1.关于Timer的三个维度
首先是 {@link java.util.Timer}，这个是最外层的类，其中包含了{@link java.util.TaskQueue}，这个是存放{@link java.util.TimerTask}的队列——a priority queue of TimerTasks。
第二层是 {@link java.util.TimerThread}，这个是{@link java.util.Timer}在初始化的时候创建并启动的一个线程，这个线程取任务并且执行。
/**
* 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();
}
2.TimerThread
Thread的子类，在run方法中循环取任务。
public void run() {
try {
mainLoop();
} 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) {
try {
TimerTask task;
boolean taskFired;
synchronized(queue) {
// Wait for queue to become non-empty
while (queue.isEmpty() && newTasksMayBeScheduled)
queue.wait();
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();
synchronized(task.lock) {
if (task.state == TimerTask.CANCELLED) {
queue.removeMin();
continue; // No action required, poll queue again
}
currentTime = System.currentTimeMillis();
executionTime = task.nextExecutionTime;
if (taskFired = (executionTime<=currentTime)) {
if (task.period == 0) { // Non-repeating, remove
queue.removeMin();
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);
}
if (taskFired) // Task fired; run it, holding no locks
task.run();
} catch(InterruptedException e) {
}
}
}
3.task的创建——schedule方法
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);
}
/**
* 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 <tt>time</tt> 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
*/
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) {
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);
if (queue.getMin() == task)
queue.notify();
}
}
1.两重锁，先锁队列queue，再锁task，task中有一个Object对象作为锁
2.设置TimerTask的下次执行时间
{@link java.util.TimerTask#nextExecutionTime} = System.currentTimeMillis()+delay
3.将任务添加到队列中
{@link java.util.Timer#queue}
4.当前任务如果是队列的一个任务，就执行
task == {@link java.util.TaskQueue#getMin}，调用queue的notify方法。如果不是，说明前面还有等待执行的task，只入队列，不用调用notify方法。
5.{@link java.util.TimerThread#mainLoop}
{@link java.util.TimerThread}会在{@link java.util.Timer}
构造的时候启动，进而调用mainLoop方法。
最开始queue是空的，所以queue.await()，当前线程挂起，当Timer中添加TimerTask任务时，
就会调用queue.notify()方法唤醒mainLoop线程。
4.根据优先级对堆进行重排序
1.TimerTask的入堆(queue)操作
delay时间设置的很长，就是为了让任务不执行，看看入队列的比较操作，period这里完全用作标志位，在debug的时候作为标记区分不同的Task，看看排序状况。
这里以3个为例，第一个入队列，index是1，第二个入队列，index是2,2>>1=1，然后拿queue[2]和queue[1]比较下次执行时间，queue[2]比queue[1]早，所以交换顺序。
第三个入队列，index是3,3>>1=1，和第一个比，queue[3]比queue[1]要早，所以交换顺序，所以现在queue[1]最早执行，queue[2]和queue[3]的顺序没有考虑。每次入队列的重排序操作在 {@link java.util.TaskQueue#fixUp} 方法中进行
/**
* 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;
}
}
2.mainLoop执行操作：
每次取queue的第一个task，如果该task还没到执行时间，就等待对应的时间queue.wait(executionTime - currentTime)。
if (!taskFired) // Task hasn't yet fired; wait
queue.wait(executionTime - currentTime);
如果这期间又来了一个优先级更高(执行顺序更靠前)的task，入队列时调用fixUp把当前task排到队列头(优先级更高)，然后notify这个queue打断这个wait，重新去取优先级更高的task。
/**
* Adds a new task to the priority queue.
*/
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);
}
如果到了执行时间(wait结束)，在下次循环的时候，就执行该task。
如果是非重复任务，调用removeMin移除当前任务，在removeMin中fixDown，进行堆重排序。
如果是重复任务的话，还要调用rescheduleMin设置下次执行的时间，在rescheduleMin中调用fixDown，进行堆重排序。
3.{@link java.util.TaskQueue#fixDown}
/**
* 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;
}
}
对剩下的Task进行重新排序，把下次执行时间最小的转移到第一个任务的位置。
测试用例：
public static void test1() {
timer.schedule(new TimerTask() {
@Override public void run() {
System.out.println("Time's up 1!---" + new Date().toString());
// SleepUtil.sleep(30000);
}
}, 2000 * 1000, 11 * 1000);
timer.schedule(new TimerTask() {
@Override public void run() {
System.out.println("Time's up 2!---" + new Date().toString());
// SleepUtil.sleep(30000);
}
}, 1500 * 1000, 22 * 1000);
timer.schedule(new TimerTask() {
@Override public void run() {
System.out.println("Time's up 3!---" + new Date().toString());
// SleepUtil.sleep(30000);
}
}, 5 * 1000, 333333 * 1000);
}
5.TimerTask的执行顺序
可以得知Timer内部是单线程执行task的，一个timer对象只会启用一个TimerThread的。
当一个timer执行多个任务时，如果一个任务执行的时间过长，后面任务执行的时间可能就不是你预期执行的时间了，因为一个任务执行完了才会执行下个任务。
测试用例：
public static void test3() {
timer.schedule(new TimerTask() {
@Override public void run() {
System.out.println("Time's up 1!---" + new Date().toString());
SleepUtil.sleep(0);
}
}, 0, 2 * 1000);
timer.schedule(new TimerTask() {
@Override public void run() {
System.out.println("Time's up 2!---" + new Date().toString());
SleepUtil.sleep(5000);
}
}, 0, 2 * 1000);
// EvictionTimer.schedule(evictor, delay, delay);
}
TimerTask执行时间过长，超过了period，执行5s，period是2s，这样period相当于失效了。因为下次执行的时间是这样计算的，
{@link java.util.TimerTask#nextExecutionTime} = System.currentTimeMillis()+delay
所以，当本次任务执行结束(过了超过5s)，到下次任务取出来判断的执行时间的时候，肯定已经超过了原本应该执行的时间。
根据mainLoop中的逻辑，当判断执行时间比当前时间要早的话，直接执行本次任务。