Java Timer定时器原理

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...
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;

/**
* Created by gxf on 2017/6/21.
*/
public class TestTimer {
public static void main(String[] args) {
Timer timer = new Timer();
Task task = new Task();
timer.schedule(task, new Date(), 1000);
}
}

class Task extends TimerTask{

@Override
public void run() {
System.out.println("Do work...");
}
}

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 cancelled
synchronized(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-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) {
}
}