epubjs
Version:
Render ePub documents in the browser, across many devices
61 lines (55 loc) • 6.76 kB
HTML
<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Timers</title><link rel="stylesheet" href="core.css" type="text/css"/><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"/></head><body><div class="sect1" title="Timers"><div class="titlepage"><div><div><h1 class="title"><a id="learnjava3-CHP-11-SECT-3"/>Timers</h1></div></div></div><p><a id="idx10636" class="indexterm"/> <a id="idx10674" class="indexterm"/>Java includes two handy classes for timed code execution. If
you write a clock application, for example, you might want to update the
display every second. You might want to play an alarm sound at some
predetermined time. You could accomplish these tasks using multiple
threads and calls to <code class="literal">Thread.sleep()</code>.
But the <a id="I_indexterm11_id741007" class="indexterm"/><code class="literal">java.util.Timer</code> and
<a id="I_indexterm11_id741017" class="indexterm"/><code class="literal">java.util.TimerTask</code>
classes handle this for you.</p><p>The <code class="literal">Timer</code> class is a scheduler.
Each instance of <code class="literal">Timer</code> has a single
thread that runs in the background, watching the clock and executing one
or more <code class="literal">TimerTask</code>s at appropriate
times. You could, for example, schedule a task to run once at a specific
time like this:</p><a id="I_11_tt703"/><pre class="programlisting"> <code class="kn">import</code> <code class="nn">java.util.*</code><code class="o">;</code>
<code class="kd">public</code> <code class="kd">class</code> <code class="nc">Y2K</code> <code class="o">{</code>
<code class="kd">public</code> <code class="kd">static</code> <code class="kt">void</code> <code class="nf">main</code><code class="o">(</code><code class="n">String</code><code class="o">[]</code> <code class="n">args</code><code class="o">)</code> <code class="o">{</code>
<code class="n">Timer</code> <code class="n">timer</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Timer</code><code class="o">();</code>
<code class="n">TimerTask</code> <code class="n">task</code> <code class="o">=</code> <code class="k">new</code> <code class="n">TimerTask</code><code class="o">()</code> <code class="o">{</code>
<code class="kd">public</code> <code class="kt">void</code> <code class="nf">run</code><code class="o">()</code> <code class="o">{</code>
<code class="n">System</code><code class="o">.</code><code class="na">out</code><code class="o">.</code><code class="na">println</code><code class="o">(</code><code class="s">"Y2K!"</code><code class="o">);</code>
<code class="o">}</code>
<code class="o">};</code>
<code class="n">Calendar</code> <code class="n">cal</code> <code class="o">=</code> <code class="k">new</code> <code class="n">GregorianCalendar</code><code class="o">(</code> <code class="mi">2000</code><code class="o">,</code> <code class="n">Calendar</code><code class="o">.</code><code class="na">JANUARY</code><code class="o">,</code> <code class="mi">1</code> <code class="o">);</code>
<code class="n">timer</code><code class="o">.</code><code class="na">schedule</code><code class="o">(</code> <code class="n">task</code><code class="o">,</code> <code class="n">cal</code><code class="o">.</code><code class="na">getTime</code><code class="o">());</code>
<code class="o">}</code>
<code class="o">}</code></pre><p><code class="literal">TimerTask</code> implements the <code class="literal">Runnable</code> interface. To create a task, you can
simply subclass <code class="literal">TimerTask</code> and supply a
<code class="literal">run()</code> method. Here, we’ve created a
simple anonymous subclass of <code class="literal">TimerTask</code>
that prints a message to <code class="literal">System.out</code>.
Using the <a id="I_indexterm11_id741096" class="indexterm"/><code class="literal">schedule()</code> method of
<code class="literal">Timer</code>, we’ve asked that the task be run
on January 1, 2000. If the scheduled time has already passed (as in our
example), the task is run immediately.</p><p>There are some other varieties of <a id="I_indexterm11_id741117" class="indexterm"/><code class="literal">schedule()</code>; you can run
tasks once or at recurring intervals. There are two kinds of recurring
tasks—fixed delay and fixed rate. <span class="emphasis"><em>Fixed delay</em></span> means
that a fixed amount of time elapses between the end of the task’s
execution and the beginning of the next execution. <span class="emphasis"><em>Fixed
rate</em></span> means that the task should begin execution at fixed time
intervals. The difference comes into play when the time to execute the
task is long relative to the interval. Keep in mind that tasks are
executed by the <code class="literal">Timer</code>’s single
scheduler thread. If one task takes a very long time, other tasks may be
delayed, in which case they run as soon as the thread becomes
available.</p><p>You could, for example, update a clock display every second with
code like this:</p><a id="I_11_tt704"/><pre class="programlisting"> <code class="n">Timer</code> <code class="n">timer</code> <code class="o">=</code> <code class="k">new</code> <code class="n">Timer</code><code class="o">();</code>
<code class="n">TimerTask</code> <code class="n">task</code> <code class="o">=</code> <code class="k">new</code> <code class="n">TimerTask</code><code class="o">()</code> <code class="o">{</code>
<code class="kd">public</code> <code class="kt">void</code> <code class="nf">run</code><code class="o">()</code> <code class="o">{</code>
<code class="n">repaint</code><code class="o">();</code> <code class="c1">// update the clock display</code>
<code class="o">}</code>
<code class="o">};</code>
<code class="n">timer</code><code class="o">.</code><code class="na">scheduleAtFixedRate</code><code class="o">(</code> <code class="n">task</code><code class="o">,</code> <code class="mi">0</code><code class="o">,</code> <code class="mi">1000</code> <code class="o">);</code></pre><p>A <code class="literal">TimerTask</code> can be canceled
before its execution with its <a id="I_indexterm11_id741168" class="indexterm"/><code class="literal">cancel()</code>
method.<a id="I_indexterm11_id741178" class="indexterm"/><a id="I_indexterm11_id741185" class="indexterm"/></p></div></body></html>