【源代码】Timer和TimerTask源代码剖析

Timer是java.util包中的一个工具类，提供了定时器的功能。

我们能够构造一个Timer对象，然后调用其schedule方法在某个特定的时间或者若干延时之后去运行一个特定的任务。甚至你能够让其以特定频率一直运行某个任务，这个任务用TimerTask描写叙述，我们将须要的操作写在TimerTask类的run方法中就可以。

本着“知其然。知其所以然”的心态，我决定研究下这个类的源代码。

打开Timer类的源代码我发现了这样两个成员变量：

 /**
     * 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.
     */
    private final TaskQueue queue = new TaskQueue();//任务队列
    /**
     * The timer thread.
     */
    private final TimerThread thread = new TimerThread(queue);//运行线程

TaskQueue是一个优先级队列。存放了我们将要运行的TimerTask对象。TimerTask对象是通过Timer类的一系列schedule方法增加队列的，TimerThread负责不断取出TaskQueue中的任务，然后运行之，也就是说。全部的任务都是是在子线程中运行的。TaskQueue队列是以其下次运行时间的先后排序的，TimerThread每次取出的都是须要最先运行的TimerTask。

（跟android中的Handler机制非常类似~）

优先级队列跟普通队列的最大差别就是，优先级队列每次出队的都是优先级最高的元素，并非按先进先出的方式。这里优先级队列的实现使用的是堆结构（当然，你也能够使用普通链表，可是每次出队得花O(n)的时间遍历链表找到优先级最大的元素,不划算）。插入及更新操作都能维持在O(logn)：

class TaskQueue {
    /**
     * Priority queue represented as a balanced binary heap: the two children
     * of queue[n] are queue[2*n] and queue[2*n+1].  The priority queue is
     * ordered on the nextExecutionTime field: The TimerTask with the lowest
     * nextExecutionTime is in queue[1] (assuming the queue is nonempty).  For
     * each node n in the heap, and each descendant of n, d,
     * n.nextExecutionTime <= d.nextExecutionTime.
     */
    private TimerTask[] queue = new TimerTask[128];//使用数组存储堆元素,最大值128
    /**
     * The number of tasks in the priority queue.  (The tasks are stored in
     * queue[1] up to queue[size]).
     */
    private int size = 0;//任务数
    /**
     * Returns the number of tasks currently on the queue.
     */
    int size() {
        return size;
    }
    /**
     * Adds a new task to the priority queue.
     */
    void add(TimerTask task) {//将TimerTask任务加入到此队列中
        // Grow backing store if necessary
        if (size + 1 == queue.length)
            queue = Arrays.copyOf(queue, 2*queue.length);
        queue[++size] = task;
        fixUp(size);//调整堆结构---->所谓的上滤
    }
    /**
     * Return the "head task" of the priority queue.  (The head task is an
     * task with the lowest nextExecutionTime.)
     */
    TimerTask getMin() {//优先级最高的元素始终在第一个位置
        return queue[1];
    }
    /**
     * Return the ith task in the priority queue, where i ranges from 1 (the
     * head task, which is returned by getMin) to the number of tasks on the
     * queue, inclusive.
     */
    TimerTask get(int i) {
        return queue[i];
    }
    /**
     * Remove the head task from the priority queue.
     */
    void removeMin() {
        queue[1] = queue[size];
        queue[size--] = null;  // Drop extra reference to prevent memory leak
        fixDown(1);//调整堆结构----->所谓的下滤
    }
    /**
     * Removes the ith element from queue without regard for maintaining
     * the heap invariant.  Recall that queue is one-based, so
     * 1 <= i <= size.
     */
    void quickRemove(int i) {
        assert i <= size;
        queue[i] = queue[size];
        queue[size--] = null;  // Drop extra ref to prevent memory leak
    }
    /**
     * Sets the nextExecutionTime associated with the head task to the
     * specified value, and adjusts priority queue accordingly.
     */
    void rescheduleMin(long newTime) {
        queue[1].nextExecutionTime = newTime;
        fixDown(1);
    }
    /**
     * Returns true if the priority queue contains no elements.
     */
    boolean isEmpty() {
        return size==0;
    }
    /**
     * Removes all elements from the priority queue.
     */
    void clear() {
        // Null out task references to prevent memory leak
        for (int i=1; i<=size; i++)
            queue[i] = null;
        size = 0;
    }
    /**
     * Establishes the heap invariant (described above) assuming the heap
     * satisfies the invariant except possibly for the leaf-node indexed by k
     * (which may have a nextExecutionTime less than its parent's).
     *
     * This method functions by "promoting" queue[k] up the hierarchy
     * (by swapping it with its parent) repeatedly until queue[k]'s
     * nextExecutionTime is greater than or equal to that of its parent.
     */
    private void fixUp(int k) {
        while (k > 1) {
            int j = k >> 1;
            if (queue[j].nextExecutionTime <= queue[k].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }
    /**
     * Establishes the heap invariant (described above) in the subtree
     * rooted at k, which is assumed to satisfy the heap invariant except
     * possibly for node k itself (which may have a nextExecutionTime greater
     * than its children's).
     *
     * This method functions by "demoting" queue[k] down the hierarchy
     * (by swapping it with its smaller child) repeatedly until queue[k]'s
     * nextExecutionTime is less than or equal to those of its children.
     */
    private void fixDown(int k) {
        int j;
        while ((j = k << 1) <= size && j > 0) {
            if (j < size &&
                queue[j].nextExecutionTime > queue[j+1].nextExecutionTime)
                j++; // j indexes smallest kid
            if (queue[k].nextExecutionTime <= queue[j].nextExecutionTime)
                break;
            TimerTask tmp = queue[j];  queue[j] = queue[k]; queue[k] = tmp;
            k = j;
        }
    }
    /**
     * Establishes the heap invariant (described above) in the entire tree,
     * assuming nothing about the order of the elements prior to the call.
     */
    void heapify() {//建堆操作，从第一个非叶子结点開始。
        for (int i = size/2; i >= 1; i--)
            fixDown(i);
    }
}

TaskQueue内部是一个TimerTask数组。数组元素从1開始，这个数组就是所谓的堆，还是个小顶堆，堆顶元素

始终为第一个元素，每次加入TimerTask都会调用fixup上滤操作，维持堆的特性，每次删除堆顶元素后须要调用fixdown下滤操作。维持堆的特性。heapify是一个建堆函数（类似堆排序中的建堆操作），从第一个非叶子结点開始。

了解TaskQueue后，再看TimerThread类：

class TimerThread extends Thread {
    /**
     * This flag is set to false by the reaper to inform us that there
     * are no more live references to our Timer object.  Once this flag
     * is true and there are no more tasks in our queue, there is no
     * work left for us to do, so we terminate gracefully.  Note that
     * this field is protected by queue's monitor!
     */
    boolean newTasksMayBeScheduled = true;
    /**
     * Our Timer's queue.  We store this reference in preference to
     * a reference to the Timer so the reference graph remains acyclic.
     * Otherwise, the Timer would never be garbage-collected and this
     * thread would never go away.
     */
    private TaskQueue queue;//持有任务队列的引用
    TimerThread(TaskQueue queue) {
        this.queue = queue;
    }
    public void run() {
        try {
            mainLoop();//运行一个死循环，不断从队列中取出任务并运行，没有任务时会堵塞
        } finally {
            // Someone killed this Thread, behave as if Timer cancelled
            synchronized(queue) {
                newTasksMayBeScheduled = false;
                queue.clear();  // Eliminate obsolete references
            }
        }
    }
    /**
     * The main timer loop.  (See class comment.)
     */
    private void mainLoop() {
        while (true) {//死循环
            try {
                TimerTask task;
                boolean taskFired;
                synchronized(queue) {//线程安全
                    // Wait for queue to become non-empty
                    while (queue.isEmpty() && newTasksMayBeScheduled)//没有任务时
                        queue.wait();//等待
                    if (queue.isEmpty())
                        break; // Queue is empty and will forever remain; die
                    // Queue nonempty; look at first evt and do the right thing
                    long currentTime, executionTime;
                    task = queue.getMin();//取出优先级最高的任务
                    synchronized(task.lock) {
                        if (task.state == TimerTask.CANCELLED) {//任务被取消
                            queue.removeMin();//干掉这个任务
                            continue;  // No action required, poll queue again
                        }
                        currentTime = System.currentTimeMillis();
                        executionTime = task.nextExecutionTime;
                        if (taskFired = (executionTime<=currentTime)) {//任务是否已经运行过了
                            if (task.period == 0) { // Non-repeating, remove
                                queue.removeMin();//已经运行过的任务会从队列中移除
                                task.state = TimerTask.EXECUTED;
                            } else { // Repeating task, reschedule
                                queue.rescheduleMin(
                                  task.period<0 ?
 
currentTime   - task.period
                                                : executionTime + task.period);
                            }
                        }
                    }
                    if (!taskFired) // Task hasn't yet fired; wait
                        queue.wait(executionTime - currentTime);//没到运行时间久等待
                }
                if (taskFired)  // Task fired; run it, holding no locks
                    task.run();//运行该任务
            } catch(InterruptedException e) {
            }
        }
    }
}

凝视写的非常明确。TimerThread会在run方法中调用mainloop方法。这是一个死循环，不断从任务队列中取出任务。运行之，假设没有任务可运行，将会wait，等待队列非空，而Timer类的schedule方法会调用notify唤醒该线程。运行任务。

private void sched(TimerTask task, long time, long period) {//全部的schedule方法都会调用此方法
        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) {
                if (task.state != TimerTask.VIRGIN)
                    throw new IllegalStateException(
                        "Task already scheduled or cancelled");
                task.nextExecutionTime = time;
                task.period = period;
                task.state = TimerTask.SCHEDULED;
            }
            queue.add(task);//增加任务队列
            if (queue.getMin() == task)
                queue.notify();//唤醒任务运行线程
        }
    }

那么TimerThread何时被启动的呢？猜猜也能知道，肯定是Timer被创建时运行的：

public Timer(String name) {
        thread.setName(name);
        thread.start();//启动线程
    }

当我们主线程运行完成后。Timer线程可能仍然处于堵塞或者其它状态，有时这不是我们希望看到的，Timer类有这样一个构造器，能够让任务运行线程以守护线程的方式运行。这样当主线程运行完成后。守护线程也会停止。

 public Timer(boolean isDaemon) {
        this("Timer-" + serialNumber(), isDaemon);
    }

以上就是Timer类的源代码分析过程。最后贴上一张图。帮助理解：