analyse the source code of Timer

Timer Class Introduction

在JDK库中Timer类主要负责计划任务的功能,也就是在指定的时间开始执行某任务。

A Simple Example

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import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;

/**
* @author Junlan Shuai[shuaijunlan@gmail.com].
* @date Created on 9:53 2017/7/26.
*/
public class TimerTest {
// define a timer
private static Timer timer = new Timer();

// define MyTask class
static public class MyTask extends TimerTask{
private String str;
public MyTask(String str){
this.str = str;
}

@Override
public void run() {
System.out.println(this.str + "running:" + new Date());
}
}
// test main
public static void main(String[] args) throws ParseException {
MyTask myTask1 = new MyTask("task1");
MyTask myTask2 = new MyTask("task2");
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");


String dateString1 = "2017-07-26 10:06:01";
Date date1 = sdf.parse(dateString1);

String dateString2 = "2017-07-26 10:05:01";
Date date2 = sdf.parse(dateString2);

timer.schedule(myTask1, date1);
timer.schedule(myTask2, date2);
}
}

在这个例子中,定义了两个TimerTask,并且设定执行时间,调用Timer的schedule()方法,传入任务和时间,执行正确。

Timer源码分析

首先我们以timer.schedule()方法为入口,进一步剖析整个执行流程。看看Timer的schedule()的源码:

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public void schedule(TimerTask task, Date time) {
sched(task, time.getTime(), 0);
}

再进一步看sched()方法的源码

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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();
}
}
  • 在这个函数里面涉及到了两个重要的变量(queue和task);
  • 首先是获取queue同步锁,设置task的基本属性,包括nextExecutionTime、perid、state;
  • 将task添加到queue中,等待task被执行;

TaskQueue源码分析

  • 底层是定义了一个private TimerTask[] queue = new TimerTask[128];数组,用来存储TimerTask,默认值为128;
  • TaskQueue是直接定义在Timer.java的类,是一个优先级队列,是根据nextExecutionTime排序的;最小的nextExecutionTime
  • 如果queue不是空的,最小的TimeTask.nextExecutionTime就是queue[1];
  • 主要理解两个函数:fixUp()和fixDown();
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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;
}
}

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;
}
}
  • 每次调用add()方法,先将任务添加到quene的最后一个,然后调用fixUp()方法,调整整个queue,将拥有最小nextExecutionTime的TimerTask调整到queue[1]位置,如果位置不够,则需要扩容,按照原来容量的两倍扩容;
# java

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