Timer 源码解读 (2) TaskQueue 最小堆实现的优先队列

Timer中的定义：

private final TaskQueue queue = new TaskQueue();

定义了一个队列：

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];

Taskqueue是对一个队列的封装，这个队列实现的一个数据结构 a balanced binary heap 平衡二叉堆 并且根据TimerTask中的nextExecutionTime进行排序，最小值在堆顶

添加元素并排序：  

  void add(TimerTask task) {        // Grow backing store if necessary        if (size + 1 == queue.length)            queue = Arrays.copyOf(queue, 2*queue.length);        queue[++size] = task;        fixUp(size);    }

获得最小元素并排序：

    void removeMin() {        queue[1] = queue[size];        queue[size--] = null;  // Drop extra reference to prevent memory leak        fixDown(1);    }

整体重排序：

    void heapify() {        for (int i = size/2; i >= 1; i--)            fixDown(i);    }

fixUp和fixDown是堆排序方法：当插入元素，移除元素的时候都需要重新排序，以使得最小元素在堆顶

    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;        }    }

    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;        }    }