📄 01-01.html
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<P>There is a problem with single tasking, however: if a certain task is slow or blocking, it can slow or halt the entire execution of a program. Consider code that implements an input monitor for a user-interface library and blocks until the user types a character, as shown below.
</P>
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<!-- CODE //-->
<PRE>
main(int argc, char **argv)
{
int c;
while (1)
{
c = getchar();
if (c == ESC)
break;
ProcessKeyboardInput (c);
ProcessMouseInput();
}
exit (0);
}
</PRE>
<!-- END CODE //-->
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<P><FONT SIZE="+1"><B>Other Examples</B></FONT></P>
<P>Another example of a single-tasking application is a spreadsheet that performs automatic recalculation. The recalculation process may suspend user interaction (with the spreadsheet file or the entire operating system) until the recalculation is complete.
</P>
<P>How can this undesirable situation be alleviated? How can you write an application that allows for particular application events to occur while not freezing the user’s system?</P>
<H4 ALIGN="LEFT"><A NAME="Heading4"></A><FONT COLOR="#000077">“Simulated” Multitasking</FONT></H4>
<P>Back in the “good old days” of programming, application developers utilized a variety of techniques to simulate multitasking in order to avoid blocking waits.
</P>
<P>Among the techniques used to avoid blocking program waits were:</P>
<DL>
<DD><B>•</B> Event loops
<DD><B>•</B> the <B>select()</B> function
<DD><B>•</B> timeouts (using alarms)
<DD><B>•</B> context swapping
</DL>
<P>Writing your own multitasking system can be 1) tedious and time-consuming (making it uneconomical) and 2) nonportable to other operating systems. Solutions have traditionally been proprietary in nature.
</P>
<P>What is necessary and practical is a method of implementing multitasking within an application that is:</P>
<DL>
<DD><B>•</B> portable
<DD><B>•</B> standardized
</DL>
<H4 ALIGN="LEFT"><A NAME="Heading5"></A><FONT COLOR="#000077">Lightweight Processes & Threads</FONT></H4>
<P>UNIX and other operating systems have pioneered the concept of <I>lightweight processes</I>. Lightweight processes are more commonly called <I>threads</I>.</P>
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<TD VALIGN="TOP" WIDTH="5%" ALIGN="LEFT"><IMG SRC="images/01-01i.jpg"></TD>
<TD VALIGN="TOP" ALIGN="LEFT"><B>Thread:</B> also known as a lightweight process, this is a single sequential flow of control within an application or applet process.</TD>
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<P>Multithreaded programs typically perform different tasks simultaneously. This creates the illusion of different parts of an application executing simultaneously on different computers or processors.
</P>
<H3><A NAME="Heading6"></A><FONT COLOR="#000077">Threads & Data Space</FONT></H3>
<P>A system that has a single task with multiple threads is different than a true multitasking system.
</P>
<H4 ALIGN="LEFT"><A NAME="Heading7"></A><FONT COLOR="#000077">What Tasks Contain</FONT></H4>
<P>Each task in a multitasking system contains:
</P>
<DL>
<DD><B>•</B> its own data space
<DD><B>•</B> its own register set
</DL>
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<TD VALIGN="TOP" WIDTH="5%" ALIGN="LEFT"><IMG SRC="images/01-02i.jpg"></TD>
<TD VALIGN="TOP" ALIGN="LEFT">Process switches are slow and expensive.</TD>
</TR>
</TABLE>
<H4 ALIGN="LEFT"><A NAME="Heading8"></A><FONT COLOR="#000077">Thread Characteristics</FONT></H4>
<P>In a multithreaded system, each thread:
</P>
<DL>
<DD><B>•</B> shares the same global data space
<DD><B>•</B> can access the same exact variable in heap memory
<DD><B>•</B> can have private data variables that are allocated on the stack
</DL>
<P><FONT SIZE="+1"><B>Decreased Overhead</B></FONT></P>
<P>In a multithreaded application, each thread runs within the same process space; thus, the overhead of switching between different <I>threads</I> is much less than the overhead of switching between different <I>processes</I> in a multitasking system.</P>
<P>One beneficial aspect of threads is that it is easy for different threads to share global data. In a true multitasking system, it is usually tedious to share data between different processes.</P>
<TABLE WIDTH="90%"><TR>
<TD VALIGN="TOP" WIDTH="5%" ALIGN="LEFT"><IMG SRC="images/01-03i.jpg"></TD>
<TD VALIGN="TOP" ALIGN="LEFT">A system that has a single task with multiple threads is different than a true multitasking system.</TD>
</TR>
</TABLE>
<H3><A NAME="Heading9"></A><FONT COLOR="#000077">Concurrent Execution</FONT></H3>
<P>Each thread in an application can run concurrently. A program typically has a single thread of execution. As the program runs, you can create more threads, as needed. Each thread is typically used to carry out some type of background task.
</P>
<H4 ALIGN="LEFT"><A NAME="Heading10"></A><FONT COLOR="#000077">An Example</FONT></H4>
<P>For example, in the spreadsheet example used earlier, a separate thread could be created which performs recalculations in the background. Another thread could be used to wait for user input.
</P>
<P>The following characteristics apply to threads:</P>
<DL>
<DD><B>•</B> A thread can run continuously in a loop for the entire duration of the program (for example, a thread can continually poll the mouse for movements and button clicks).
<DD><B>•</B> Alternately, a thread can contain a single set of instructions that is executed just once before the thread terminates (such as a one-time spreadsheet recalculation).
</DL>
<TABLE WIDTH="90%">
<TR>
<TD VALIGN="TOP" WIDTH="5%" ALIGN="LEFT"><IMG SRC="images/01-04i.jpg"></TD>
<TD VALIGN="TOP" ALIGN="LEFT">A Java program will run until all nondaemon threads have been terminated.</TD>
</TR>
</TABLE>
<H3><A NAME="Heading11"></A><FONT COLOR="#000077">Writing Threaded Java Apps</FONT></H3>
<H4 ALIGN="LEFT"><A NAME="Heading12"></A><FONT COLOR="#000077">Exercise 1-1: Writing your first threaded Java application</FONT></H4>
<P>The first threaded Java application you will write is a simple counter. This app will count from 1 to 10 and then exit. This file will be a simple console application, run by the stand-alone Java interpreter. Name the file <SMALL>COUNTER1.JAVA</SMALL>, copying the code shown below.</P>
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<TR><TD>
<!-- CODE //-->
<PRE>
class Counter1
{
public static void main(String args[])
{
Counter1Thread t = new Counter1Thread();
t.start();
System.out.println(“main() has finished.”);
}
}
class Counter1Thread extends Thread
{
Counter1Thread()
{
}
public void run()
{
for (int i = 1; i <= 10; i++)
{
System.out.println(“i is ” + i);
{
System.out.println(“The thread is finished”);
}
}
</PRE>
<!-- END CODE //-->
</TD>
</TR>
</TABLE>
<P>The output from the <SMALL>COUNTER1.JAVA</SMALL> program is as follows:</P>
<TABLE BORDER="2" BORDERCOLOR="#0000" ALIGN="CENTER">
<TR><TD>
<!-- CODE //-->
<PRE>
main () has finished.
i is 1
i is 2
i is 3
i is 4
i is 5
i is 6
i is 7
i is 8
i is 9
i is 10
The thread is finished
</PRE>
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</TD>
</TR>
</TABLE>
<H4 ALIGN="LEFT"><A NAME="Heading13"></A><FONT COLOR="#000077">Exercise 1-2: Writing a Java app with two threads</FONT></H4>
<P>As a variation on the first application, the following exercise adds another thread. Each thread in the application will count from 1 to 10 and then exit. Name this app <SMALL>COUNTER2.JAVA</SMALL>.</P>
<TABLE BORDER="2" BORDERCOLOR="#0000" ALIGN="CENTER">
<TR><TD>
<!-- CODE //-->
<PRE>
class Counter2
{
public static void main(String args[])
{
Counter2Thread t1 = new Counter2Thread (“First Thread”);
Counter2Thread t2 = new Counter2Thread (“Second Thread”);
t1.start();
t2.start();
System.out.println(“main() has finished.”);
}
}
class Counter2Thread extends Thread
{
Counter2Thread(String szName)
{
super (szName);
}
public void run()
{
for (int i = 1; i <= 10; i++)
{
System.out.println(getName() + “:\ti is ” + i);
}
System.out.println(getName() + “ is finished”);
}
}
</PRE>
<!-- END CODE //-->
</TD>
</TR>
</TABLE>
<P>The output from this program is as follows:
</P>
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<TR><TD>
<!-- CODE //-->
<PRE>
main () has finished.
First Thread: i is 1
First Thread: i is 2
First Thread: i is 3
First Thread: i is 4
First Thread: i is 5
First Thread: i is 6
First Thread: i is 7
First Thread: i is 8
First Thread: i is 9
First Thread: i is 10
Second Thread: i is 1
Second Thread: i is 2
Second Thread: i is 3
Second Thread: i is 4
Second Thread: i is 5
Second Thread: i is 6
Second Thread: i is 7
Second Thread: i is 8
Second Thread: i is 9
Second Thread: i is 10
Second Thread is finished
First Thread is finished
</PRE>
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</TD>
</TR>
</TABLE>
<P><FONT SIZE="+1"><B>Starvation</B></FONT></P>
<P>This Java application illustrates one of the interesting problems with multiple threads—<I>starvation</I>. The subject of starvation will be discussed later in this course.</P><P><BR></P>
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