Java多线程三

1. 基本概念

  java.util.Timer：是一个实用工具类，该类用来调度一个线程(schedule a thread)，使它可以在将来某一时刻执行。 Java的Timer类可以调度一个任务运行一次，或定期循环运行。 Timer tasks should complete quickly. 即定时器中的操作要尽可能花费短的时间。

  java.util.TimerTask：是一个抽象类，它实现了Runnable接口。我们需要扩展该类以便创建自己的TimerTask，这个TimerTask可以被Timer调度。

  注意：默认, task执行线程不是daemon线程, 任务执行完，主线程（或其他启动定时器的线程）结束时，task线程并没有结束。如果调用者想要快速终止计时器的任务执行线程，调用者应该调用

timer.cancel()

方法。

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">timerTest</span>() {
Timer myTimer = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Timer();
myTimer.schedule(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimerTask() {
@Override
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"1s后运行"</span>);
myTimer.cancel(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 需要手动cancel</span>
}
}, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1000</span>);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

2. 源码解析及案例

  Timer类是线程安全的，多进程不需要外部同步机制就可以共享同一个Timer对象。创建Timer对象，会创建一个java.util.TaskQueue实例，在执行定时任务时，将taskqueue对象作为锁，在指定时间间隔添加任务，在任何时刻只能有一个线程执行TimerTask。

  Timer类使用对象的wait和notify方法来调度任务。

  查看Timer源代码：

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// TaskQueue队列，内部就是一个TimerTask[]数组</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> TaskQueue queue = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TaskQueue();
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Timer内部维持一个叫TimerThread的线程，传递TaskQueue队列</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> TimerThread thread = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimerThread(queue);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 创建Timer即启动线程</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-title" style="box-sizing: border-box;">Timer</span>(String name) {
thread.setName(name);
thread.start();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 启动线程，后面有分析TimerThread的run方法</span>
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">schedule</span>(TimerTask task, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> delay) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (delay < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IllegalArgumentException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Negative delay."</span>);
sched(task, System.currentTimeMillis()+delay, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 核心调度方法，time表示执行的绝对时间</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">sched</span>(TimerTask task, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> time, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> period) {
...
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// timer对象中的queue作为锁</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">synchronized</span>(queue) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!thread.newTasksMayBeScheduled)
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IllegalStateException(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Timer already cancelled."</span>);

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">synchronized</span>(task.lock) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (task.state != TimerTask.VIRGIN)
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">throw</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> IllegalStateException(
<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Task already scheduled or cancelled"</span>);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 设置下次执行的时间</span>
task.nextExecutionTime = time;
task.period = period;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 设置task为调度状态</span>
task.state = TimerTask.SCHEDULED;
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 将当前待执行的task添加到队列中</span>
queue.add(task);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 队列中取出的head task为当前task</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (queue.getMin() == task)
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 在任何时刻只能有一个线程执行TimerTask</span>
queue.notify();
}
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li></ul>

  其中Timer中启动的TimerThread的run方法：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> TaskQueue <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>;
TimerThread(TaskQueue <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span> = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> run() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
mainLoop();
} finally {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Someone killed this Thread, behave as if Timer cancelled</span>
synchronized(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>) {
newTasksMayBeScheduled = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>;
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.clear();  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Eliminate obsolete references</span>
}
}
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> mainLoop() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">true</span>) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">try</span> {
TimerTask task;
boolean taskFired;
synchronized(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 如果queue队列为空，则将线程阻塞，等待task</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.isEmpty() && newTasksMayBeScheduled)
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.wait();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.isEmpty())
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">break</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Queue is empty and will forever remain; die</span>

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Queue nonempty; look at first evt and do the right thing</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> currentTime, executionTime;
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 队列中获取task</span>
task = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.getMin();
synchronized(task.lock) {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 此处说明可以通过cancel()设置终止task的执行，但TimerThread并没有终止</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (task.state == TimerTask.CANCELLED) {
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.removeMin(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 从队列中移除</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">continue</span>;  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// No action required, poll queue again</span>
}
currentTime = System.currentTimeMillis();
executionTime = task.nextExecutionTime;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (taskFired = (executionTime<=currentTime)) {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (task.period == <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Non-repeating, remove</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.removeMin();
task.state = TimerTask.EXECUTED;
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> { <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Repeating task, reschedule</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.rescheduleMin(
task.period<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> ? currentTime   - task.period
: executionTime + task.period);
}
}
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 还未到执行时间，则等待相应的时间</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (!taskFired) <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Task hasn't yet fired; wait</span>
<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">queue</span>.wait(executionTime - currentTime);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (taskFired)  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// Task fired; run it, holding no locks</span>
task.run(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 执行task的run方法！</span>
} <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">catch</span>(InterruptedException e) {
}
}
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li><li style="box-sizing: border-box; padding: 0px 5px;">55</li><li style="box-sizing: border-box; padding: 0px 5px;">56</li><li style="box-sizing: border-box; padding: 0px 5px;">57</li><li style="box-sizing: border-box; padding: 0px 5px;">58</li><li style="box-sizing: border-box; padding: 0px 5px;">59</li><li style="box-sizing: border-box; padding: 0px 5px;">60</li><li style="box-sizing: border-box; padding: 0px 5px;">61</li></ul>

  TimerTask的调度流程：由以上分析可知，当我们创建一个Timer时，同时内部创建了一个TimerThread线程，并启动它，该线程会不断扫描从Timer传递进来的task队列，如果为空，则wait()阻塞该线程；当timer调用shedule方法的时候，将传递的task添加到队列中，同时调用

queue.notify()

方法唤醒TimerThread线程，则从队列中取出task根据给定的等待时间wait等待，等待完成后执行

task.run();

启动任务。（这种结构可以应用到简单的爬虫中）

  


  如何终止Timer线程：

  默认情况下，创建的timer线程会一直执行，一般通过下面的方式来终止timer线程：

调用timer的cancle方法
把timer线程设置成daemon线程，（new Timer(true)创建daemon线程），在jvm里，如果所有用户线程结束，那么守护线程也会被终止，不过这种方法一般不用。

  下面给出一个Timer执行多个task的简单案例：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">multileTasksShareOneTimer</span>() {
Timer myTimer = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Timer();
TimerTask task1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimerTask() {
@Override
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(Thread.currentThread().getName() +<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 1s后执行task1"</span>);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//myTimer.cancel();</span>
}
};

TimerTask task2 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimerTask() {
@Override
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(Thread.currentThread().getName() +<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 2s后执行task2"</span>);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//myTimer.cancel();</span>
}
};
myTimer.schedule(task1, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1000</span>);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// task2.cancel();</span>
myTimer.schedule(task2, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2000</span>);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

  运行输出：注意此时Timer的TimerThread并没有结束，因为在mainLoop()等待task而wait进入阻塞状态！



  如果设置了task2.cancel();则调度执行task2会抛出异常：



  定时器实际使用的场景还是很多的，比如下面的，在每天零晨备份数据库，等等。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
* 每天0晨备份数据库
*/</span>
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@SuppressWarnings</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"deprecation"</span>)
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">backupDatabase</span>() {
Timer timer = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Timer();
TimerTask task = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> TimerTask() {

<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 备份数据库</span>
System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"数据库备份..."</span>);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// other operation</span>
}
};
Date firstTime = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Date();
firstTime.setHours(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
firstTime.setMinutes(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
firstTime.setSeconds(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">long</span> oneDayPeriod = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">24</span> * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">60</span> * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">60</span> * <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1000</span>;
timer.scheduleAtFixedRate(task, firstTime, oneDayPeriod);
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// timer.schedule(task, firstTime, oneDayPeriod);</span>
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

  最后：如果是简单的定时调度，使用Timer就够了，如果复杂的调度任务，可以考虑使用Quartz