Timer.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * under the terms of the GNU General Public License version 2 only, as
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 * particular file as subject to the "Classpath" exception as provided
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * accompanied this code).
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 * 2 along with this work; if not, write to the Free Software Foundation,
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 *
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package java.util;

import java.util.concurrent.atomic.AtomicInteger;

/**
 * A facility for threads to schedule tasks for future execution in a
 * background thread.  Tasks may be scheduled for one-time execution, or for
 * repeated execution at regular intervals.
 *
 * <p>Corresponding to each {@code Timer} object is a single background
 * thread that is used to execute all of the timer's tasks, sequentially.
 * Timer tasks should complete quickly.  If a timer task takes excessive time
 * to complete, it "hogs" the timer's task execution thread.  This can, in
 * turn, delay the execution of subsequent tasks, which may "bunch up" and
 * execute in rapid succession when (and if) the offending task finally
 * completes.
 *
 * <p>After the last live reference to a {@code Timer} object goes away
 * <i>and</i> all outstanding tasks have completed execution, the timer's task
 * execution thread terminates gracefully (and becomes subject to garbage
 * collection).  However, this can take arbitrarily long to occur.  By
 * default, the task execution thread does not run as a <i>daemon thread</i>,
 * so it is capable of keeping an application from terminating.  If a caller
 * wants to terminate a timer's task execution thread rapidly, the caller
 * should invoke the timer's {@code cancel} method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its {@code stop} method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * {@code IllegalStateException}, as if the timer's {@code cancel}
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * {@code Timer} object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using the {@code Object.wait(long)} method.
 *
 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
 * one of the concurrency utilities therein is the {@link
 * java.util.concurrent.ScheduledThreadPoolExecutor
 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
 * executing tasks at a given rate or delay.  It is effectively a more
 * versatile replacement for the {@code Timer}/{@code TimerTask}
 * combination, as it allows multiple service threads, accepts various
 * time units, and doesn't require subclassing {@code TimerTask} (just
 * implement {@code Runnable}).  Configuring {@code
 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
 * {@code Timer}.
 *
 * <p>Implementation note: This class scales to large numbers of concurrently
 * scheduled tasks (thousands should present no problem).  Internally,
 * it uses a binary heap to represent its task queue, so the cost to schedule
 * a task is O(log n), where n is the number of concurrently scheduled tasks.
 *
 * <p>Implementation note: All constructors start a timer thread.
 *
 * @author Josh Bloch
 * @see TimerTask
 * @see Object#wait(long)
 * @since 1.3
 */
// 定时器
public class Timer {
    /**
     * This ID is used to generate thread names.
     */
    private static final AtomicInteger nextSerialNumber = new AtomicInteger(0);
    
    /**
     * The timer task queue.  This data structure is shared with the timer
     * thread.  The timer produces tasks, via its various schedule calls,
     * and the timer thread consumes, executing timer tasks as appropriate,
     * and removing them from the queue when they're obsolete.
     */
    // 定时器任务队列
    private final TaskQueue queue = new TaskQueue();
    
    /**
     * The timer thread.
     */
    // 定时器线程
    private final TimerThread thread = new TimerThread(queue);
    
    /**
     * This object causes the timer's task execution thread to exit gracefully
     * when there are no live references to the Timer object and no tasks in the timer queue.
     * It is used in preference to a finalizer on Timer as such a finalizer would be susceptible
     * to a subclass's finalizer forgetting to call it.
     */
    private final Object threadReaper = new Object() {
        @SuppressWarnings("deprecation")
        protected void finalize() throws Throwable {
            synchronized(queue) {
                // 取消定时器
                thread.newTasksMayBeScheduled = false;
                // 唤醒定时器线程
                queue.notify(); // In case queue is empty.
            }
        }
    };
    
    
    
    /*▼ 构造方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Creates a new timer.  The associated thread does <i>not</i>
     * {@linkplain Thread#setDaemon run as a daemon}.
     */
    public Timer() {
        this("Timer-" + serialNumber());
    }
    
    /**
     * Creates a new timer whose associated thread has the specified name.
     * The associated thread does <i>not</i>
     * {@linkplain Thread#setDaemon run as a daemon}.
     *
     * @param name the name of the associated thread
     *
     * @throws NullPointerException if {@code name} is null
     * @since 1.5
     */
    public Timer(String name) {
        thread.setName(name);
        thread.start();
    }
    
    /**
     * Creates a new timer whose associated thread may be specified to
     * {@linkplain Thread#setDaemon run as a daemon}.
     * A daemon thread is called for if the timer will be used to
     * schedule repeating "maintenance activities", which must be
     * performed as long as the application is running, but should not
     * prolong the lifetime of the application.
     *
     * @param isDaemon true if the associated thread should run as a daemon.
     */
    public Timer(boolean isDaemon) {
        this("Timer-" + serialNumber(), isDaemon);
    }
    
    /**
     * Creates a new timer whose associated thread has the specified name,
     * and may be specified to
     * {@linkplain Thread#setDaemon run as a daemon}.
     *
     * @param name     the name of the associated thread
     * @param isDaemon true if the associated thread should run as a daemon
     *
     * @throws NullPointerException if {@code name} is null
     * @since 1.5
     */
    public Timer(String name, boolean isDaemon) {
        thread.setName(name);
        thread.setDaemon(isDaemon);
        thread.start();
    }
    
    /*▲ 构造方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /*▼ 执行任务 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Schedules the specified task for execution after the specified delay.
     *
     * @param task  task to be scheduled.
     * @param delay delay in milliseconds before task is to be executed.
     *
     * @throws IllegalArgumentException if {@code delay} is negative, or
     *                                  {@code delay + System.currentTimeMillis()} is negative.
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} is null
     */
    // 安排一次性任务在delay延时后开始执行
    public void schedule(TimerTask task, long delay) {
        if(delay<0) {
            throw new IllegalArgumentException("Negative delay.");
        }
        
        sched(task, System.currentTimeMillis() + delay, 0);
    }
    
    /**
     * Schedules the specified task for execution at the specified time.  If
     * the time is in the past, the task is scheduled for immediate execution.
     *
     * @param task task to be scheduled.
     * @param time time at which task is to be executed.
     *
     * @throws IllegalArgumentException if {@code time.getTime()} is negative.
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} or {@code time} is null
     */
    // 安排一次性任务在time时间时开始执行
    public void schedule(TimerTask task, Date time) {
        sched(task, time.getTime(), 0);
    }
    
    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying {@code Object.wait(long)} is accurate).
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     *
     * @throws IllegalArgumentException if {@code delay < 0}, or
     *                                  {@code delay + System.currentTimeMillis() < 0}, or
     *                                  {@code period <= 0}
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} is null
     */
    // 安排【固定延时】任务在delay延时后开始执行
    public void schedule(TimerTask task, long delay, long period) {
        if(delay<0) {
            throw new IllegalArgumentException("Negative delay.");
        }
        
        if(period<=0) {
            throw new IllegalArgumentException("Non-positive period.");
        }
        
        sched(task, System.currentTimeMillis() + delay, -period);
    }
    
    /**
     * Schedules the specified task for repeated <i>fixed-delay execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-delay execution, each execution is scheduled relative to
     * the actual execution time of the previous execution.  If an execution
     * is delayed for any reason (such as garbage collection or other
     * background activity), subsequent executions will be delayed as well.
     * In the long run, the frequency of execution will generally be slightly
     * lower than the reciprocal of the specified period (assuming the system
     * clock underlying {@code Object.wait(long)} is accurate).  As a
     * consequence of the above, if the scheduled first time is in the past,
     * it is scheduled for immediate execution.
     *
     * <p>Fixed-delay execution is appropriate for recurring activities
     * that require "smoothness."  In other words, it is appropriate for
     * activities where it is more important to keep the frequency accurate
     * in the short run than in the long run.  This includes most animation
     * tasks, such as blinking a cursor at regular intervals.  It also includes
     * tasks wherein regular activity is performed in response to human
     * input, such as automatically repeating a character as long as a key
     * is held down.
     *
     * @param task      task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period    time in milliseconds between successive task executions.
     *
     * @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or
     *                                  {@code period <= 0}
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} or {@code firstTime} is null
     */
    // 安排【固定延时】任务在firstTime时间时开始执行
    public void schedule(TimerTask task, Date firstTime, long period) {
        if(period<=0) {
            throw new IllegalArgumentException("Non-positive period.");
        }
        
        sched(task, firstTime.getTime(), -period);
    }
    
    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning after the specified delay.  Subsequent executions take place
     * at approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying {@code Object.wait(long)} is accurate).
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task   task to be scheduled.
     * @param delay  delay in milliseconds before task is to be executed.
     * @param period time in milliseconds between successive task executions.
     *
     * @throws IllegalArgumentException if {@code delay < 0}, or
     *                                  {@code delay + System.currentTimeMillis() < 0}, or
     *                                  {@code period <= 0}
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} is null
     */
    // 安排【固定周期】任务在delay延时后开始执行
    public void scheduleAtFixedRate(TimerTask task, long delay, long period) {
        if(delay<0) {
            throw new IllegalArgumentException("Negative delay.");
        }
        
        if(period<=0) {
            throw new IllegalArgumentException("Non-positive period.");
        }
        
        sched(task, System.currentTimeMillis() + delay, period);
    }
    
    /**
     * Schedules the specified task for repeated <i>fixed-rate execution</i>,
     * beginning at the specified time. Subsequent executions take place at
     * approximately regular intervals, separated by the specified period.
     *
     * <p>In fixed-rate execution, each execution is scheduled relative to the
     * scheduled execution time of the initial execution.  If an execution is
     * delayed for any reason (such as garbage collection or other background
     * activity), two or more executions will occur in rapid succession to
     * "catch up."  In the long run, the frequency of execution will be
     * exactly the reciprocal of the specified period (assuming the system
     * clock underlying {@code Object.wait(long)} is accurate).  As a
     * consequence of the above, if the scheduled first time is in the past,
     * then any "missed" executions will be scheduled for immediate "catch up"
     * execution.
     *
     * <p>Fixed-rate execution is appropriate for recurring activities that
     * are sensitive to <i>absolute</i> time, such as ringing a chime every
     * hour on the hour, or running scheduled maintenance every day at a
     * particular time.  It is also appropriate for recurring activities
     * where the total time to perform a fixed number of executions is
     * important, such as a countdown timer that ticks once every second for
     * ten seconds.  Finally, fixed-rate execution is appropriate for
     * scheduling multiple repeating timer tasks that must remain synchronized
     * with respect to one another.
     *
     * @param task      task to be scheduled.
     * @param firstTime First time at which task is to be executed.
     * @param period    time in milliseconds between successive task executions.
     *
     * @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or {@code period <= 0}
     * @throws IllegalStateException    if task was already scheduled or cancelled,
     *                                  timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} or {@code firstTime} is null
     */
    // 安排【固定周期】任务在firstTime时间时开始执行
    public void scheduleAtFixedRate(TimerTask task, Date firstTime, long period) {
        if(period<=0) {
            throw new IllegalArgumentException("Non-positive period.");
        }
        
        sched(task, firstTime.getTime(), period);
    }
    
    /**
     * Schedule the specified timer task for execution at the specified
     * time with the specified period, in milliseconds.  If period is
     * positive, the task is scheduled for repeated execution; if period is
     * zero, the task is scheduled for one-time execution. Time is specified
     * in Date.getTime() format.  This method checks timer state, task state,
     * and initial execution time, but not period.
     *
     * @throws IllegalArgumentException if {@code time} is negative.
     * @throws IllegalStateException    if task was already scheduled or
     *                                  cancelled, timer was cancelled, or timer thread terminated.
     * @throws NullPointerException     if {@code task} is null
     */
    /*
     * 安排任务以待执行
     *
     * time  ：任务触发时间
     *
     * period：任务的重复模式：
     *         零：非重复任务：只执行一次
     *         正数：重复性任务：固定周期，从任务初次被触发开始，以后每隔period时间就被触发一次
     *         负数：重复性任务：固定延时，任务下次的开始时间=任务上次结束时间+(-period)
     */
    private void sched(TimerTask task, long time, long period) {
        if(time<0) {
            throw new IllegalArgumentException("Illegal execution time.");
        }
        
        // Constrain value of period sufficiently to prevent numeric
        // overflow while still being effectively infinitely large.
        if(Math.abs(period)>(Long.MAX_VALUE >> 1)) {
            period >>= 1;
        }
        
        synchronized(queue) {
            if(!thread.newTasksMayBeScheduled) {
                throw new IllegalStateException("Timer already cancelled.");
            }
            
            synchronized(task.lock) {
                // 如果任务已经开始执行或已被取消，抛出异常
                if(task.state != TimerTask.VIRGIN) {
                    throw new IllegalStateException("Task already scheduled or cancelled");
                }
                
                // 记录任务触发时间
                task.nextExecutionTime = time;
                
                // 任务的重复模式
                task.period = period;
                
                // 任务进入【排队】状态
                task.state = TimerTask.SCHEDULED;
            }
            
            // 将任务加入任务队列
            queue.add(task);
    
            // 如果当前任务排在队头
            if(queue.getMin() == task) {
                // 唤醒定时器线程
                queue.notify();
            }
        }
    }
    /*▲ 执行任务 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取消定时器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Terminates this timer, discarding any currently scheduled tasks.
     * Does not interfere with a currently executing task (if it exists).
     * Once a timer has been terminated, its execution thread terminates
     * gracefully, and no more tasks may be scheduled on it.
     *
     * <p>Note that calling this method from within the run method of a
     * timer task that was invoked by this timer absolutely guarantees that
     * the ongoing task execution is the last task execution that will ever
     * be performed by this timer.
     *
     * <p>This method may be called repeatedly; the second and subsequent
     * calls have no effect.
     */
    // 取消定时器
    public void cancel() {
        synchronized(queue) {
            // 不再执行任务
            thread.newTasksMayBeScheduled = false;
            // 清空任务队列
            queue.clear();
            // 唤醒定时器线程
            queue.notify();  // In case queue was already empty.
        }
    }
    
    /**
     * Removes all cancelled tasks from this timer's task queue.  <i>Calling
     * this method has no effect on the behavior of the timer</i>, but
     * eliminates the references to the cancelled tasks from the queue.
     * If there are no external references to these tasks, they become
     * eligible for garbage collection.
     *
     * <p>Most programs will have no need to call this method.
     * It is designed for use by the rare application that cancels a large
     * number of tasks.  Calling this method trades time for space: the
     * runtime of the method may be proportional to n + c log n, where n
     * is the number of tasks in the queue and c is the number of cancelled
     * tasks.
     *
     * <p>Note that it is permissible to call this method from within
     * a task scheduled on this timer.
     *
     * @return the number of tasks removed from the queue.
     *
     * @since 1.5
     */
    // 清除所有进入【取消】状态的任务
    public int purge() {
        int result = 0;
        
        synchronized(queue) {
            // 遍历所有任务
            for(int i = queue.size(); i>0; i--) {
                // 找出已取消的任务
                if(queue.get(i).state == TimerTask.CANCELLED) {
                    // 快速移除索引i处的任务（没有重建小顶堆）
                    queue.quickRemove(i);
                    result++;
                }
            }
            
            // 将重建小顶堆的时间推迟到这里
            if(result != 0) {
                // 重建小顶堆
                queue.heapify();
            }
        }
        
        return result;
    }
    
    /*▲ 取消定时器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    // 返回一个定时器编号
    private static int serialNumber() {
        return nextSerialNumber.getAndIncrement();
    }
    
}