task.java

java高级使用教程 全书一共分六章

// Task - a periodic action
//
// Copyright (C) 1996 by Jef Poskanzer <[e-mail]>.  All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.
//
// Visit the ACME Labs Java page for up-to-date versions of this and other
// fine Java utilities: http://www.acme.com/java/

package Acme;

import java.util.*;

/// A periodic action.
// <P>
// A Task represents a bit of code that you want to run repeatedly at
// equally-timed intervals.  Doing the timing yourself via sleeps is
// either inaccurate, since it depends on how long the task takes to
// run, or else complicated.  Well, this class does the timing accurately
// but hides the complexity from you.
// <P>
// Tasks use the Runnable interface, just like Threads.  This means that
// just like with Threads, there are two different ways you can create
// a Task:
// <UL>
// <LI>
// You can have your class implement the Runnable interface and add
// a run() method:
// <BLOCKQUOTE><CODE><PRE>
// class Anim implements Runnable
//     {
//     public void run()
//         {
//         // do an animation frame
//         }
//     }
// </PRE></CODE></BLOCKQUOTE>
// To start this task you would use something like this:
// <BLOCKQUOTE><CODE><PRE>
// Anim a = new Anim();
// Acme.Task t = new Acme.Task( a, 1000 );
// </PRE></CODE></BLOCKQUOTE>
// Or if, as is likely, your class starts the task in its constructor:
// <BLOCKQUOTE><CODE><PRE>
// Acme.Task t = new Acme.Task( this, 1000 );
// </PRE></CODE></BLOCKQUOTE>
// <LI>
// The other way to create a Task is to make a subclass of Task and override
// the run() method:
// <BLOCKQUOTE><CODE><PRE>
// class AnimTask extends Acme.Task
//     {
//     public void run()
//         {
//         // do an animation frame
//         }
//     }
// </PRE></CODE></BLOCKQUOTE>
// Then you would start it like so:
// <BLOCKQUOTE><CODE><PRE>
// Acme.Task t = new AnimTask( 1000 );
// </PRE></CODE></BLOCKQUOTE>
// </UL>
// <P>
// <A HREF="/resources/classes/Acme/Task.java">Fetch the software.</A><BR>
// <A HREF="/resources/classes/Acme.tar.gz">Fetch the entire Acme package.</A>

public class Task implements Runnable
    {

    /*private*/ protected Runnable runnable;
    /*private*/ protected long msPeriod;
    /*private*/ protected TaskRunner runner;
    /*private*/ protected long nextRun;

    /// Constructor for subclasses that override run().
    public Task( long msPeriod )
	{
	this( null, msPeriod );
	}
    
    /// Constructor with a Runnable specified.
    public Task( Runnable runnable, long msPeriod )
	{
	this.runnable = runnable;
	this.msPeriod = msPeriod;
	runner = new TaskRunner( this );
	TaskTimer.add( this );
	}
    
    /// The body of the Task.
    // You must either override this method by subclassing class Task,
    // or you must create the Task with a Runnable target.
    public void run()
	{
	if ( runnable == null )
	    throw new RuntimeException();	// didn't override
	runnable.run();
	}
    
    /// Stops a Task from running in the future.
    public synchronized void deschedule()
	{
	TaskTimer.deschedule( this );
	}

    }


// TaskRunner - thread for running a task
//
// There's one of these for each Task.  However, we can't put these methods
// into the Task class itself because the run() method has to be a loop here.

class TaskRunner extends Thread
    {

    private Task task;
    /*private*/ protected boolean done;

    public TaskRunner( Task task )
	{
	this.task = task;
	done = false;
	this.start();
	}
    
    public synchronized void run()
	{
	while ( true )
	    {
	    try
		{
		wait();
		}
	    catch ( InterruptedException e ) { }
	    if ( done )
		break;
	    task.run();
	    }
	}

    public synchronized void notifier()
	{
	notify();
	}

    }


// TaskTimer - trigger TaskRunners at the right moment
//
// Only one of these gets created, and it handles the timing of all the
// Tasks.

class TaskTimer extends Thread
    {

    private static boolean done = true;
    private static Vector tasks = null;
    private static TaskTimer timer = null;

    public static synchronized void add( Task task )
	{
	task.nextRun = System.currentTimeMillis() + task.msPeriod;
	if ( tasks == null )
	    tasks = new Vector();
	tasks.addElement( task );
	sortTasks( tasks );
	if ( timer == null )
	    {
	    done = false;
	    timer = new TaskTimer();
	    }
	}
    
    public static synchronized void deschedule( Task task )
	{
	task.runner.done = true;
	task.runner.notifier();
	tasks.removeElement( task );
	if ( tasks.size() == 0 )
	    {
	    done = true;
	    tasks = null;
	    timer.notifier();
	    timer = null;
	    }
	}

    public TaskTimer()
	{
	this.setPriority( Thread.MAX_PRIORITY );
	this.start();
	}
    
    public synchronized void run()
	{
	while ( true )
	    {
	    if ( done )
		break;

	    // The tasks are kept sorted by nextRun, so the next runnable
	    // task is the first one in the list.
	    Task nextTask = (Task) tasks.elementAt( 0 );
	    long nextRun = nextTask.nextRun;

	    // Sleep until it's time to run it.
	    try
		{
		wait( nextRun - System.currentTimeMillis() );
		}
	    catch ( InterruptedException e ) { }

	    if ( done )
		break;

	    // Run all tasks that should be run.
	    long now = System.currentTimeMillis();
	    for ( int i = 0; i < tasks.size(); ++i )
		{
		Task task = (Task) tasks.elementAt( i );
		if ( task.nextRun > now )
		    break;
		task.runner.notifier();
		task.nextRun += task.msPeriod;
		}
	    // And re-sort, since the nextRuns have changed.
	    sortTasks( tasks );
	    }
	}
    
    /// Stupid bubble sort.  Maybe someday Java will have a Sortable interface.
    private static void sortTasks( Vector tasks )
	{
	for ( int i = 0; i < tasks.size() - i; ++i )
	    {
	    Task taski = (Task) tasks.elementAt( i );
	    for ( int j = i + 1; j < tasks.size(); ++j )
		{
		Task taskj = (Task) tasks.elementAt( j );
		if ( taskj.nextRun < taski.nextRun )
		    {
		    tasks.setElementAt( taskj, i );
		    tasks.setElementAt( taski, j );
		    taski = taskj;
		    }
		}
	    }
	}

    public synchronized void notifier()
	{
	notify();
	}

    }