Re: use JNI to start JVM, then trying to change the java -- RPG400-L


On 22/07/2005, at 7:07 PM, Larry Ducie wrote:

How does this work with multi-threading though? I mean, you can havemany threads running within java, but the iSeries creates a BCI jobfor each thread (I believe).

Not true. A Batch-Immediate (BCI) job is the result of using one of thespawn() family of APIs. spawn() is functionally equivalent to thefork() and exec() combination so common in Unix-land. A spawned job isa separate job just like fork() creates a separate process. Spawn canbe used to create the illusion of multi-threading at an applicationlevel but then so can SBMJOB.

fork() tends to result in schizoid programs that behave differentlydepending on whether they run in the parent process or the childprocess. exec() helps avoid that by allowing the forked process to loadand run a different program.

Threads however are not spawned but are started by pthread_create().These are true kernel threads and run in the same job as the job thatcreated them. There is a main thread plus 0 or more child threads eachof which can in turn start additional threads. Threads can only run ina job that is capable of supporting multiple threads which means aBatch (BCH) job that has the ALWMLTTHD attribute set to *YES or a BCIjob that has the SPAWN_SETTHREAD_NP set on. Thus there is a linkbetween threads and batch jobs but threads are not implemented as batchjobs.

One of the few decent books on Posix threads is "Programming with POSIXThreads" by David R. Butenhof.

Does the main thread run within the same job as the JVM (and RPGprogram) but all other threads run as BCI jobs along-side it?

Depends on how the JVM was started. If started in the current job (viaJNI) then it runs in parallel with the main thread. Other threads,whether application threads or JVM threads, run in the same job.

If started via RUNJVA or QSHELL then the JVM is in a separate BCI joband any Java threads, including the garbage collector thread, run inthat other job. Application threads run in the parent job.

Also, what about the garbage collector? Doesn't that run in a separatethread (job) too?


Garbage collection runs as a thread in the same job as the JVM.

Regards,
Simon Coulter.
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