chapter14.html

java 是一个很好的网络开发环境。由于它是通过解释的方法

  <font color=#0000ff>private</font> <font color=#0000ff>int</font> threadNumber;
  <font color=#0000ff>private</font> <font color=#0000ff>static</font> <font color=#0000ff>int</font> threadCount = 0;
  <font color=#0000ff>public</font> SimpleThread() {
    threadNumber = ++threadCount;
    System.out.println(<font color=#004488>"Making "</font> + threadNumber);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> run() {
    <font color=#0000ff>while</font>(<font color=#0000ff>true</font>) {
      System.out.println(<font color=#004488>"Thread "</font> + 
        threadNumber + <font color=#004488>"("</font> + countDown + <font color=#004488>")"</font>);
      <font color=#0000ff>if</font>(--countDown == 0) <font color=#0000ff>return</font>;
    }
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    <font color=#0000ff>for</font>(<font color=#0000ff>int</font> i = 0; i &lt; 5; i++)
      <font color=#0000ff>new</font> SimpleThread().start();
    System.out.println(<font color=#004488>"All Threads Started"</font>);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">A <B>run(&#160;)</B> method
virtually always has some kind of loop that continues until the thread is no
longer necessary, so you must establish the condition on which to break out of
this loop (or, in the case above, simply <B>return</B> from <B>run(&#160;)</B>).
Often, <B>run(&#160;)</B> is cast in the form of an infinite loop, which means
that, barring some external call to <B>stop(&#160;)</B> or
<B>destroy(&#160;)</B> for that thread, it will run forever (until the program
completes).</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">In <B>main(&#160;) </B>you can see
a number of threads being created and run. The special method that comes with
the <A NAME="Index2449"></A><B>Thread</B> class is <B>start(&#160;)</B>, which
performs special initialization for the thread and then calls
<B>run(&#160;)</B>. So the steps are: the constructor is called to build the
object, then <B>start(&#160;)</B> configures the thread and calls
<B>run(&#160;)</B>. If you don&#8217;t call <B>start(&#160;)</B> (which you can
do in the constructor, if that&#8217;s appropriate) the thread will never be
started.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output for one run of this
program (it will be different every time) is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>Making 1
Making 2
Making 3
Making 4
Making 5
Thread 1(5)
Thread 1(4)
Thread 1(3)
Thread 1(2)
Thread 2(5)
Thread 2(4)
Thread 2(3)
Thread 2(2)
Thread 2(1)
Thread 1(1)
All Threads Started
Thread 3(5)
Thread 4(5)
Thread 4(4)
Thread 4(3)
Thread 4(2)
Thread 4(1)
Thread 5(5)
Thread 5(4)
Thread 5(3)
Thread 5(2)
Thread 5(1)
Thread 3(4)
Thread 3(3)
Thread 3(2)
Thread 3(1)</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You&#8217;ll notice that nowhere in
this example is <B>sleep(&#160;)</B> called, and yet the output indicates that
each thread gets a portion of the CPU&#8217;s time in which to execute. This
shows that <B>sleep(&#160;)</B>, while it relies on the existence of a thread in
order to execute, is not involved with either enabling or disabling threading.
It&#8217;s simply another method.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can also see that the
<A NAME="Index2450"></A>threads are not run in the order that they&#8217;re
created. In fact, the order that the CPU attends to an existing set of threads
is indeterminate, unless you go in and adjust the priorities using
<B>Thread</B>&#8217;s <B>setPriority(&#160;)</B> method.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">When <B>main(&#160;)</B> creates
the <B>Thread</B> objects it isn&#8217;t capturing the handles for any of them.
An ordinary object would be fair game for garbage collection, but not a
<B>Thread</B>. Each <B>Thread</B> &#8220;registers&#8221; itself so there is
actually a reference to it someplace and the garbage collector can&#8217;t clean
it up.</FONT><A NAME="_Toc375545474"></A><A NAME="_Toc408018747"></A><BR></P></DIV>
<A NAME="Heading483"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
Threading for a responsive interface<BR><A NAME="Index2451"></A></H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Now it&#8217;s possible to solve
the problem in <B>Counter1.java </B>with a thread. The trick is to place the
subtask &#8211; that is, the loop that&#8217;s inside <B>go(&#160;)</B> &#8211;
inside the <B>run(&#160;)</B> method of a thread. When the user presses the
<B>start</B> button, the thread is started, but then the <I>creation</I> of the
thread completes, so even though the thread is running, the main job of the
program (watching for and responding to user-interface events) can continue.
Here&#8217;s the solution:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Counter2.java</font>
<font color=#009900>// A responsive user interface with threads</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.awt.event.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>class</font> SeparateSubTask <font color=#0000ff>extends</font> Thread {
  <font color=#0000ff>private</font> <font color=#0000ff>int</font> count = 0;
  <font color=#0000ff>private</font> Counter2 c2;
  <font color=#0000ff>private</font> <font color=#0000ff>boolean</font> runFlag = <font color=#0000ff>true</font>;
  <font color=#0000ff>public</font> SeparateSubTask(Counter2 c2) {
    <font color=#0000ff>this</font>.c2 = c2;
    start();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> invertFlag() { runFlag = !runFlag;}
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> run() {
    <font color=#0000ff>while</font> (<font color=#0000ff>true</font>) {
     <font color=#0000ff>try</font> {
      sleep(100);
     } <font color=#0000ff>catch</font> (InterruptedException e){}
     <font color=#0000ff>if</font>(runFlag) 
       c2.t.setText(Integer.toString(count++));
    }
  }
} 

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Counter2 <font color=#0000ff>extends</font> Applet {
  TextField t = <font color=#0000ff>new</font> TextField(10);
  <font color=#0000ff>private</font> SeparateSubTask sp = <font color=#0000ff>null</font>;
  <font color=#0000ff>private</font> Button 
    onOff = <font color=#0000ff>new</font> Button(<font color=#004488>"Toggle"</font>),
    start = <font color=#0000ff>new</font> Button(<font color=#004488>"Start"</font>);
  <font color=#0000ff>public</font> <font color=#0000ff>void</font> init() {
    add(t);
    start.addActionListener(<font color=#0000ff>new</font> StartL());
    add(start);
    onOff.addActionListener(<font color=#0000ff>new</font> OnOffL());
    add(onOff);
  }
  <font color=#0000ff>class</font> StartL <font color=#0000ff>implements</font> ActionListener {
    <font color=#0000ff>public</font> <font color=#0000ff>void</font> actionPerformed(ActionEvent e) {
      <font color=#0000ff>if</font>(sp == <font color=#0000ff>null</font>)
        sp = <font color=#0000ff>new</font> SeparateSubTask(Counter2.<font color=#0000ff>this</font>);
    }
  }
  <font color=#0000ff>class</font> OnOffL <font color=#0000ff>implements</font> ActionListener {
    <font color=#0000ff>public</font> <font color=#0000ff>void</font> actionPerformed(ActionEvent e) {
      <font color=#0000ff>if</font>(sp != <font color=#0000ff>null</font>)
        sp.invertFlag();
    }
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    Counter2 applet = <font color=#0000ff>new</font> Counter2();
    Frame aFrame = <font color=#0000ff>new</font> Frame(<font color=#004488>"Counter2"</font>);
    aFrame.addWindowListener(
      <font color=#0000ff>new</font> WindowAdapter() {
        <font color=#0000ff>public</font> <font color=#0000ff>void</font> windowClosing(WindowEvent e) {
          System.exit(0);
        }
      });
    aFrame.add(applet, BorderLayout.CENTER);
    aFrame.setSize(300,200);
    applet.init();
    applet.start();
    aFrame.setVisible(<font color=#0000ff>true</font>);
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia"><B>Counter2</B> is now a
straightforward program, whose job is only to set up and maintain the user
interface. But now, when the user presses the <B>start</B> button, a method is
not called. Instead a thread of class <B>SeparateSubTask</B> is created (the
constructor starts it, in this case), and then the <B>Counter2</B> event loop
continues. Note that the handle to the <B>SeparateSubTask</B> is stored so that
when you press the <B>onOff</B> button it can toggle the <B>runFlag</B> inside
the <B>SeparateSubTask</B> object. That thread (when it looks at the flag) can
then start and stop itself. (This could also have been accomplished by making
<B>SeparateSubTask</B> an inner class.)</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The class <B>SeparateSubTask</B> is
a simple extension of <B>Thread</B> with a constructor (that stores the
<B>Counter2</B> handle and then runs the thread by calling <B>start(&#160;)</B>)
and a <B>run(&#160;)</B> that essentially contains the code from inside
<B>go(&#160;)</B> in <B>Counter1.java</B>. Because <B>SeparateSubTask</B> knows
that it holds a handle to a <B>Counter2</B>, it can reach in and access
<B>Counter2</B>&#8217;s <B>TextField</B> when it needs to.</FONT><BR></P></DIV>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">When you press the <B>onOff</B>
button, you&#8217;ll see a virtually instant response. Of course, the response
isn&#8217;t really instant, not like that of a system that&#8217;s driven by
interrupts. The counter stops only when the thread has the CPU and notices that
the flag has changed.</FONT><A NAME="_Ref403700451"></A><BR></P></DIV>
<A NAME="Heading484"></A><FONT FACE = "Verdana"><H4 ALIGN="LEFT">
Improving the code with an inner class</H4></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">As an aside, look at the coupling
that occurs between the <B>SeparateSubTask</B> and <B>Counter2</B> classes. The
<B>SeparateSubTask</B> is intimately tied to <B>Counter2</B> &#8211; it must
keep a handle to its &#8220;parent&#8221; <B>Counter2</B> object so it can call
back and manipulate it. And yet the two classes shouldn&#8217;t really merge
together into a single class (although in the next section you&#8217;ll see that
Java provides a way to combine them) because they&#8217;re doing separate things
and are created at different times. They are tightly connected (what I call a
&#8220;<A NAME="Index2452"></A>couplet&#8221;) and this makes the coding
awkward. This is a situation in which an
<A NAME="Index2453"></A><A NAME="Index2454"></A><A NAME="Index2455"></A>inner
class can improve the code significantly:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: Counter2i.java</font>
<font color=#009900>// Counter2 using an inner class for the thread</font>
<font color=#0000ff>import</font> java.awt.*;
<font color=#0000ff>import</font> java.awt.event.*;
<font color=#0000ff>import</font> java.applet.*;

<font color=#0000ff>public</font> <font color=#0000ff>class</font> Counter2i <font color=#0000ff>extends</font> Applet {
  <font color=#0000ff>private</font> <font color=#0000ff>class</font> SeparateSubTask <font color=#0000ff>extends</font> Thread {
    <font color=#0000ff>int</font> count = 0;
    <font color=#0000ff>boolean</font> runFlag = <font color=#0000ff>true</font>;
    SeparateSubTask() { start(); }
    <font color=#0000ff>public</font> <font color=#0000ff>void</font> run() {
      <font color=#0000ff>while</font> (<font color=#0000ff>true</font>) {
       <font color=#0000ff>try</font> {
        sleep(100);
       } <font color=#0000ff>catch</font> (InterruptedException e){}
       <font color=#0000ff>if</font>(runFlag) 
         t.setText(Integer.toString(count++));
      }
    }
  } 
  <font color=#0000ff>private</font> SeparateSubTask sp = <font color=#0000ff>null</font>;