tij0159.html

学习java的经典书籍

</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>implements
Runnable
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
holds a number of color blocks and randomly chooses ones to update. Then a
number of these 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects are created, depending roughly on the grid dimension you choose. As a
result, you have far fewer threads than color blocks, so if there&#8217;s a
speedup we&#8217;ll know it was because there were too many threads in the
previous example:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: ColorBoxes2.java</font>
<font color="#009900">// Balancing thread use</font>
<font color="#0000ff">import</font> java.awt.*;
<font color="#0000ff">import</font> java.awt.event.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> CBox2 <font color="#0000ff">extends</font> Canvas {
  <font color="#0000ff">private</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> Color[] colors = { 
    Color.black, Color.blue, Color.cyan, 
    Color.darkGray, Color.gray, Color.green,
    Color.lightGray, Color.magenta, 
    Color.orange, Color.pink, Color.red, 
    Color.white, Color.yellow 
  };
  <font color="#0000ff">private</font> Color cColor = newColor();
  <font color="#0000ff">private</font> <font color="#0000ff">static</font> <font color="#0000ff">final</font> Color newColor() {
    <font color="#0000ff">return</font> colors[
      (<font color="#0000ff">int</font>)(Math.random() * colors.length)
    ];
  }
  <font color="#0000ff">void</font> nextColor() {
    cColor = newColor();
    repaint();
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> paint(Graphics  g) {
    g.setColor(cColor);
    Dimension s = getSize();
    g.fillRect(0, 0, s.width, s.height);
  }
}

<font color="#0000ff">class</font> CBoxVector 
  <font color="#0000ff">extends</font> Vector <font color="#0000ff">implements</font> Runnable {
  <font color="#0000ff">private</font> Thread t;
  <font color="#0000ff">private</font> <font color="#0000ff">int</font> pause;
  <font color="#0000ff">public</font> CBoxVector(<font color="#0000ff">int</font> pause) {
    <font color="#0000ff">this</font>.pause = pause;
    t = <font color="#0000ff">new</font> Thread(<font color="#0000ff">this</font>);
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> go() { t.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">int</font> i = (<font color="#0000ff">int</font>)(Math.random() * size());
      ((CBox2)elementAt(i)).nextColor();
      <font color="#0000ff">try</font> {
        t.sleep(pause);
      } <font color="#0000ff">catch</font>(InterruptedException e) {}
    } 
  }
}

<font color="#0000ff">public</font> <font color="#0000ff">class</font> ColorBoxes2 <font color="#0000ff">extends</font> Frame {
  <font color="#0000ff">private</font> CBoxVector[] v;
  <font color="#0000ff">public</font> ColorBoxes2(<font color="#0000ff">int</font> pause, <font color="#0000ff">int</font> grid) {
    setTitle("ColorBoxes2");
    setLayout(<font color="#0000ff">new</font> GridLayout(grid, grid));
    v = <font color="#0000ff">new</font> CBoxVector[grid];
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; grid; i++)
      v[i] = <font color="#0000ff">new</font> CBoxVector(pause);
    <font color="#0000ff">for</font> (<font color="#0000ff">int</font> i = 0; i &lt; grid * grid; i++) {
      v[i % grid].addElement(<font color="#0000ff">new</font> CBox2());
      add((CBox2)v[i % grid].lastElement());
    }
    <font color="#0000ff">for</font>(<font color="#0000ff">int</font> i = 0; i &lt; grid; i++)
      v[i].go();
    addWindowListener(<font color="#0000ff">new</font> WindowAdapter() {
      <font color="#0000ff">public</font> <font color="#0000ff">void</font> windowClosing(WindowEvent e) {
        System.exit(0);
      }
    });
  }   
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#009900">// Shorter default pause than ColorBoxes:</font>
    <font color="#0000ff">int</font> pause = 5;
    <font color="#0000ff">int</font> grid = 8;
    <font color="#0000ff">if</font>(args.length &gt; 0) 
      pause = Integer.parseInt(args[0]);
    <font color="#0000ff">if</font>(args.length &gt; 1)
      grid = Integer.parseInt(args[1]);
    Frame f = <font color="#0000ff">new</font> ColorBoxes2(pause, grid);
    f.setSize(500, 400);
    f.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">In
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ColorBoxes2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an array of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is created and initialized to hold 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>grid</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s,
each of which knows how long to sleep. An equal number of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Cbox2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
objects is then added to each 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and each vector is told to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>go(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
which starts its thread.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBox2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is similar to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBox</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
it paints itself with a randomly-chosen color. But that&#8217;s 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>all</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBox2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
does. All of the threading has been moved into 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
could also have inherited 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and had a member object of type 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
That design has the advantage that the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>addElement(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
methods could then be given specific argument and return value types instead of
generic 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Object</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">s.
(Their names could also be changed to something shorter.) However, the design
used here seemed at first glance to require less code. In addition, it
automatically retains all the other behaviors of a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Vector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
With all the casting and parentheses necessary for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>elementAt(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
this might not be the case as your body of code grows.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
before, when you implement 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Runnable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
you don&#8217;t get all of the equipment that comes with 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so you have to create a new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Thread</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and hand yourself to its constructor in order to have something to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>start(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
as you can see in the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>CBoxVector</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
constructor and in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>go(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
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 simply chooses a random element number within the vector and calls 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>nextColor(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for that element to cause it to choose a new randomly-selected color.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Upon
running this program, you see that it does indeed run faster and respond more
quickly (for instance, when you interrupt it, it stops more quickly), and it
doesn&#8217;t seem to bog down as much at higher grid sizes. Thus, a new factor
is added into the threading equation: you must watch to see that you
don&#8217;t have &#8220;too many threads&#8221; (whatever that turns out to
mean for your particular program and platform). If you do, you must try to use
techniques like the one above to &#8220;balance&#8221; the number of threads in
your program. If you see performance problems in a multithreaded program you
now have a number of issues to examine:
</FONT><P></DIV>
<OL>
<LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Do
you have enough calls to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sleep(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>yield(&#160;),</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and/or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>wait(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">?</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Are
calls to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>sleep(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
long enough?
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Are
you running too many threads?
</FONT><LI><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">	Have
you tried different platforms and JVMs?
</FONT></OL><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Issues
like this are one reason that multithreaded programming is often considered an
art.
</FONT><a name="_Toc375545489"></a><a name="_Toc408018762"></a><P></DIV>

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