定时器Timer源码解析
一 Timer
1、Timer
Timer较之Quartz结构相对简单，其原理更容易动，并且两个会有相似之处，可以在了解Timer之后在看Quartz可能会相对容易通透一点，在Quartz之前先了解一下Timer定时器，以下是JDK Api中的介绍：
- 线程调度任务以供将来在后台线程中执行的功能。 任务可以安排一次执行，或定期重复执行。
- 对应于每个Timer对象是单个后台线程，用于依次执行所有定时器的所有任务。 计时器任务应该快速完成。 如果一个定时器任务需要花费很多时间来完成，它会“计时”计时器的任务执行线程。 这可能会延迟随后的任务的执行，这些任务在（和）如果违规任务最后完成时，可能会“束起来”并快速执行。
- 在最后一次对Timer对象的引用后*，所有未完成的任务已完成执行，定时器的任务执行线程正常终止（并被收集到垃圾回收）。但是，这可能需要任意长时间的发生。默认情况下，任务执行线程不作为守护程序线程*运行，因此它能够使应用程序终止。如果主叫方想要快速终止定时器的任务执行线程，则调用者应该调用定时器的cancel方法
- 这个类是线程安全的：多个线程可以共享一个单独的Timer对象，而不需要外部同步。
- 如果定时器的任务执行线程意外终止，例如，因为它调用了stop方法，那么在计时器上安排任务的任何进一步的尝试将会产生一个IllegalStateException ，就像定时器的cancel方法被调用一样。
2、源码解析
2.1 Timer
初始化Timer时，会将其中的TaskQueue以及TimerThread也进行相应的初始化并且，会启动线程，使thread在一个wait状态。
/**
* 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();
/**
* The timer thread.
*/
private final TimerThread thread = new TimerThread(queue);
/**
* 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
*/
//初始化时会将thead线程启动，使其在一个wait状态。
public Timer(String name) {
thread.setName(name);
thread.start();
}
/**
* 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 <tt>delay</tt> is negative, or
* <tt>delay + System.currentTimeMillis()</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
*/
public void schedule(TimerTask task, long delay) {
if (delay < 0)
throw new IllegalArgumentException("Negative delay.");
sched(task, System.currentTimeMillis()+delay, 0);
}
其中的核心schedule方法提供多个重载方法，提供给使用者，最终会调用sched方法。参数含义如下：task（具体执行动作），time（下一次执行时间），perid（每次执行时间间隔）。
/**
* 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.
* 计划指定的计时器任务，以便在指定的时间和指定的时间段执行，以毫秒为单位。
* 如果周期为正，则计划重复执行任务；如果周期为零，则计划一次性执行任务。
* 时间以日期指定。getTime（）格式。此方法检查计时器状态、任务状态、和初始执行时间，但不检查周期。
*
* @param task 具体执行动作。
* @param time 下次执行时间，时间戳。
* @param period 每次执行时间间隔，时间戳。
*/
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.
// 限制时间间隔的最大值
if (Math.abs(period) > (Long.MAX_VALUE >> 1))
period >>= 1;
//同一个Timer并发调用sched时加重锁
synchronized(queue) {
if (!thread.newTasksMayBeScheduled)
throw new IllegalStateException("Timer already cancelled.");
//不同Timer调用同一个task时，task加锁应用，task.lock为TimerTask类中的加锁标识。
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;
}
//将task加入执行计划queue中。
queue.add(task);
if (queue.getMin() == task)
//getMin()获取执行计划中下一次执行，如果当前task为下次执行，则通知queue，不需要在wait
queue.notify();
}
}
2.2 TimerThread（thread）
TimerThread主要用于管理task重复的启用以及非重复task任务的删除，在上述Timer中，初始化时会启动以下线程，使thread到达运行状态，执行mainLoop方法,进而在该方法中在到等待状态。
/**
* 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!
*/
//是否继续执行，true标识继续，false标识结束，最终结束定时器，还需要清空执行计划queue。
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;
TimerThread(TaskQueue queue) {
this.queue = queue;
}
public void run() {
try {
mainLoop();
} finally {
// Someone killed this Thread, behave as if Timer cancelled
//上述mainLoop执行结束
synchronized(queue) {
newTasksMayBeScheduled = false;
queue.clear(); // Eliminate obsolete references
}
}
}
/**
* The main timer loop. (See class comment.)
*/
private void mainLoop() {
while (true) {
//？？？？？？无线执行for循环完成自动服务。
try {
TimerTask task;
boolean taskFired;
//将一个执行计划queue，初始化多个TimerThread，并启动线程时，需要将queue加锁
synchronized(queue) {
// Wait for queue to become non-empty
while (queue.isEmpty() && newTasksMayBeScheduled)
//执行计划中五可执行任务，并且为可执行状态时，则执行计划等待任务加入
queue.wait();
if (queue.isEmpty())
//如果如果queue为空，则退出不在进行，clear时
break; // Queue is empty and will forever remain; die
// Queue nonempty; look at first evt and do the right thing
//executionTime 下一次执行时间
long currentTime, executionTime;
//获取下一次执行计划
task = queue.getMin();
//场景同一个task列入多个执行计划时，
synchronized(task.lock) {
if (task.state == TimerTask.CANCELLED) {
//task状态为取消状态时，queue执行计划需要将task删除，并将queue按下一次执行时间进行排序。
queue.removeMin();
continue; // No action required, poll queue again
}
currentTime = System.currentTimeMillis();
executionTime = task.nextExecutionTime;
if (taskFired = (executionTime<=currentTime)) {
//taskFired（true） 下次执行时间小于等于当前时间时
if (task.period == 0) { // Non-repeating, remove
//不存在时间间隔时，并且次执行时间小于等于当前时间时，标识非重复任务，则将之从执行计划中删除
queue.removeMin();
//并更新该task的执行状态
task.state = TimerTask.EXECUTED;
} else { // Repeating task, reschedule
//改task任务为重复执行任务，则更新下次执行时间，并且将queue执行计划按下次执行时间进行排序。
queue.rescheduleMin(
task.period<0 ? currentTime - task.period
: executionTime + task.period);
}
}
}
if (!taskFired) // Task hasn't yet fired; wait
//taskFired（false） 下次执行时间大于当前时间时，则等待
queue.wait(executionTime - currentTime);
}
if (taskFired) // Task fired; run it, holding no locks
//满足条件则执行task
task.run();
} catch(InterruptedException e) {
}
}
}
2.3 TaskQueue(queue)
/**
* 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.
*/
//默认数组长度128
private TimerTask[] queue = new TimerTask[128];
/**
* Adds a new task to the priority queue.
*/
//判断长度，并采用数组扩容
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);
}
/**
* Return the "head task" of the priority queue. (The head task is an
* task with the lowest nextExecutionTime.)
*/
TimerTask getMin() {
return queue[1];
}
/**
* 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);
}
/**
* 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);
}
/**
* 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.
*/
//将将执行计划queue[k]与queue[1]进行比较，执行计划靠前的则放入queue[1]中
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;
}
}
简单示例
public class SimpleTimer {
public static void main(String[] args) {
Timer simpleTimer = new Timer("firstTime");
SimpleTask task = new SimpleTask();
//Object lock = new Object();
//1000毫秒后开始执行，每次间隔2000毫秒
simpleTimer.schedule(task,1000,2000);
simpleTimer.schedule(task,1000,2000);
}
public static class SimpleTask extends TimerTask{
Integer index = 0;
@Override
public void run() {
index = index + 1;
System.out.println("************"+index);
if(index == 10 ){
cancel();
}
}
}
}
注：由于Timer采用单线程执行，未采用线程池，在遇到耗时较长的Job工作时，时间校准会出现误差