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【源码】Timer和TimerTask源码剖析
Timer是java.util包中的一个工具类，提供了定时器的功能。我们可以构造一个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执行一个特定的任务，甚至你可以让其以特定频率一直执行某个任务，这个任务用TimerTask描述，我们将需要的操作写在TimerTask类的run方法中即可。
注释写的很明白，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，执行之，如果没有任务可执行，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，执行任务。
那么TimerThread何时被启动的呢？猜猜也能知道，肯定是Timer被创建时执行的：
当我们主线程执行完毕后，Timer线程可能仍然处于阻塞或者其他状态，有时这不是我们希望看到的，Timer类有这样一个构造器，可以让任务执行线程以守护线程的方式运行，这样当主线程执行完毕后，守护线程也会停止。
以上就是Timer类的源码分析过程，最后贴上一张图，帮助理解：
本着“知其然，知其所以然”的心态，我决定研究下这个类的源码。
打开Timer类的源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);//执行线程TaskQueue是一个优先级队列，存放了我们将要执行的TimerTask对象，TimerTask对象是通过Timer类的一系列schedule方法加入队列的，TimerThread负责不断取出TaskQueue中的任务，然后执行之，也就是说，所有的任务都是是在子线程中执行的。TaskQueue队列是以其下次执行时间的先后排序的，TimerThread每次取出的都是需要最先执行的TimerTask。（跟android中的Handler机制很类似~）
优先级队列跟普通队列的最大区别就是，优先级队列每次出队的都是优先级最高的元素，并不是按先进先出的方式。这里优先级队列的实现使用的是堆结构（当然，你也可以使用普通链表，但是每次出队得花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任务添加到此队列中
// 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开始。
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开始，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，每次添加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需要调用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开始。
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();//执行一个死循环，不断从队列中取出任务并执行，没有任务时会阻塞
} 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)) {//任务是否已经执行过了
if (task.period == 0) { // Non-repeating, remove
queue.removeMin();//已经执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);//没到执行时间久等待
}
if (taskFired) // Task fired; run it, holding no locks
task.run();//执行该任务
} catch(InterruptedException e) {
}
}
}
}注释写的很明白，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，执行之，如果没有任务可执行，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，执行任务。
private void sched(TimerTask task, long time, long period) {//所有的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);//加入任务队列
if (queue.getMin() == task)
queue.notify();//唤醒任务执行线程
}
}
那么TimerThread何时被启动的呢？猜猜也能知道，肯定是Timer被创建时执行的：
public Timer(String name) {
thread.setName(name);
thread.start();//启动线程
}当我们主线程执行完毕后，Timer线程可能仍然处于阻塞或者其他状态，有时这不是我们希望看到的，Timer类有这样一个构造器，可以让任务执行线程以守护线程的方式运行，这样当主线程执行完毕后，守护线程也会停止。
public Timer(boolean isDaemon) {
this("Timer-" + serialNumber(), isDaemon);
}以上就是Timer类的源码分析过程，最后贴上一张图，帮助理解：
【源码】Timer和TimerTask源码剖析
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