Java Webapp: strange memory statistics

I'm running a Java web application(J2EE+Tomcat6+struts2+hibernate) on a Linux Centos machine. The problem is the "top" command says java is using 14% of memory(out of 8G) but in the summary information section, says the overall free memory is 300M! (just Mem, not swap)

    PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  SWAP   TIME CODE DATA nFLT COMMAND                                                                                                                                 
    666 root      20   0 1352m 1.2g  10m S  7.6 14.7 663:00.37 158m 663:00   3开发者_开发技巧6 1.3g   65 java
   3170 mysql     18   0  145m  33m 5044 S  0.7  0.4 753:51.96 111m 753:51 6496 132m  100 mysqld                                                                                                                                  
  17340 root      15   0  2332 1080  800 R  0.3  0.0   0:00.05 1252   0:00   56  468    0 top                                                                                                                                     
      1 root      15   0  2068  628  536 S  0.0  0.0   0:02.58 1440   0:02   32  280   20 init                                                                                                                                    
      2 root      RT  -5     0    0    0 S  0.0  0.0   0:00.18    0   0:00    0    0    0 migration/0                                                                                                                             
      3 root      34  19     0    0    0 S  0.0  0.0   0:00.00    0   0:00    0    0    0 ksoftirqd/0                                                                                                                             
      4 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00    0   0:00    0    0    0 watchdog/0                                                                                                                              
      5 root      RT  -5     0    0    0 S  0.0  0.0   0:00.01    0   0:00    0    0    0 migration/1                                                                                                                             
      6 root      34  19     0    0    0 S  0.0  0.0   0:00.00    0   0:00    0    0    0 ksoftirqd/1                                                                                                                             
      7 root      RT  -5     0    0    0 S  0.0  0.0   0:00.00    0   0:00    0    0    0 watchdog/1                                                                                                                              
      8 root      RT  -5     0    0    0 S  0.0  0.0   0:00.01    0   0:00    0    0    0 migration/2

.
.
.
and so on...



Mem:   8300688k total,  7998720k used,   301968k free,    52452k buffers
Swap: 16779884k total,       60k used, 16779824k free,  6511228k cached

what is the problem? what could be using memory covertly? non-heap or what?! thanks buddies.

6511228k cached means ~6.5GB of RAM is used by your OS for caching.

If you're using Java 5+ you should be able to connect to the JVM using jconsole. There you should be able to see all the memory that the JVM uses. Unless there's a bug in the JVM implementation (which I would doubt), the numbers you see there is all this JVM uses.

If that's low enough, then some other process would use that memory.