做项目很多时候会用到定时任务，比如在深夜，流量较小的时候，做一些统计工作。早上定时发送邮件，更新数据库等。这里可以用Java的Timer或线程池实现。Timer可以实现，不过Timer存在一些问题。他起一个单线程，如果有异常产生，线程将退出，整个定时任务就失败。
下面是一个Timer实现的定时任务Demo，会向控制台每隔一秒输出Do work...
1 import java.util.Date;2 import java.util.Timer;3 import java.util.TimerTask;4 5 /**6 * Created by gxf on 2017/6/21.7 */8 public class TestTimer {9 public static void main(String[] args) {
10 Timer timer = new Timer();
11 Task task = new Task();
12 timer.schedule(task, new Date(), 1000);
13 }
14 }
15
16 class Task extends TimerTask{
17
18 @Override
19 public void run() {
20 System.out.println("Do work...");
21 }
22 } 控制台输出
Do work... Do work... Do work... Do work...
我们将进入JDK源码分析一下，Timer原理
Timer源码
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); 这里可以看出，有一个队列（其实是个最小堆），和一个线程对象
我们在看一下Timer的构造函数
/*** Creates a new timer. The associated thread does <i>not</i>* {@linkplain Thread#setDaemon run as a daemon}.*/public Timer() {this("Timer-" + serialNumber());} 这里调用了有参构造函数，进入查看
/*** 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();} 这里可以看到，起了一个线程
ok，我们再看一下，TimerTask这个类
/*** A task that can be scheduled for one-time or repeated execution by a Timer.** @author Josh Bloch* @see Timer* @since 1.3*/public abstract class TimerTask implements Runnable { 虽然代码不多，也不贴完，这里看出，是一个实现了Runable接口的类，也就是说可以放到线程中运行的任务
这里就清楚了，Timer是一个线程，TimerTask是一个Runable实现类，那只要提交TimerTask对象就可以运行任务了。
public void schedule(TimerTask task, Date firstTime, long period) {if (period <= 0)throw new IllegalArgumentException("Non-positive period.");sched(task, firstTime.getTime(), -period);} 进入Timer shed(task, firstTime, period)
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();}} 这里主要是queue.add(task)将任务放到最小堆里面，并queue.notity()唤醒在等待的线程
那么我们进入Timer类的TimerThread对象查看run方法，因为Timer类里面有个TimerThread 对象是一个线程
public void run() {try {mainLoop();} finally {// Someone killed this Thread, behave as if Timer cancelledsynchronized(queue) {newTasksMayBeScheduled = false;queue.clear(); // Eliminate obsolete references}}} 这里可以看出，在执行一个mainLoop()循环，进入这个循环
/*** 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-emptywhile (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 thinglong 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, removequeue.removeMin();task.state = TimerTask.EXECUTED;} else { // Repeating task, reschedulequeue.rescheduleMin(task.period<0 ? currentTime - task.period: executionTime + task.period);}}}if (!taskFired) // Task hasn't yet fired; waitqueue.wait(executionTime - currentTime);}if (taskFired) // Task fired; run it, holding no lockstask.run();} catch(InterruptedException e) {}} 这里忘了说明，TimerTask是按nextExecutionTime进行堆排序的。每次取堆中nextExecutionTime和当前系统时间进行比较，如果当前时间大于nextExecutionTime则执行，如果是单次任务，会将任务从最小堆，移除。否则，更新nextExecutionTime的值
至此，Timer定时任务原理基本理解，单线程 + 最小堆 + 不断轮询
