com.ccg.util
Class Scheduler

java.lang.Object
  java.lang.Thread
      com.ccg.util.Scheduler

All Implemented Interfaces:

Runnable

public class Scheduler
extends Thread

Class to do something periodically.

Java supports threads very nicely. However, there are times when threads are overkill. For example, if you find yourself wanting to check something every few seconds, you would typically start a thread that would run in the background. It would do its check, then sleep for a period of time and continue on. For each one of these types of checks, you typically create a separate running thread, leaving your system with a lot of threads which are typically in a sleep state.

This class attempts to reduce these types of situations to a single thread. It works in the following manner:

• You create a Schedulable object (typically derived from the ScheduleEntry class.
• You provide a run() method in your object which does not run forever (just does its thing one time and exits).
• For each Schedulable object created, you add them to a Scheduler, or add them to the system wide (shared) Scheduler.
• You then start the Scheduler (unless you used the system wide default Scheduler - which is automatically started when first used).
• You then forget about it. The Scheduler class takes care of invoking the run() method of each Schedulable item added.

This results in one thread capable of doing a lot of "periodic" things.

IMPORTANT NOTE: You can really "muck-up" the Scheduler by adding a Schedulable item that does not exit from its run() method.

A sample program SchedTest.java has been provided to demonstrate how one can use this class to cause periodic actions to occur.

Since:

1.0

Version:

$Revision: 1.13 $

Author:

$Author: pkb $

See Also:

addToDefault(com.ccg.util.Schedulable), Schedulable, ScheduleEntry, MemMonitor

Nested Class Summary

Nested classes/interfaces inherited from class java.lang.Thread

Thread.State, Thread.UncaughtExceptionHandler

Field Summary

Fields inherited from class java.lang.Thread

MAX_PRIORITY, MIN_PRIORITY, NORM_PRIORITY

Constructor Summary

Scheduler()
Default constructor for the Scheduler.

Method Summary

void	add(Schedulable s) Add a new Schedulable object to the Scheduler This method takes a new Schedulable item and inserts it into the schedule list such that the item's run method will be invoked after the amount of time specified by its delay time has been reached.
static void	addToDefault(Schedulable s) Add a new Schedulable item to the default system Scheduler.
void	clear() Stops the scheduler thread AND clear all of the scheduled entries.
void	remove(Schedulable s) Remove a Schedulable object from the Scheduler.
static void	removeFromDefault(Schedulable s) Remove a Schedulable item from the default system Scheduler.
void	run() The main loop of the Scheduler object.
void	shutdown() Nicely stops the Scheduler.

Methods inherited from class java.lang.Thread

activeCount, checkAccess, countStackFrames, currentThread, destroy, dumpStack, enumerate, getAllStackTraces, getContextClassLoader, getDefaultUncaughtExceptionHandler, getId, getName, getPriority, getStackTrace, getState, getThreadGroup, getUncaughtExceptionHandler, holdsLock, interrupt, interrupted, isAlive, isDaemon, isInterrupted, join, join, join, resume, setContextClassLoader, setDaemon, setDefaultUncaughtExceptionHandler, setName, setPriority, setUncaughtExceptionHandler, sleep, sleep, start, stop, stop, suspend, toString, yield

Methods inherited from class java.lang.Object

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

Constructor Detail

Scheduler

public Scheduler()

Default constructor for the Scheduler. This default constructor initializes the Scheduler and prepares it such that it can be run in the foreground, started as a background thread, and/or have additional Schedulable items added to it.

Since:

1.0

See Also:

add(com.ccg.util.Schedulable), run()

Method Detail

addToDefault

public static void addToDefault(Schedulable s)

Add a new Schedulable item to the default system Scheduler. This static method does the following:

• Creates a new system wide (shared) Scheduler object (if it hasn't been created yet).
• Adds your specified Schedulable item to the default system Scheduler.
• If the system wide (shared) Scheduler was created by this invocation, it will be started as a background thread.

Since most users of this class will want all of their Schedulable objects to be run by the same Scheduler, this is the easiest way to accomplish it. You simply keep calling this static method with your new Schedulable objects which you want to have run periodically - everything else is automatic.

Parameters:

s - New Schedulable item to insert into the schedule list.

Since:

1.0

See Also:

add(com.ccg.util.Schedulable)

removeFromDefault

public static void removeFromDefault(Schedulable s)

Remove a Schedulable item from the default system Scheduler. This static method is used to remove your schedulable item from the system scheduler.

Parameters:

s - Schedulable item to remove from the schedule list.

Since:

1.0

See Also:

addToDefault(com.ccg.util.Schedulable)

add

public void add(Schedulable s)

Add a new Schedulable object to the Scheduler This method takes a new Schedulable item and inserts it into the schedule list such that the item's run method will be invoked after the amount of time specified by its delay time has been reached.

Note: The Scheduler depends on all of its Schedulable items behaving properly. If a Schedulable's run() method blocks or loops indefinitely, it will prevent the Scheduler from running any of its other Schedulable items.

After each invocation, the Schedulable items delay time will be fetched. If the delay time returns a value greater than 0, then the Schedulable item will be re-inserted into the schedule table according to the delay time. For example, if the Schedulable.getDelayMillis() always returns 5000 for the Schedulable item, then the Schedulable item will be run every 5 seconds.

Parameters:

s - New Schedulable item to insert into the schedule list.

Since:

1.0

See Also:

run(), ScheduleEntry

remove

public void remove(Schedulable s)

Remove a Schedulable object from the Scheduler.

This method allows one to remove a schedulable item from the scheduler.

Parameters:

s - The Schedulable entry which was previously added that you would like to remove.

Since:

1.0

See Also:

add(com.ccg.util.Schedulable)

run

public void run()

The main loop of the Scheduler object. This is the main loop of the Scheduler object. It runs continually and invokes the run() method of each of its contained Schedulable items as necessary. Typically you do not invoke this method directly. Typically you will use the start() method to run the Scheduler as a background thread.

Specified by:

run in interface Runnable

Overrides:

run in class Thread

Since:

1.0

See Also:

add(com.ccg.util.Schedulable)

clear

public void clear()

Stops the scheduler thread AND clear all of the scheduled entries.

Since:

1.0

See Also:

shutdown()

shutdown

public void shutdown()

Nicely stops the Scheduler. This method trys to nicely stop the Scheduler. It sets an internal flag indicating that the Scheduler should stop and then interrupts the Scheduler hoping it will detect the flag and shut down.

Since:

1.0

See Also:

run()