Class ProcessManager
- All Implemented Interfaces:
CSProcess
CSProcess
to be spawned
concurrently with the process doing the spawning.
Shortcut to the Constructor and Method Summaries.
Description
The ProcessManager class enables aCSProcess
to be spawned
concurrently with the process doing the spawning. The class provides
methods to manage the spawned process: start
,
join
and stop
. The spawned process may, of course,
be a Parallel
network of processes to any depth of nesting, in which
case the whole network comes under this management.
Spawning processes is not the normal way of creating a network in JCSP - the
normal method is to use the Parallel
class. However, when we need
to add processes in response to some run-time event, this capability is very
useful.
For completeness, ProcessManager is itself a CSProcess
- run
ning a ProcessManager simply runs the process
it is managing.
Spawning a CSProcess
This example demonstrates that the managed CSProcess is executed concurrently with the spawning process and that it dies when its manager terminates. The managed process is `infinite' and just counts and chatters. The managed process is automatically terminated if the main Java thread terminates (as in the case, eventually, below).
import org.jcsp.lang.*;
public class ProcessManagerExample1 {
public static void main (String[] argv) {
final ProcessManager manager = new ProcessManager (
new CSProcess () {
public void run () {
final CSTimer tim = new CSTimer ();
long timeout = tim.read ();
int count = 0;
while (true) {
System.out.println (count + " :-) managed process running ...");
count++;
timeout += 100;
tim.after (timeout); // every 1/10th of a second ...
}
}
}
);
final CSTimer tim = new CSTimer ();
long timeout = tim.read ();
System.out.println ("\n\n\t\t\t\t\t
*** start the managed process");
manager.start ();
for (int i = 0; i invalid input: '<' 10; i++) {
System.out.println ("\n\n\t\t\t\t\t
*** I'm still executing as well");
timeout += 1000;
tim.after (timeout); // every second ...
}
System.out.println ("\n\n\t\t\t\t\t
*** I'm finishing now!");
}
}
Stopping, Interrupting, Race-Hazards and Poison
Stopping a Java thread releases any locks it (or any sub-process) may be holding, so this reduces the danger of other threads deadlocking through a failure to acquire a needed lock. However, if the stopped process were in the middle of some synchronised transaction, the data update may be incomplete (and, hence, corrupt) depending on the precise moment of the stopping. This is a race-hazard. Further, if some other thread later needed to interact with the stopped thread, it would deadlock.
Instead of stopping a JCSP process, it is much safer to
interrupt
it.
This gives the process the chance to notice the interrupt (through an
exception handler) and tidy up.
If no such handler is provided and the JCSP process attempts any
synchronisation afterwards, the process will bomb out with
a ProcessInterruptedException
.
For historical reasons, a stop()
method is provided below –
but it is implemented as interrupt()
(and deprecated).
If the managed process has gone parallel, managing an interrupt to achieve a clean exit is more tricky. Stopping a network by setting a global volatile flag that each process polls from time to time is not safe. For example, a thread blocked on a monitor wait will remain blocked if the thread that was going to notify it spots the shut-down flag and terminates.
For JCSP processes, there is a general solution to this [`Graceful Termination and Graceful Resetting', P.H.Welch, Proceedings of OUG-10, pp. 310-317, Ed. A.W.P.Bakkers, IOS Press (Amsterdam), ISBN 90 5199 011 1, April, 1989], based on the careful distribution of poison over the network's normal communication channels.
However, JCSP now supports graceful termination of process networks and sub-networks
through a notion of poisoning synchoronisation objects (e.g. channels)
– see Poisonable
.
- Author:
- P.H. Welch, P.D. Austin
- See Also:
-
Field Summary
FieldsModifier and TypeFieldDescriptionstatic final int
The maximum priority value for running a process.static final int
The minimum priority value for running a process.static final int
The normal priority value for running a process. -
Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionint
Public accessor for obtaining theProcessManager
object's process' priority.void
Interrupt the managed process.void
join()
Join the managed process (that is wait for it to terminate).void
run()
Run the managed process (that is start it and wait for it to terminate).void
setPriority
(int priority) Public mutator for setting theProcessManager
object's process' priority.void
start()
Start the managed process (but keep running ourselves).void
start
(int priority) Start the managed process at a specified priority (but keep running ourselves).void
stop()
Deprecated.
-
Field Details
-
PRIORITY_MAX
public static final int PRIORITY_MAXThe maximum priority value for running a process.- See Also:
-
PRIORITY_NORM
public static final int PRIORITY_NORMThe normal priority value for running a process.- See Also:
-
PRIORITY_MIN
public static final int PRIORITY_MINThe minimum priority value for running a process.- See Also:
-
-
Constructor Details
-
ProcessManager
- Parameters:
proc
- theCSProcess
to be executed by this ProcessManager
-
-
Method Details
-
start
public void start()Start the managed process (but keep running ourselves). -
start
public void start(int priority) Start the managed process at a specified priority (but keep running ourselves). The priority of theProcessManager
that this is called upon will remain at the specified priority once the process has terminated. The priority should be specified as anint
betweenPRIORITY_MIN
and
PRIORITY_MAX
.
- Parameters:
priority
- the priority at which to start the process.
-
stop
public void stop()Deprecated.Stop (permanently) the managed process. This method now calls interrupt(), which will not always stop the process. -
interrupt
public void interrupt()Interrupt the managed process. This will usually cause the process to throw aProcessInterruptedException
, which will likely halt the process. -
join
public void join()Join the managed process (that is wait for it to terminate). -
run
public void run()Run the managed process (that is start it and wait for it to terminate). This will adjust the priority of the calling process to the priority of this
ProcessManager
and then return the priority to the previous value once the managed process has terminated.The managed process can be run at the caller's priority simply by directly calling the
CSProcess
object'srun()
method. -
setPriority
public void setPriority(int priority) Public mutator for setting the
ProcessManager
object's process' priority.The priority should be specified as an
int
betweenPRIORITY_MIN
and
PRIORITY_MAX
.
- Parameters:
priority
- the priority to use.
-
getPriority
public int getPriority()Public accessor for obtaining the
ProcessManager
object's process' priority.- Returns:
- the priority at which the
ProcessManager
object's process will be run.
-