org.jcsp.lang
Class CSTimer

java.lang.Object
  org.jcsp.lang.Guard
      org.jcsp.lang.CSTimer

public class CSTimer
extends Guard

This is a Guard for setting timeouts in an Alternative.

Description

CSTimer is a Guard for setting timeouts in an Alternative. It also provides the current system time and can set straight (i.e. committed) timeouts. The timeouts are in terms of absolute time values - not relative delays.

Note: for those familiar with the occam multiprocessing language, CSTimer gives the semantics of the TIMER type (including its use as a guard in an ALT construct).

Warning: a CSTimer records the timeout value for use by an Alternative. Therefore, different CSTimers must be used by different processes - the same CSTimer must not be shared.

Implementation note: all CSTimers currently use the same System.currentTimeMillis time.

Examples

The use of a CSTimer for setting timeouts on channel input is documented in the Alternative class (see the examples A Fair Multiplexor with a Timeout and A Simple Traffic Flow Regulator).

Here, we just show its use for setting committed timeouts. Regular generates a regular stream of output on its out channel. The rate of output is determined by its interval parameter. Recall that timeouts implemented by CSTimer are in terms of absolute time values. Notice that the sequence of output times maintains an arithmetic progression. Any delays in completing each cycle (e.g. caused by the process scheduler or the lateness of the process synchronising with us to accept our data) will be compensated for automatically - the output sequence always returns to its planned schedule whenever it can.

 import org.jcsp.lang.*;
  
 public class Regular implements CSProcess {
  
   final private ChannelOutput out;
   final private Integer N;
   final private long interval;
  
   public Regular (final ChannelOutput out, final int n, final long interval) {
     this.out = out;
     this.N = new Integer (n);
     this.interval = interval;
   }
  
   public void run () {
  
     final CSTimer tim = new CSTimer ();
     long timeout = tim.read ();       // read the (absolute) time once only
  
     while (true) {
       out.write (N);
       timeout += interval;            // set the next (absolute) timeout
       tim.after (timeout);            // wait until that (absolute) timeout
     }
   }
  
 }
 

For convenience, a sleep method that blocks for a specified time period (in milliseconds) is also provided. This has the same semantics as java.lang.Thread.sleep. [Note: programming a regular sequence of events is a little easier using after (as in the above) rather than sleep.]

Author:

P.D. Austin, P.H. Welch

See Also:

Alternative, Guard

Constructor Summary

CSTimer()

Method Summary

void	after(long msecs) Puts the process to sleep until an absolute time is reached.
long	getAlarm() Returns the alarm value that has been set by the previous call to setAlarm(long).
long	read() Returns the current system time in msecs.
void	set(long msecs) Deprecated. Use setAlarm(long) - this name caused confusion with the idea of setting the current time (a concept that is not supported).
void	setAlarm(long msecs) Sets the absolute timeout value that will trigger an Alternative select operation (when this CSTimer is one of the guards with which that Alternative was constructed).
void	sleep(long msecs) Puts the process to sleep for a specified time (milliseconds).

Methods inherited from class org.jcsp.lang.Guard

schedule

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

CSTimer

public CSTimer()

Method Detail

setAlarm

public void setAlarm(long msecs)

Sets the absolute timeout value that will trigger an Alternative select operation (when this CSTimer is one of the guards with which that Alternative was constructed).

Parameters:

msecs - the absolute timeout value.

getAlarm

public long getAlarm()

Returns the alarm value that has been set by the previous call to setAlarm(long).

set

public void set(long msecs)

Deprecated. Use setAlarm(long) - this name caused confusion with the idea of setting the current time (a concept that is not supported).

Sets the absolute timeout value that will trigger an Alternative select operation (when this CSTimer is one of the guards with which that Alternative was constructed).

Parameters:

msecs - the absolute timeout value.

read

public long read()

Returns the current system time in msecs.

Returns:

the current system time in msecs

after

public void after(long msecs)

Puts the process to sleep until an absolute time is reached.

Parameters:

msecs - the absolute time awaited. Note: if this time has already been reached, this returns straight away.

sleep

public void sleep(long msecs)

Puts the process to sleep for a specified time (milliseconds).

Parameters:

msecs - the length of the sleep period. Note: if this is negative, this returns straight away.