Fácil pero... jmap, jps, jstack

Siempre que hay un problema serio tenemos que recurrir a la información ofrecida por una serie de utilidades que incluye "de serie" Java 5, jmap para la información en la memoria del proceso, jps para obtener las jvm corriendo en el sistema, y jstack para la traza de los hilos.

Por ejemplo, para obtener las jvm de la máquina hacemos:

emilio@silencio:~$ /opt/java5/bin/jps
21910 Jps
21801 Bootstrap

Después, la traza de los hilos:

emilio@silencio:~$ /opt/java5/bin/jstack 21801
Attaching to process ID 21801, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 1.5.0_08-b03
Thread 21949: (state = BLOCKED)
- java.lang.Object.wait(long) @bci=0 (Interpreted frame)
- org.apache.tomcat.util.threads.ThreadPool$MonitorRunnable.run() @bci=10, line=559 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=595 (Interpreted frame)

con el que, al menos, podremos atacar el problema con más conocimiento...

Por último, jugamos con la memoria, de más o menos interés dependiendo de las necesidades, generación permanente:

emilio@silencio:~$ /opt/java5/bin/jmap -permstat 21801
Attaching to process ID 21801, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 1.5.0_08-b03
finding class loader instances ..Unknown oop at 0x9d385848
Oop's klass is null
done.
computing per loader stat ..done.
please wait.. computing liveness.................................................................done.
class_loader classes bytes parent_loader alive? type

1211 2771016 null live
0x9f1d9a38 1 1408 0x9efec8e0 dead sun/reflect/DelegatingClassLoader@0xad392da0
0x9f1ace08 1 1424 0x9efec8e0 dead sun/reflect/DelegatingClassLoader@0xad392da0
0x9f061d58 1 1408 0x9efec8e0 dead sun/reflect/DelegatingClassLoader@0xad392da0
0x9f8773e0 58 172472 0x9effbcf8 live org/apache/catalina/loader/WebappClassLoader@0xad9d8da0

histograma:

emilio@silencio:~$ /opt/jdk1.5.0_08/bin/jmap -histo 21801
Attaching to process ID 21801, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 1.5.0_08-b03
Iterating over heap. This may take a while...
Unknown oop at 0x9d355648
Oop's klass is null
Object Histogram:

Size Count Class description
-------------------------------------------------------
5527864 44548 * ConstMethodKlass
3520376 43933 char[]
3210464 44548 * MethodKlass
2764288 67280 * SymbolKlass
2216472 4001 * ConstantPoolKlass
1657864 4774 byte[]
1640944 4001 * InstanceKlassKlass
1325072 3614 * ConstantPoolCacheKlass
1225368 51057 java.lang.String
690320 8629 java.lang.reflect.Method
685512 9521 java.lang.reflect.Field
655104 5685 int[]
428448 4463 java.lang.Class
405936 16914 java.util.HashMap$Entry

y el resumen:

emilio@silencio:~$ /opt/jdk1.5.0_08/bin/jmap -heap 21801
Attaching to process ID 21801, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 1.5.0_08-b03

using thread-local object allocation.
Mark Sweep Compact GC

Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 268435456 (256.0MB)
NewSize = 655360 (0.625MB)
MaxNewSize = 4294901760 (4095.9375MB)
OldSize = 1441792 (1.375MB)
NewRatio = 8
SurvivorRatio = 8
PermSize = 16777216 (16.0MB)
MaxPermSize = 67108864 (64.0MB)

Heap Usage:
New Generation (Eden + 1 Survivor Space):
capacity = 1245184 (1.1875MB)
used = 666696 (0.6358108520507812MB)
free = 578488 (0.5516891479492188MB)
53.54196648848684% used
Eden Space:
capacity = 1114112 (1.0625MB)
used = 638656 (0.60906982421875MB)
free = 475456 (0.45343017578125MB)
57.32421875% used
From Space:
capacity = 131072 (0.125MB)
used = 28040 (0.02674102783203125MB)
free = 103032 (0.09825897216796875MB)
21.392822265625% used
To Space:
capacity = 131072 (0.125MB)
used = 0 (0.0MB)
free = 131072 (0.125MB)
0.0% used
tenured generation:
capacity = 10674176 (10.1796875MB)
used = 10395440 (9.913864135742188MB)
free = 278736 (0.2658233642578125MB)
97.38868836339218% used
Perm Generation:
capacity = 20447232 (19.5MB)
used = 20255184 (19.316848754882812MB)
free = 192048 (0.1831512451171875MB)
99.06076284555289% used

Desde Java 6 podemos hacer cosas más interesantes con jmap + herramientas como visualvm... pero esa es otra historia.