Common Heap Related Switches

There are many different command line switches that can be used with Java. This section describes some of the more commonly used switches:

  • -Xms - Sets the initial heap size for when the JVM starts
  • -Xmx - Sets the maximum heap size
  • -Xmn - Sets the size of the Young Generation
  • -XX:PermSize - Sets the starting size of the Permanent Generation
  • -XX:MaxPermSize - Sets the maximum size of the Permanent Generation

Common Heap Related Switches Demo

Enable GC log

-verbose:gc -Xloggc:gc.log
-verbose:gc -Xloggc:gc.log -XX:+PrintGCDetails
-verbose:gc -Xloggc:gc.log -XX:+PrintGCDetails -XX:+PrintGCDateStamps

GC Log Example

2013-03-12T14:47:00.733+0800: 331.682: [Full GC [PSYoungGen: 2064K->0K(14656K)] [PSOldGen: 329172K->331173K(341376K)] 331236K->331173K(356032K) [PSPermGen: 1934K->1934K(16384K)], 0.0040140 secs] [Times: user=0.00 sys=0.00, real=0.01 secs]
  • Full GC happened at 2013-03-12T14:47:00.733+0800, after JVM run 331.682 seconds
  • Current heap size was changed by GC: 331236K->331173K
  • Max heap size: 356032K
  • Current permanent size was changed: 1934K->1934K
  • Max permanent area size: 16384K
  • Full GC took 0.0040140 secs

Enable heap dump


Enable jconsole

  • jconsole monitor j2se app(including JMX MBean monitor)
  • jconsole monitor j2se app(Only monitor memory and threads)