in the building tonight...

Finding Uptime on XP
Use the systeminfo command and the look for "System Up Time"

C:\> systeminfo | find "System Up"
  System Up Time: 151 Days, 9 Hours, 45 Minutes, 8 Seconds

Finding Uptime on Solaris or Linux
Use the uptime command:

sharfah@starship:~> uptime
  5:46pm  up 2 day(s), 7:30, 1 user, load average: 3.79, 5.55, 5.41

You can also find out when your system booted up, by using who -b:

sharfah@starship:~> who -b
 system boot Dec 10 11:16