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SHAPE ="RECT" HREF ="/reference/author.html" COORDS ="361,0,440,29"><AREA SHAPE ="RECT" HREF ="/reference/basic.html" COORDS ="254,0,360,29"><AREA SHAPE ="RECT" HREF ="/reference/help.html" COORDS ="183,0,251,29"><AREA SHAPE ="RECT" HREF ="/reference/cool.html" COORDS ="100,0,181,29"><AREA SHAPE ="RECT" HREF ="/reference/mybook.html" COORDS ="0,0,100,29"></MAP><MAP NAME ="header_r1.gif.map"><AREA SHAPE ="RECT" HREF ="/reference/personalize.html" COORDS ="339,0,439,29"></MAP><p><font face="Arial,Helvetica" size="-1" color="#006666"><A HREF="/reference/r_library.html"><B>All Categories</A> :</B><b><A HREF="/reference/r_java.html">Java</A></b></font><p><META><H1><FONT SIZE=6 COLOR=#Ff0000>Chapter 31</FONT></H1><H1><FONT SIZE=6 COLOR=#Ff0000>Threads</FONT></H1><HR><P><CENTER><B><FONT SIZE=5><A NAME="CONTENTS">CONTENTS</A></FONT></B></CENTER><UL><LI><A HREF="#TwoKindsofThreads">Two Kinds of Threads</A><LI><A HREF="#ConvertingaClasstoaThread">Converting a Class to a Thread</A><UL><LI><A HREF="#DeclaringtheClassasImplementingtheIRunnableIInterface">Declaring the Class as Implementing the <I>Runnable</I> Interface</A><LI><A HREF="#ImplementingtheIrunIMethod">Implementing the <I>run()</I> Method</A><LI><A HREF="#DeclaringaIThreadIObject">Declaring a <I>Thread</I> Object</A><LI><A HREF="#CreatingandStartingtheIThreadIObject">Creating and Starting the <I>Thread</I> Object</A><LI><A HREF="#StoppingtheThread">Stopping the Thread</A><LI><A HREF="#ExampleUsingaThreadinanApplet">Example: Using a Thread in an Applet</A></UL><LI><A HREF="#DerivingaClassfromIThreadI">Deriving a Class from <I>Thread</I></A><UL><LI><A HREF="#ExampleCreatingaThreadClass">Example: Creating a Thread Class</A><LI><A HREF="#ExampleUsingaSeparateThreadinanApplet">Example: Using a Separate Thread in an Applet</A></UL><LI><A HREF="#SynchronizingMultipleThreads">Synchronizing Multiple Threads</A><UL><LI><A HREF="#ExampleUsingaSynchronizedMethod">Example: Using a Synchronized Method</A><LI><A HREF="#UnderstandingThreadApplet">Understanding ThreadApplet3</A></UL><LI><A HREF="#Summary">Summary</A><LI><A HREF="#ReviewQuestions">Review Questions</A><LI><A HREF="#ReviewExercises">Review Exercises</A></UL><HR><P>When using Windows 95 (and other modern operating systems), youknow that you can run several programs simultaneously. This abilityis called multitasking. What you may not know is that many oftoday's operating systems also allow threads, which are separateprocesses that are kind of a step down from a complete application.A thread is a lot like a subprogram. An applet can create severalthreads-several different flows of execution-and run them concurrently.This is a lot like having multitasking inside multitasking. Theuser knows that he or she can run several applications at a time.The programmer knows that each application can run several threadsat a time. In this chapter, you'll learn how to create and managethreads in your own applets.<H2><A NAME="TwoKindsofThreads"><FONT SIZE=5 COLOR=#Ff0000>Two Kinds of Threads</FONT></A></H2><P>In Java, you can create threads in a couple of ways. The simplestway is to take an existing class and turn it into a thread. Youdo this by modifying the class so that it implements the <TT>Runnable</TT>interface, which declares the <TT>run()</TT> method required byall types of threads. (The <TT>run()</TT>method contains the codeto be executed by a thread.) In the previous chapter, you learnedhow interfaces in Java enable you to add capabilities to classessimply by implementing the interface in that class. Now, you geta chance to put that idea to work for real.<P>The second way to create a thread is to write a completely separateclass derived from Java's <TT>Thread</TT> class. Because the <TT>Thread</TT>class itself implements the <TT>Runnable</TT> interface, it alreadycontains a <TT>run()</TT> method. However, <TT>Thread</TT>'s <TT>run()</TT>method doesn't do anything. You usually have to override the methodin your own class in order to create the type of thread you want.<H2><A NAME="ConvertingaClasstoaThread"><FONT SIZE=5 COLOR=#Ff0000>Converting a Class to a Thread</FONT></A></H2><P>As I mentioned in the preceding section, the first way to createa thread is to convert a class to a thread. To do this, you mustperform several steps, as listed here:<OL><LI>Declare the class as implementing the <TT>Runnable</TT> interface.<LI>Implement the <TT>run()</TT> method.<LI>Declare a <TT>Thread</TT> object as a data field of the class.<LI>Create the <TT>Thread</TT> object and call its <TT>start()</TT>method.<LI>Call the thread's <TT>stop()</TT> method to destroy the thread.</OL><P>The following sections look at each of these steps in detail.<H3><A NAME="DeclaringtheClassasImplementingtheIRunnableIInterface">Declaring the Class as Implementing the <I>Runnable</I> Interface</A></H3><P>As you can see in step 1 in the preceding section, to create athread from a regular class, the class must first be declaredas implementing the <TT>Runnable</TT> interface. For example,if your class is declared as<BLOCKQUOTE><PRE>public class MyApplet extends Applet </PRE></BLOCKQUOTE><P>you must change that declaration to<BLOCKQUOTE><PRE>public class MyApplet extends Applet implements Runnable</PRE></BLOCKQUOTE><H3><A NAME="ImplementingtheIrunIMethod">Implementing the <I>run()</I> Method</A></H3><P>Now, because you've told Java you're about to implement an interface,you must implement every method in the interface. In the caseof <TT>Runnable</TT>, that's easy because there's only one method,<TT>run()</TT>, the basic implementation of which looks like this:<BLOCKQUOTE><PRE>public void run(){}</PRE></BLOCKQUOTE><P>When you start your new thread, Java calls the thread's <TT>run()</TT>method, so it is in <TT>run()</TT> where all the action takesplace. The preceding example of the <TT>run()</TT> method is theminimum you need to compile the new source code for the thread.However, in a real program, you'll add code to <TT>run()</TT>so that the thread does what you want it to do.<H3><A NAME="DeclaringaIThreadIObject">Declaring a <I>Thread</I> Object</A></H3><P>The next step is to declare a <TT>Thread</TT> object as a datafield of the class, like this:<BLOCKQUOTE><PRE>Thread thread;</PRE></BLOCKQUOTE><P>The <TT>thread</TT> object will hold a reference to the threadwith which the applet is associated. You will be able to accessthe thread's methods through this object.<H3><A NAME="CreatingandStartingtheIThreadIObject">Creating and Starting the <I>Thread</I> Object</A></H3><P>Now it's time to write the code that creates the thread and getsit going. Assuming that your new threaded class is an applet,you'll often want to create and start the thread in the applet's<TT>start()</TT> method, as shown in Listing 31.1.<HR><BLOCKQUOTE><B>Listing 31.1 LST31_1.TXT: Creating and Startinga </B><I>Thread</I><B> Object.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>public void start(){ thread = new Thread(this); thread.start();}</PRE></BLOCKQUOTE><HR><P><CENTER><TABLE BORDER=1 WIDTH=80%><TR VALIGN=TOP><TD><B>NOTE</B></TD></TR><TR VALIGN=TOP><TD><BLOCKQUOTE>Back in <A HREF="ch15.htm" >Chapter 15</A>, "Writing a Simple Applet," you learned that <TT>start()</TT> is the method that represents the applet's second life-cycle stage. Java calls your applet's life-cycle methods in this order: <TT>init()</TT>, <TT>start()</TT>, <TT>paint()</TT>, <TT>stop()</TT>, and <TT>destroy()</TT>. Java calls the <TT>start()</TT> method whenever the applet needs to start running, usually when it's first loaded or when the user has switched back to the applet from another Web page.</BLOCKQUOTE></TD></TR></TABLE></CENTER><P><P>Look at the call to the <TT>Thread</TT> constructor in Listing31.1. Notice that the constructor's single argument is the applet's<TT>this</TT> reference. This is how Java knows with which classto associate the thread. Right after the call to the constructor,the applet calls the <TT>Thread</TT> object's <TT>start()</TT>method, which starts the thread running. When the thread startsrunning, Java calls the thread's <TT>run()</TT> method, wherethe thread's work gets done.<H3><A NAME="StoppingtheThread">Stopping the Thread</A></H3><P>When the thread's <TT>run()</TT> method ends, so does the thread.However, because threads tend to run for quite a while, controllingthings like animation in the applet, the user is likely to switchaway from your applet before the thread stops. In this case. it'sup to your applet to stop the thread. Because Java calls an applet's<TT>stop()</TT> method whenever the user switches away from theapplet, the <TT>stop()</TT> method is a good place to stop thethread, as shown in Listing 31.2.<HR><BLOCKQUOTE><B>Listing 31.2 LST31_2.TXT: Stopping a Thread.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>public void stop(){ thread.stop();}</PRE></BLOCKQUOTE><HR><H3><A NAME="ExampleUsingaThreadinanApplet">Example: Using a Thread in an Applet</A></H3><P>To understand about threads, you really have to dig in and usethem. So in this section, you'll put together an applet that associatesitself with a <TT>Thread</TT> object and runs the thread to controla very simple animated display. The animation in this case isnot a bunch of space invaders landing to take over the earth,but rather only a changing number that proves that the threadis running. Listing 31.3 is the applet in question, which is calledThreadApplet. Figure 31.1 shows the applet running under Appletviewer.<P><A HREF="f31-1.gif"><B> Figure 31.1 : </B><I>ThreadApplet uses a thread to count to 1,000.</I></A><P><HR><BLOCKQUOTE><B>Listing 31.3 ThreadApplet.java: Using a Thread inan Applet.<BR></B></BLOCKQUOTE><BLOCKQUOTE><PRE>import java.awt.*;import java.applet.*;public class ThreadApplet extends Applet implements Runnable{ Thread thread; int count;⌨️ 快捷键说明
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