tij0155.html

学习java的经典书籍

    <font color="#0000ff">while</font>(<font color="#0000ff">true</font>) {
      System.out.println("Thread " + 
        threadNumber + "(" + countDown + ")");
      <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("All Threads Started");
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>return</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">).
Often, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is cast in the form of an infinite loop, which means that, barring some
external call to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>stop(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>destroy(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for that thread, it will run forever (until the program completes).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">you
can see a number of threads being created and run. The special method that
comes with the <A NAME="Index2461"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
class is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which performs special initialization for the thread and then calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
So the steps are: the constructor is called to build the object, then 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
configures the thread and calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
If you don&#8217;t call 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(which you can do in the constructor, if that&#8217;s appropriate) the thread
will never be started.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output for one run of this program (it will be different every time) is:
</FONT><P></DIV>

<font color="#990000"><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><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You&#8217;ll
notice that nowhere in this example is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sleep(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sleep(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
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><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can also see that the <A NAME="Index2462"></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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">&#8217;s
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>setPriority(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">When
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
creates the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
&#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><P></DIV>
<A NAME="Heading484"></A><H3 ALIGN=LEFT>
Threading
for a responsive interface
<P><A NAME="Index2463"></A></H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Now
it&#8217;s possible to solve the problem in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Counter1.java
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">with
a thread. The trick is to place the subtask &#8211; that is, the loop
that&#8217;s inside 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>go(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
&#8211; inside the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>run(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
method of a thread. When the user presses the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
button, the thread is started, but then the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>creation</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
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><P></DIV>

<font color="#990000"><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("Toggle"),
    start = <font color="#0000ff">new</font> Button("Start");
  <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("Counter2");
    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">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Counter2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is now a straightforward program, whose job is only to set up and maintain the
user interface. But now, when the user presses the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
button, a method is not called. Instead a thread of class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>SeparateSubTask</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is created (the constructor starts it, in this case), and then the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Counter2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">