Introduction to Java Shared classes on z/OS

Java shared classes provides the facility for Java to store classes, and JITted classes in shared memory which persists across the restart of a Java application. You can harden this shared memory to disk, and reload it after an IPL.

If shared classes are being used, then as the JVM starts it looks for the classes in the shared cache, and if not found, loads them from the file system, processes them and stores them in the shared classes memory. With this scheme, the second start of the JVM gains from the shared classes.

See the introduction documentation and some practical documentation.

Background

To support the use of the shared memory, Java uses standard facilities for shared memory, and for semaphores (to ensure single threading of updates).

I have my shared classes in directory /u/tmp/zowec. When the shared classes are used, two files are created.

• javasharedresources/C290M17F1A64_semaphore_zoweGW_G43L00
• javasharedresources/C290M17F1A64_memory_zoweGW_G43L00

Listing them gives

672 -rw-r--r--   1 ZWESVUSR ZWEADMIN ... C290M17F1A64_semaphore_zoweGW_G43L00  
673 -rw-r--r--   1 ZWESVUSR ZWEADMIN ... C290M17F1A64_memory_zoweGW_G43L00     

The files are owned by userid ZWESVUSR and group ZAWADMIN from my started task userid. The semaphore file has an inode of 672 (0x2a0), and the shared memory file has an inode of 673(0x2a1).

The shared memory and semaphores use InterProcess Communication (ipc) facilities.

The id in hex of the files is used as part of the ipc key.

The command ipcs gave

Shared Memory:                                                
T         ID     KEY        MODE       OWNER    GROUP 
m     204805 0x6102a131 --rw------- ZWESVUSR ZWEADMIN         
        
Semaphores:                                                   
T         ID     KEY        MODE       OWNER    GROUP         
s     200708 0x8102a031 --ra------- ZWESVUSR ZWEADMIN         
        

These resources are owned by userid:ZWESVUSR, and group:ZWEADMIN. The userid and group comes from the userid that created the shared classes cache.

If you need to “clean up” you can delete these using the ipcrm command.