Timer和ScheduledThreadPoolExecutor的区别及源码分析
Timer
基于单线程、系统时间实现的延时、定期任务执行类。具体可以看下面红色标注的代码。
public class Timer { /** * 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);
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; TimerThread(TaskQueue queue) { this.queue = queue; } 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) { } } } }
Timer延时、定时任务的实现采用单线程，在主循环(mainLoop)中循环遍历任务队列（TaskQueue）,如果执行时间小于等于当前系统时间则执行任务，否则继续等待（执行时间-当前时间）。
ScheduledThreadPoolExecutor
基于多线程、JVM时间实现的延时、定期任务执行类。具体可以看下面红色标注的代码。
public ScheduledThreadPoolExecutor(int corePoolSize) { super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS, new DelayedWorkQueue()); }
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) { if (command == null || unit == null) throw new NullPointerException(); if (delay <= 0) throw new IllegalArgumentException(); ScheduledFutureTask<Void> sft = new ScheduledFutureTask<Void>(command, null, triggerTime(initialDelay, unit), unit.toNanos(-delay)); RunnableScheduledFuture<Void> t = decorateTask(command, sft); sft.outerTask = t; delayedExecute(t); return t; }
private void delayedExecute(RunnableScheduledFuture<?> task) { if (isShutdown()) reject(task); else { super.getQueue().add(task); if (isShutdown() && !canRunInCurrentRunState(task.isPeriodic()) && remove(task)) task.cancel(false); else ensurePrestart(); } }
void ensurePrestart() { int wc = workerCountOf(ctl.get()); if (wc < corePoolSize) addWorker(null, true); else if (wc == 0) addWorker(null, false); }
private boolean addWorker(Runnable firstTask, boolean core) { retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; for (;;) { int wc = workerCountOf(c); if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; if (compareAndIncrementWorkerCount(c)) break retry; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } } boolean workerStarted = false; boolean workerAdded = false; Worker w = null; try { w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { // Recheck while holding lock. // Back out on ThreadFactory failure or if // shut down before lock acquired. int rs = runStateOf(ctl.get()); if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); workers.add(w); int s = workers.size(); if (s > largestPoolSize) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } if (workerAdded) { t.start(); workerStarted = true; } } } finally { if (! workerStarted) addWorkerFailed(w); } return workerStarted; }
final void runWorker(Worker w) { Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; w.unlock(); // allow interrupts boolean completedAbruptly = true; try { while (task != null || (task = getTask()) != null) { w.lock(); // If pool is stopping, ensure thread is interrupted; // if not, ensure thread is not interrupted. This // requires a recheck in second case to deal with // shutdownNow race while clearing interrupt if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) wt.interrupt(); try { beforeExecute(wt, task); Throwable thrown = null; try { task.run(); } catch (RuntimeException x) { thrown = x; throw x; } catch (Error x) { thrown = x; throw x; } catch (Throwable x) { thrown = x; throw new Error(x); } finally { afterExecute(task, thrown); } } finally { task = null; w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { processWorkerExit(w, completedAbruptly); } }
private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); // Are workers subject to culling? boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
private class ScheduledFutureTask<V> extends FutureTask<V> implements RunnableScheduledFuture<V> { /** Sequence number to break ties FIFO */ private final long sequenceNumber; /** The time the task is enabled to execute in nanoTime units */ private long time; /** * Period in nanoseconds for repeating tasks. A positive * value indicates fixed-rate execution. A negative value * indicates fixed-delay execution. A value of 0 indicates a * non-repeating task. */ private final long period; /** The actual task to be re-enqueued by reExecutePeriodic */ RunnableScheduledFuture<V> outerTask = this; /** * Index into delay queue, to support faster cancellation. */ int heapIndex; /** * Creates a one-shot action with given nanoTime-based trigger time. */ ScheduledFutureTask(Runnable r, V result, long ns) { super(r, result); this.time = ns; this.period = 0; this.sequenceNumber = sequencer.getAndIncrement(); } /** * Creates a periodic action with given nano time and period. */ ScheduledFutureTask(Runnable r, V result, long ns, long period) { super(r, result); this.time = ns; this.period = period; this.sequenceNumber = sequencer.getAndIncrement(); } /** * Creates a one-shot action with given nanoTime-based trigger time. */ ScheduledFutureTask(Callable<V> callable, long ns) { super(callable); this.time = ns; this.period = 0; this.sequenceNumber = sequencer.getAndIncrement(); } public long getDelay(TimeUnit unit) { return unit.convert(time - now(), NANOSECONDS); } public int compareTo(Delayed other) { if (other == this) // compare zero if same object return 0; if (other instanceof ScheduledFutureTask) { ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other; long diff = time - x.time; if (diff < 0) return -1; else if (diff > 0) return 1; else if (sequenceNumber < x.sequenceNumber) return -1; else return 1; } long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS); return (diff < 0) ? -1 : (diff > 0) ? 1 : 0; } /** * Returns {@code true} if this is a periodic (not a one-shot) action. * * @return {@code true} if periodic */ public boolean isPeriodic() { return period != 0; } /** * Sets the next time to run for a periodic task. */ private void setNextRunTime() { long p = period; if (p > 0) time += p; else time = triggerTime(-p); } public boolean cancel(boolean mayInterruptIfRunning) { boolean cancelled = super.cancel(mayInterruptIfRunning); if (cancelled && removeOnCancel && heapIndex >= 0) remove(this); return cancelled; } /** * Overrides FutureTask version so as to reset/requeue if periodic. */ public void run() { boolean periodic = isPeriodic(); if (!canRunInCurrentRunState(periodic)) cancel(false); else if (!periodic) ScheduledFutureTask.super.run(); else if (ScheduledFutureTask.super.runAndReset()) { setNextRunTime(); reExecutePeriodic(outerTask); } } }
ScheduledThreadPoolExecutor执行流程总结：
1.schedule（定期执行方法)
2.new Task(ScheduledFutureTask)（构建任务作业）
3.delayedExecute(task) （延时执行任务）
4.workQueue.add(task) (任务作业添加到阻塞队列)
5.addWorker(null, true or false) (添加核心作业线程，当核心线程数设置为0时则启动一个非核心线程)
6.runWorker(运行作业线程)
7.循环：getTask ->workQueue.take（获取作业）
8.task.run (作业运行)->ScheduledFutureTask.run(reExecutePeriodic(outerTask)-> workQueue.add(task)......) (周期作业重复调用)
（核心线程 或 非核心线程循环从队列中获取Task执行，周期任务则将任务重新排队）
DelayedWorkQueue中的take方法
public RunnableScheduledFuture<?> take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { for (;;) { RunnableScheduledFuture<?> first = queue[0]; if (first == null) available.await(); else { long delay = first.getDelay(NANOSECONDS); if (delay <= 0) return finishPoll(first); first = null; // don't retain ref while waiting if (leader != null) available.await(); else { Thread thisThread = Thread.currentThread(); leader = thisThread; try { available.awaitNanos(delay); } finally { if (leader == thisThread) leader = null; } } } } } finally { if (leader == null && queue[0] != null) available.signal(); lock.unlock(); } }
public long getDelay(TimeUnit unit) { return unit.convert(time - now(), NANOSECONDS); }
/** * Returns current nanosecond time. */ final long now() { return System.nanoTime(); }
ScheduledThreadPoolExecutor执行延时、定期任务，核心代码就在runWorker,循环获取任务队列中的任务然后执行，在获取任务的时候如果任务的执行时间没到，则进行等待。延时时间的计算都是基于System.nanoTime(),即JVM时间。
ThreadPoolExecutor执行流程（参考）
submit（task）-> execute(task)
->1 当前线程数<核心线程数：addWorker(task,true) (添加核心工作者线程，任务并没有进入队列排队)->runWorker-> task.run (核心线程直接执行任务)
->2 当前线程数>=核心线程数：workQueue.add(task) (任务作业添加到阻塞队列)->
2.1 任务排队成功：addWorker(null, false)（非核心工作者线程)-> 循环【getTask(workQueue.take)->task.run】（非核心线程循环从队列中获取Task执行）
2.2 任务排队失败：addWorker(task, false) (非核心工作者线程)-> task.run （尝试添加非核心线程执行任务）
优缺点：
1.Timer单线程，执行周期任务时，一次出错，则TimerThread线程终止, 所有任务将无法执行。而且任务的执行时间可能会影响周期的准确性。
2.Timer基于系统时间，系统时间的修改会影响任务的执行。在以系统时间为准的场景中(public void schedule(TimerTask task, Date time))使用非常合适，使用周期性任务则受到极大影响，因为时间间隔被破坏！
3.ScheduledThreadPoolExecutor多线程，任务的执行不会相互影响，且能保证执行时间间隔的准确性。
4.ScheduledThreadPoolExecutor基于JVM时间，该时间本身无任何意义，仅用来计算时间间隔，不受系统时间影响。所以用来计算周期间隔特别合适，而且单位是纳秒更加精确。因此延时任务、周期任务采用它比Timer更加靠谱！
总结：
Timer的使用场景，仅在基于系统时间为准的场景中非常合适（依赖当前系统时间进行判断任务的执行）。
ScheduledThreadPoolExecutor的使用场景则更为广泛，对延时任务、周期任务使用此类更靠谱（依赖时间间隔(JVM时间差值计算得到)进行判断任务的执行）。基于系统时间执行的任务则无法精确（因为系统时间可以随时调整）！
