标签：init import res flow cti over get produce max
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 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) { } }
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
标签：init import res flow cti over get produce max
原文地址：http://www.cnblogs.com/luckygxf/p/7078662.html
