Article directory
1 Overview
There are many implementation methods and frameworks for task scheduling, such as DelayQueue with thread pool; Spring; Quartz; ScheduledThreadPool, etc. Today, let’s study the principles and cases of the implementation of Java’s own scheduling tool class Timer.
2. Case
Let's first implement a demo that specifies delayed execution + periodic scheduling + stops at a specified time
@Slf4j
public class TimerDemo1 {
public static void main(String[] args) {
// 创建一个定时器
Timer timer = new Timer();
TimerTask job1 = new TimerTask() {
@Override
public void run() {
log.info("{} - 开始 job1 任务 >>>>>>>>>>>", Thread.currentThread().getName());
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("<<<<<<<<<<<<< job1 任务完成 - {} ...", Thread.currentThread().getName());
}
};
Runnable scheduleThread1 = ()->{
log.info("调度线程{} - 开始调度...", Thread.currentThread().getName());
// 延时3s开始执行，执行周期为5s
timer.schedule(job1, 3000, 5000);
};
Runnable scheduleThread2 = ()->{
try {
TimeUnit.SECONDS.sleep(30);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("调度线程{} - 结束调度...", Thread.currentThread().getName());
timer.cancel();
};
scheduleThread1.run();
scheduleThread2.run();
}
}
Effect: Start execution with a delay of 3s, the execution cycle is 5s, and end after running for 30s
16:02:06.583 [main] INFO com.example.demo202206.schedule.timer.TimerDemo1 - 调度线程main - 开始调度...
16:02:09.603 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:10.610 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:14.618 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:15.630 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:19.626 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:20.631 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:24.641 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:25.644 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:29.649 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:30.663 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:34.661 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - Timer-0 - 开始 job1 任务 >>>>>>>>>>>
16:02:35.670 [Timer-0] INFO com.example.demo202206.schedule.timer.TimerDemo1 - <<<<<<<<<<<<< job1 任务完成 - Timer-0 ...
16:02:36.591 [main] INFO com.example.demo202206.schedule.timer.TimerDemo1 - 调度线程main - 结束调度...
The Timer class is very simple to use. It mainly uses the Timer tool class for timing execution, and TimerTask implements specific business logic.
3. Source code analysis
3.1 Properties and structures
The core attributes are mainly the following two: priority queue + consumer thread
// 根据时间进行优先排序的队列
private final TaskQueue queue = new TaskQueue();
// 对TaskQueue里面的定时任务进行编排和触发执行，它是一个内部无限循环的线程。
private final TimerThread thread = new TimerThread(queue);
The TaskQueue task queue is a priority queue based on an array
TaskQueue此类表示计时器任务队列：TimerTasks的优先级队列，
在nextExecutionTime上订阅。每个Timer对象都有其中一个，它与TimerThread共享。
在内部，该类使用堆为add、removeMin和rescheduleMin
提供日志（n）性能以及getMin操作的恒定时间性能。
优先级队列表现为平衡二进制堆的优先级队列：两个子队列
队列[n]是队列[2*n], 和队列[2*n+1]。
优先级队列为在nextExecutionTime字段上排序：具有最低值的TimerTask
nextExecutionTime在队列[1]中（假定队列为非空）。对于
堆中的每个节点n，n.nextExecutionTime<=d.nextExecutionTime。
3.2 TaskQueue priority queue
Implement a queue sorted according to execution time when adding tasks, and each consumption is to obtain the task with the smallest execution time
class TaskQueue {
// 默认128长度数组的队列
private TimerTask[] queue = new TimerTask[128];
private int size = 0;
void add(TimerTask var1) {
// 扩容
if (this.size + 1 == this.queue.length) {
this.queue = (TimerTask[])Arrays.copyOf(this.queue, 2 * this.queue.length);
}
// 加到数组最后位置
this.queue[++this.size] = var1;
// 调整
this.fixUp(this.size);
}
TimerTask getMin() {
return this.queue[1];
}
// 堆插入操作 时间复杂度为o(logn)
private void fixUp(int var1) {
while(true) {
if (var1 > 1) {
int var2 = var1 >> 1;
// 通过任务执行时间进行比较排序
if (this.queue[var2].nextExecutionTime > this.queue[var1].nextExecutionTime) {
TimerTask var3 = this.queue[var2];
this.queue[var2] = this.queue[var1];
this.queue[var1] = var3;
var1 = var2;
continue;
}
}
return;
}
}
}
3.3 TimerThread consumption thread
The consumption thread is mainly to implement a consumption thread that consumes TaskQueue queue tasks
/**
* The main timer loop. (See class comment.)
*/
private void mainLoop() {
while (true) {
try {
TimerTask task;
boolean taskFired;
synchronized(queue) {
// 如果queue里面没有要执行的任务，则挂起TimerThread线程
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;
// 获取queue队列里面下一个要执行的任务（根据时间排序，也就是接下来最近要执行的任务）
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;
// 执行条件
if (taskFired = (executionTime<=currentTime)) {
// 是否需要周期运行？
if (task.period == 0) {
// Non-repeating, remove
// 执行完便从队列中移除
queue.removeMin();
task.state = TimerTask.EXECUTED;
} else {
// Repeating task, reschedule
//针对task.period不等于0的任务，则计算它的下次执行时间点
//task.period<0表示是fixed delay模式的任务
//task.period>0表示是fixed rate模式的任务
queue.rescheduleMin(
task.period<0 ? currentTime - task.period
: executionTime + task.period);
}
}
}
// // 如果任务的下次执行时间还没有到达，则挂起TimerThread线程executionTime - currentTime毫秒数，到达执行时间点再自动激活
if (!taskFired) // Task hasn't yet fired; wait
queue.wait(executionTime - currentTime);
}
// 如果任务的下次执行时间到了，则执行任务
// 注意：这里任务执行没有另起线程，还是在TimerThread线程执行的，所以当有任务在同时执行时会出现阻塞
if (taskFired) // Task fired; run it, holding no locks
// 注意：这里没有try catch异常，当TimerTask抛出异常会导致整个TimerThread跳出循环，从而导致Timer失效
task.run();
} catch(InterruptedException e) {
}
}
}
}
4. Process summary
The principle of Timer is mainly to implement a small top-heap TaskQueue task queue, and then use TimerThread to continuously obtain the execution time of the first task in the queue and compare it with the current time. If the time is up, first check whether this task is a periodic task. If it is, modify the current task time to the next execution time, and if it is not a periodic task, remove the task from the priority queue. Finally perform the task. If the time is not up yet, call wait() to wait.
5. Insufficient
Timer belongs to a relatively early timing scheduling tool class in Java. According to its source code analysis, there are still the following deficiencies
- Timer only has one background thread for scheduling, so it does not support concurrent execution, and there are multiple tasks that need to be run at the same time or the running time of a certain task is greater than the periodic scheduling time, which will cause the execution effect to not meet expectations.
- The exception of the task is not handled well in the TimerThread, so the implementation of each TimerTask must try catch to prevent the exception from being thrown, resulting in the overall failure of the Timer
- The insertion and deletion time complexity of the queue is o(logn), which is not suitable for a large number of task operations.