1Usage: jcmd <pid | main class> <command ...|PerfCounter.print|-f file>
2   or: jcmd -l                                                    
3   or: jcmd -h                                                    
4                                                                  
5  command must be a valid jcmd command for the selected jvm.      
6  Use the command "help" to see which commands are available.   
7  If the pid is 0, commands will be sent to all Java processes.   
8  The main class argument will be used to match (either partially 
9  or fully) the class used to start Java.                         
10  If no options are given, lists Java processes (same as -l).     
11                                                                  
12  PerfCounter.print display the counters exposed by this process  
13  -f  read and execute commands from the file                     
14  -l  list JVM processes on the local machine                     
15  -h  this help                                                   
16