定時器的實現原理及參考

摘要： 如果讓你來實現一個定時器的功能，簡單點就是，每隔n秒，去執行一次A任務，你打算怎麼實現？ 我覺得一般都能想到，使用一個死迴圈，然後每次迴圈比較時間，時間到了就去執行A任務就好了。但是這樣會不會有問題？每次迴圈會不會效能消耗太大？別人都是怎麼做的？如果有語言提供的工具，那我自然更加...

如果讓你來實現一個定時器的功能，簡單點就是，每隔n秒，去執行一次A任務，你打算怎麼實現？

我覺得一般都能想到，使用一個死迴圈，然後每次迴圈比較時間，時間到了就去執行A任務就好了。但是這樣會不會有問題？每次迴圈會不會效能消耗太大？別人都是怎麼做的？如果有語言提供的工具，那我自然更加相信他而不是自己去實現。

好吧，用程式語言自身提供的工具一般情況下自然是比較明智的選擇，因為別人本來就比你厲害啊。

那麼，java中的定時器？不用說，timer。是怎麼做的呢？他到底比自己好在哪裡，他肯定是用了什麼我不知道的高深莫測的演算法幹出來的。好吧，你可以把一切不知道的東西歸之於大神。但是正確的開啟方式是這樣的，去看一下他怎麼幹的就好了。

timer原始碼閱讀：

demo:

public class DebugerTest {

public static void main(String[] args) {
DebugerTest test = new DebugerTest();
Timer timer = new Timer();
timer.schedule(new TimerTask() {
@Override
public void run() {
try {
test.moveABrick();
} catch (Exception e) {
e.printStackTrace();
}
}
}, 1000, 5000);

timer.schedule(new TimerTask() {
@Override
public void run() {
Long nowTimestamp = System.currentTimeMillis() / 1000;
System.out.println(nowTimestamp + " [" + Thread.currentThread().getName() + "] " + ": hello, new schedule...");
}
}, 1000, 1000);
}

// 去搬磚
public void moveABrick() {
int i = 0;
while (true) {
if(i++ < 3) {
Long nowTimestamp = System.currentTimeMillis() / 1000;
System.out.println(nowTimestamp + " [" + Thread.currentThread().getName() + "] " + ": moving step +" + i);
// 用於展示併發效果, 驗證結果是，正常情況下並不會存在併發
try {
Thread.sleep(3000L);
} catch (InterruptedException e) {
// interrupt
}
}
else {
break;
}
}
System.out.println("move over.");
}
}

開啟方式： new Timer().schedule(xx, 1000, 5000);

然後他就吭哧吭哧的每過xx秒就去做事了。

看第一句new，其實他建立了一個例項級的執行緒，並把他打開了，然後，接下來就看想幹啥了。這裡schedule, 他自然就線上程開工裡去判定了。

// java.util.Timer, 構造方法

private final TimerThread thread = new TimerThread(queue);

public Timer() {
this("Timer-" + serialNumber());
}

public Timer(String name) {
thread.setName(name);
// 看到了吧，只要new一個定時器，就會有一個執行緒在跑了，所以沒事別搞那麼多 timer出來哈哈
thread.start();
}

// start 之後，幹啥去了呢？

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 {
// 就幹一件事，去迴圈輪詢，當然還要做一些善後工作
mainLoop();
} finally {
// Someone killed this Thread, behave as if Timer cancelled
synchronized(queue) {
newTasksMayBeScheduled = false;
queue.clear();// Eliminate obsolete references
}
}
}

private void mainLoop() {
while (true) {
try {
TimerTask task;
boolean taskFired;
// 由於queue是非執行緒安全的，所以要使用同步鎖定
synchronized(queue) {
// 如果佇列為空則一直等待，如果發生了異常，則結束任務
while (queue.isEmpty() && newTasksMayBeScheduled)
// 此等待為 Object 類的阻塞等等，與 synchronized 一起使用
queue.wait();
if (queue.isEmpty())
break;

long currentTime, executionTime;
// 獲取佇列頭的任務（最早可能執行的任務），進行判定
task = queue.getMin();
synchronized(task.lock) {
// 如果任務已設定取消，則移除佇列
if (task.state == TimerTask.CANCELLED) {
queue.removeMin();
continue;
}
currentTime = System.currentTimeMillis();
executionTime = task.nextExecutionTime;
// 時間判定，如果小於當前時間，則可以執行任務
if (taskFired = (executionTime<=currentTime)) {
// period=0,意味著不需要再迴圈任務了
if (task.period == 0) { 
queue.removeMin();
task.state = TimerTask.EXECUTED;
} else {
// 如果是需要多次執行的任務，則重新讓把佇列加入，然後重排序
queue.rescheduleMin(
task.period<0 ? currentTime- task.period
: executionTime + task.period);
}
}
}
// 任務執行時間還沒有到，阻塞等待，超時時間到時，也就是任務開始執行的時刻到了
if (!taskFired) 
queue.wait(executionTime - currentTime);
}
// 經過前面的檢查，到此處一般就可以執行任務了，同步呼叫
if (taskFired) 
// 注意是同步呼叫, 原因嘛，我也不知道
task.run();
} catch(InterruptedException e) {
}
}
}
}

至此，我們已經new完了，好累啊！

下面來看一下 schedule(xx, 1000, 5000), 設定任務方式。

// 以定時間隔的方式重複執行
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);
}
// 呼叫內部封裝好的任務計劃 
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");
// 設定任務執行時間，狀態
task.nextExecutionTime = time;
task.period = period;
task.state = TimerTask.SCHEDULED;
}

// 新增任務到佇列，則判定如果當前任務就是第一個(有且僅有時，定時器處理阻塞等待狀態)的話，觸發一次notify(), 使用執行緒的 wait() 開始執行。
queue.add(task);
if (queue.getMin() == task)
queue.notify();
}
}

最後，有一個關鍵的點我們沒有考慮到，那就是當有多個任務時，怎樣確定任務的先級，為什麼每次只要取出第一個任務執行即可？

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.
* 佇列的容器，是經過按時間排序的陣列
*/
private TimerTask[] queue = new TimerTask[128];

private int size = 0;

int size() {
return size;
}

// 新增任務，必要時進行擴容
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);
}

/**
* 獲取佇列頭的任務，即第一個元素
*/
TimerTask getMin() {
return queue[1];
}

TimerTask get(int i) {
return queue[i];
}

/**
* 執行完成後，刪除隊頭
*/
void removeMin() {
queue[1] = queue[size];
queue[size--] = null;// Drop extra reference to prevent memory leak
fixDown(1);
}

/**
* 快速刪除佇列
*/
void quickRemove(int i) {
assert i <= size;

queue[i] = queue[size];
queue[size--] = null;// Drop extra ref to prevent memory leak
}

/**
* 將隊頭任務重新入隊，僅改下下次執行時間即可，每次新增更新完成都需要做一次重排序
*/
void rescheduleMin(long newTime) {
queue[1].nextExecutionTime = newTime;
fixDown(1);
}

boolean isEmpty() {
return size==0;
}

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.
* 佇列重排序，增加元素時使用
*/
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.
* 佇列重排序，減少元素時使用
*/
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() {
for (int i = size/2; i >= 1; i--)
fixDown(i);
}
}

最後，我們還要看一下具體的任務結構是什麼樣的：

public abstract class TimerTask implements Runnable {

// 同步鎖
final Object lock = new Object();

int state = VIRGIN;

// 定義任務的幾種狀態用以判定是否需要執行
static final int VIRGIN = 0;

static final int SCHEDULED= 1;

static final int EXECUTED= 2;

static final int CANCELLED= 3;

long nextExecutionTime;

/**
* Period in milliseconds 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.
* 正數代表以固定速度執行，負數代表以固定時間延遲執行，0代表不重複執行
*/
long period = 0;

/**
* Creates a new timer task.
*/
protected TimerTask() {
}

/**
* The action to be performed by this timer task.
* 實現類只要實現這個方法，就可以執行指定的任務了，
*其他方法一般情況下，統一由父抽象類實現即可
*/
public abstract void run();

public boolean cancel() {
synchronized(lock) {
boolean result = (state == SCHEDULED);
state = CANCELLED;
return result;
}
}

public long scheduledExecutionTime() {
synchronized(lock) {
return (period < 0 ? nextExecutionTime + period
: nextExecutionTime - period);
}
}
}

完整原始碼如下,有興趣請展開(Timer主要由三個內部類組成: Timer, Timer$TimerThread, Timer$TimerThread)：

package java.util;
import java.util.Date;
import java.util.concurrent.atomic.AtomicInteger;

/**
 * A facility for threads to schedule tasks for future execution in a
 * background thread.Tasks may be scheduled for one-time execution, or for
 * repeated execution at regular intervals.
 *
 * <p>Corresponding to each <tt>Timer</tt> object is a single background
 * thread that is used to execute all of the timer's tasks, sequentially.
 * Timer tasks should complete quickly.If a timer task takes excessive time
 * to complete, it "hogs" the timer's task execution thread.This can, in
 * turn, delay the execution of subsequent tasks, which may "bunch up" and
 * execute in rapid succession when (and if) the offending task finally
 * completes.
 *
 * <p>After the last live reference to a <tt>Timer</tt> object goes away
 * <i>and</i> all outstanding tasks have completed execution, the timer's task
 * execution thread terminates gracefully (and becomes subject to garbage
 * collection).However, this can take arbitrarily long to occur.By
 * default, the task execution thread does not run as a <i>daemon thread</i>,
 * so it is capable of keeping an application from terminating.If a caller
 * wants to terminate a timer's task execution thread rapidly, the caller
 * should invoke the timer's <tt>cancel</tt> method.
 *
 * <p>If the timer's task execution thread terminates unexpectedly, for
 * example, because its <tt>stop</tt> method is invoked, any further
 * attempt to schedule a task on the timer will result in an
 * <tt>IllegalStateException</tt>, as if the timer's <tt>cancel</tt>
 * method had been invoked.
 *
 * <p>This class is thread-safe: multiple threads can share a single
 * <tt>Timer</tt> object without the need for external synchronization.
 *
 * <p>This class does <i>not</i> offer real-time guarantees: it schedules
 * tasks using the <tt>Object.wait(long)</tt> method.
 *
 * <p>Java 5.0 introduced the {@code java.util.concurrent} package and
 * one of the concurrency utilities therein is the {@link
 * java.util.concurrent.ScheduledThreadPoolExecutor
 * ScheduledThreadPoolExecutor} which is a thread pool for repeatedly
 * executing tasks at a given rate or delay.It is effectively a more
 * versatile replacement for the {@code Timer}/{@code TimerTask}
 * combination, as it allows multiple service threads, accepts various
 * time units, and doesn't require subclassing {@code TimerTask} (just
 * implement {@code Runnable}).Configuring {@code
 * ScheduledThreadPoolExecutor} with one thread makes it equivalent to
 * {@code Timer}.
 *
 * <p>Implementation note: This class scales to large numbers of concurrently
 * scheduled tasks (thousands should present no problem).Internally,
 * it uses a binary heap to represent its task queue, so the cost to schedule
 * a task is O(log n), where n is the number of concurrently scheduled tasks.
 *
 * <p>Implementation note: All constructors start a timer thread.
 *
 * @authorJosh Bloch
 * @seeTimerTask
 * @seeObject#wait(long)
 * @since1.3
 */

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);

/**
* This object causes the timer's task execution thread to exit
* gracefully when there are no live references to the Timer object and no
* tasks in the timer queue.It is used in preference to a finalizer on
* Timer as such a finalizer would be susceptible to a subclass's
* finalizer forgetting to call it.
*/
private final Object threadReaper = new Object() {
protected void finalize() throws Throwable {
synchronized(queue) {
thread.newTasksMayBeScheduled = false;
queue.notify(); // In case queue is empty.
}
}
};

/**
* This ID is used to generate thread names.
*/
private final static AtomicInteger nextSerialNumber = new AtomicInteger(0);
private static int serialNumber() {
return nextSerialNumber.getAndIncrement();
}

/**
* Creates a new timer.The associated thread does <i>not</i>
* {@linkplain Thread#setDaemon run as a daemon}.
*/
public Timer() {
this("Timer-" + serialNumber());
}

/**
* Creates a new timer whose associated thread may be specified to
* {@linkplain Thread#setDaemon run as a daemon}.
* A daemon thread is called for if the timer will be used to
* schedule repeating "maintenance activities", which must be
* performed as long as the application is running, but should not
* prolong the lifetime of the application.
*
* @param isDaemon true if the associated thread should run as a daemon.
*/
public Timer(boolean isDaemon) {
this("Timer-" + serialNumber(), isDaemon);
}

/**
* 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();
}

/**
* Creates a new timer whose associated thread has the specified name,
* and may be specified to
* {@linkplain Thread#setDaemon run as a daemon}.
*
* @param name the name of the associated thread
* @param isDaemon true if the associated thread should run as a daemon
* @throws NullPointerException if {@code name} is null
* @since 1.5
*/
public Timer(String name, boolean isDaemon) {
thread.setName(name);
thread.setDaemon(isDaemon);
thread.start();
}

/**
* Schedules the specified task for execution after the specified delay.
*
* @param tasktask 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);
}

/**
* Schedules the specified task for execution at the specified time.If
* the time is in the past, the task is scheduled for immediate execution.
*
* @param task task to be scheduled.
* @param time time at which task is to be executed.
* @throws IllegalArgumentException if <tt>time.getTime()</tt> is negative.
* @throws IllegalStateException if task was already scheduled or
*cancelled, timer was cancelled, or timer thread terminated.
* @throws NullPointerException if {@code task} or {@code time} is null
*/
public void schedule(TimerTask task, Date time) {
sched(task, time.getTime(), 0);
}

/**
* Schedules the specified task for repeated <i>fixed-delay execution</i>,
* beginning after the specified delay.Subsequent executions take place
* at approximately regular intervals separated by the specified period.
*
* <p>In fixed-delay execution, each execution is scheduled relative to
* the actual execution time of the previous execution.If an execution
* is delayed for any reason (such as garbage collection or other
* background activity), subsequent executions will be delayed as well.
* In the long run, the frequency of execution will generally be slightly
* lower than the reciprocal of the specified period (assuming the system
* clock underlying <tt>Object.wait(long)</tt> is accurate).
*
* <p>Fixed-delay execution is appropriate for recurring activities
* that require "smoothness."In other words, it is appropriate for
* activities where it is more important to keep the frequency accurate
* in the short run than in the long run.This includes most animation
* tasks, such as blinking a cursor at regular intervals.It also includes
* tasks wherein regular activity is performed in response to human
* input, such as automatically repeating a character as long as a key
* is held down.
*
* @param tasktask to be scheduled.
* @param delaydelay in milliseconds before task is to be executed.
* @param period time in milliseconds between successive task executions.
* @throws IllegalArgumentException if {@code delay < 0}, or
*{@code delay + System.currentTimeMillis() < 0}, or
*{@code period <= 0}
* @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, 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);
}

/**
* Schedules the specified task for repeated <i>fixed-delay execution</i>,
* beginning at the specified time. Subsequent executions take place at
* approximately regular intervals, separated by the specified period.
*
* <p>In fixed-delay execution, each execution is scheduled relative to
* the actual execution time of the previous execution.If an execution
* is delayed for any reason (such as garbage collection or other
* background activity), subsequent executions will be delayed as well.
* In the long run, the frequency of execution will generally be slightly
* lower than the reciprocal of the specified period (assuming the system
* clock underlying <tt>Object.wait(long)</tt> is accurate).As a
* consequence of the above, if the scheduled first time is in the past,
* it is scheduled for immediate execution.
*
* <p>Fixed-delay execution is appropriate for recurring activities
* that require "smoothness."In other words, it is appropriate for
* activities where it is more important to keep the frequency accurate
* in the short run than in the long run.This includes most animation
* tasks, such as blinking a cursor at regular intervals.It also includes
* tasks wherein regular activity is performed in response to human
* input, such as automatically repeating a character as long as a key
* is held down.
*
* @param tasktask to be scheduled.
* @param firstTime First time at which task is to be executed.
* @param period time in milliseconds between successive task executions.
* @throws IllegalArgumentException if {@code firstTime.getTime() < 0}, or
*{@code period <= 0}
* @throws IllegalStateException if task was already scheduled or
*cancelled, timer was cancelled, or timer thread terminated.
* @throws NullPointerException if {@code task} or {@code firstTime} is null
*/
public void schedule(TimerTask task, Date firstTime, long period) {
if (period <= 0)
throw new IllegalArgumentException("Non-positive period.");
sched(task, firstTime.getTime(), -period);
}

/**
* Schedules the specified task for repeated <i>fixed-rate execution</i>,
* beginning after the specified delay.Subsequent executions take place
* at approximately regular intervals, separated by the specified period.
*
* <p>In fixed-rate execution, each execution is scheduled relative to the
* scheduled execution time of the initial execution.If an execution is
* delayed for any reason (such as garbage collection or other background
* activity), two or more executions will occur in rapid succession to
* "catch up."In the long run, the frequency of execution will be
* exactly the reciprocal of the specified period (assuming the system
* clock underlying <tt>Object.wait(long)</tt> is accurate).
*
* <p>Fixed-rate execution is appropriate for recurring activities that
* are sensitive to <i>absolute</i> time, such as ringing a chime every
* hour on the hour, or running scheduled maintenance every day at a
* particular time.It is also appropriate for recurring activities
* where the total time to perform a fixed number of executions is
* important, such as a countdown timer that ticks once every second for
* ten seconds.Finally, fixed-rate execution is appropriate for
* scheduling multiple repeating timer tasks that must remain synchronized
* with respect to one another.
*
* @param tasktask to be scheduled.
* @param delaydelay in milliseconds before task is to be executed.
* @param period time in milliseconds between successive task executions.
* @throws IllegalArgumentException if {@code delay < 0}, or
*{@code delay + System.currentTimeMillis() < 0}, or
*{@code period <= 0}
* @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 scheduleAtFixedRate(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);
}

/**
* Schedules the specified task for repeated <i>fixed-rate execution</i>,
* beginning at the specified time. Subsequent executions take place at
* approximately regular intervals, separated by the specified period.
*
* <p>In fixed-rate execution, each execution is scheduled relative to the
* scheduled execution time of the initial execution.If an execution is
* delayed for any reason (such as garbage collection or other background
* activity), two or more executions will occur in rapid succession to
* "catch up."In the long run, the frequency of execution will be
* exactly the reciprocal of the specified period (assuming the system
* clock underlying <tt>Object.wait(long)</tt> is accurate).As a
* consequence of the above, if the scheduled first time is in the past,
* then any "missed" executions will be scheduled for immediate "catch up"
* execution.
*
* <p>Fixed-rate execution is appropriate for recurring activities that
* are sensitive to <i>absolute</i> time, such as ringing a chime every
* hour on the hour, or running scheduled maintenance every day at a
* particular time.It is also appropriate for recurring activities
* where the total time to perform a fixed number of executions is
* important, such as a countdown timer that ticks once every second for
* ten seconds.Finally, fixed-rate execution is appropriate for
* scheduling multiple repeating timer tasks that must remain synchronized
* with respect to one another.
*
* @param tasktask to be scheduled.
* @param firstTime First time at which task is to be executed.
* @param period time in milliseconds between successive task executions.
* @throws IllegalArgumentException if {@code firstTime.getTime() < 0} or
*{@code period <= 0}
* @throws IllegalStateException if task was already scheduled or
*cancelled, timer was cancelled, or timer thread terminated.
* @throws NullPointerException if {@code task} or {@code firstTime} is null
*/
public void scheduleAtFixedRate(TimerTask task, Date firstTime,
long period) {
if (period <= 0)
throw new IllegalArgumentException("Non-positive period.");
sched(task, firstTime.getTime(), 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();
}
}

/**
* Terminates this timer, discarding any currently scheduled tasks.
* Does not interfere with a currently executing task (if it exists).
* Once a timer has been terminated, its execution thread terminates
* gracefully, and no more tasks may be scheduled on it.
*
* <p>Note that calling this method from within the run method of a
* timer task that was invoked by this timer absolutely guarantees that
* the ongoing task execution is the last task execution that will ever
* be performed by this timer.
*
* <p>This method may be called repeatedly; the second and subsequent
* calls have no effect.
*/
public void cancel() {
synchronized(queue) {
thread.newTasksMayBeScheduled = false;
queue.clear();
queue.notify();// In case queue was already empty.
}
}

/**
* Removes all cancelled tasks from this timer's task queue.<i>Calling
* this method has no effect on the behavior of the timer</i>, but
* eliminates the references to the cancelled tasks from the queue.
* If there are no external references to these tasks, they become
* eligible for garbage collection.
*
* <p>Most programs will have no need to call this method.
* It is designed for use by the rare application that cancels a large
* number of tasks.Calling this method trades time for space: the
* runtime of the method may be proportional to n + c log n, where n
* is the number of tasks in the queue and c is the number of cancelled
* tasks.
*
* <p>Note that it is permissible to call this method from within a
* a task scheduled on this timer.
*
* @return the number of tasks removed from the queue.
* @since 1.5
*/
public int purge() {
int result = 0;

synchronized(queue) {
for (int i = queue.size(); i > 0; i--) {
if (queue.get(i).state == TimerTask.CANCELLED) {
queue.quickRemove(i);
result++;
}
}

if (result != 0)
queue.heapify();
}

return result;
}
}

/**
 * This "helper class" implements the timer's task execution thread, which
 * waits for tasks on the timer queue, executions them when they fire,
 * reschedules repeating tasks, and removes cancelled tasks and spent
 * non-repeating tasks from the 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) {
}
}
}
}

/**
 * This class represents a timer task queue: a priority queue of TimerTasks,
 * ordered on nextExecutionTime.Each Timer object has one of these, which it
 * shares with its TimerThread.Internally this class uses a heap, which
 * offers log(n) performance for the add, removeMin and rescheduleMin
 * operations, and constant time performance for the getMin operation.
 */
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];

/**
* The number of tasks in the priority queue.(The tasks are stored in
* queue[1] up to queue[size]).
*/
private int size = 0;

/**
* 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) {
// 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];
}

/**
* 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);
}

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

View Code

好了，到這裡，神祕面紗已經不存在了，是不是自信心更增加了一點呢？

當然，語言級別提供東西，對大部分同學來說，已經可以奉若神靈了。但是，要想有更進一步的提升，則你需要思考的更多。

語言如果就是完美的，那要升級有啥用呢？

未來終究還是你們這些年輕人的啊！哈哈