第十五章 Java多线程机制

本章内容❏

线程的基本概念❏

线程的创建和启动❏

线程的调度和优先级❏

线程的状态控制❏

线程的应用线程的基本概念线程是一个程序内部的顺序控制流。线程和进程的区别每个进程都有独立的代码和数据空间(进程上下文)，进程间的切换会有较大的开销。线程可以看成时轻量级的进程，同一类线程共享代码和数据空间，每个线程有独立的运行栈和程序计数器(PC)，线程切换的开销小。多进程:在操作系统中能同时运行多个任务(程序)多线程:在同一应用程序中有多个顺序流同时执行线程的基本概念Java的线程是通过java.lang.Thread类来实现的。VM

启动时会有一个由主方法（public

staticvoid

main(){}）所定义的线程。可以通过创建Thread

的实例来创建新的线程。每个线程都是通过某个特定Thread对象所对应的方法run()来完成其操作的，方法run()称为线程体。通过调用Thead类的start()方法来启动一个线程。线程的创建和启动可以有两种方式创建新的线程。ThreadmyThread

＝newThead（target）其中target为java.lang.Runnable接口类型。

Runnable中只有一个方法：public

void

run（）;用以定义线程运行体。可以定义一个Thread的子类并重写其run方法如：class

MyThread

extends

Thead

{public

void

run(){…}}然后生成该类的对象：MyThread

myThread＝new

MyThead(…)线程的创建和启动举例public

class

Test

{publicstatic

void

main(String[]

args)

{MyThread

thread1

=

new

MyThread("thread1");MyThread

thread2

=

new

MyThread("thread2");MyThread

thread3

=

new

MyThread("thread3");thread1.start();

thread2.start();thread3.start();}}class

MyThread

extends

Thread

{MyThread(String

name){super(name);}public

void

run(){for(int

i

=1;i<=1000;i++){System.out.println("i

am

"+getName()+".");}}}线程的创建和启动举例public

class

Test

{publicstatic

void

main(String[]

args)

{Thread

thread1

=

new

Thread(new

Runner("thread1"));Thread

thread2

=

new

Thread(new

Runner("thread2"));Thread

thread3

=

new

Thread(new

Runner("thread3"));thread1.start();

thread2.start();thread3.start();}}class

Runner

implements

Runnable

{private

String

name;Runner(String

name){

=

name;}public

void

run(){for(int

i

=1;i<=1000;i++){System.out.println("i

am

"+name+".");}}}课堂练习分别使用两种方法创建一个线程，在屏幕上打印出1～20的阶乘。import

java.util.*;public

class

Test

{public

static

void

main(String[]

args)

{MyThread

addThread

=new

MyThread("addThread",MyThread.ADD);MyThread

subThread

=new

MyThread("subThread",MyThread.SUB);MyThread

mulThread

=new

MyThread("mulThread",MyThread.MUL);addThread.start();subThread.start();mulThread.start();}}线程的创建举例1class

MyThread

extends

Thread

{public

static

final

int

ADD

=

1;public

static

final

int

SUB

=

2;public

static

final

int

MUL

=

3;private

int

operator

=

ADD;privateString

threadName

=

null;MyThread(String

name,

int

op)

{super(name);

operator

=

op;}public

voidrun(){Random

random

=

new

Random();for

(int

i

=

1;

i

<=

1000;

i++)

{int

x

=

random.nextInt(100);

int

y

=

random.nextInt(100);

switch

(operator)

{case

ADD

:System.out.println(getName()

+

":

"+x+"+"+y+"="+(x

+

y));

break;case

SUB

:System.out.println(getName()

+

":"

+x+"-"+y+"="+(x

-

y));

break;case

MUL

:

System.out.println(getName()

+

":

"+x+"*"+y+"="+(x

*

y));

break;default

:

System.out.println("Can"t

support");}}}}线程的创建举例1（续）课堂练习使用Runnable接口的方法，实现上面的例子。使用Runnable在实现Runnable接口的类的run方法定义中可以使用

Thread的静态方法：public

static

Thread

currentThread()获取当前线程的引用。Runnable举例public

class

Test

{publicstatic

void

main(String[]

args)

{String[]

p

=

{"Mircosoft","IBM","Sun","Apple","HP"};Products

products

=

new

Products(p);Thread

upper

=

new

Thread(products,"upper");Thread

lower

=

new

Thread(products,"lower");upper.start();

lower.start();}}class

Products

implements

Runnable

{String[]

p;Products(String[]

p){this.p

=

p;}public

void

run(){String

name

=

Thread.currentThread().getName();if(name.equals("upper")){for(int

i=0;i<p.length;i++){System.out.println(p[i].toUpperCase());}}else

if(name.equals("lower")){for(int

i=0;i<p.length;i++){System.out.println(p[i].toLowerCase());}}}}线程状态转换方法功

能isAlive()判断线程是否还“活”着，即线程是否还未终止。getPriority()获得线程的优先级数值setPriority()设置线程的优先级数值Thread.sleep()将当前线程睡眠指定毫秒数join()调用某线程的该方法，将当前线程与该线程“合并”，即等待该线程结束，再恢复当前线程的运行。yield()让出CPU，当前线程进入就绪队列等待调度。wait()当前线程进入对象的wait

pool。notify()/notifyAll()唤醒对象的wait

pool中的一个/所有等待线程。线程控制基本方法线程的优先级别Java提供一个线程调度器来监控程序中启动后进入就绪状态的所有线程。线程调度器按照线程的优先级决定应调度哪个线程来执行。线程的优先级用数字来表示，范围从1到10，一个线程的缺省优先级是5。Thread.MIN_PRIORITY

=

1Thread.MAX_PRIORITY

=

10Thread.NORM_PRIORITY

=

5使用下述线方法获得或设置线程对象的优先级。int

getPriority();void

setPriority(int

newPriority);import

java.awt.*;public

class

Test

{public

static

voidmain(String[]

args)

{Frame

f

=

new

Frame("thread");f.setLayout(null);f.setBounds(300,60,300,200);

f.setVisible(true);f.add(new

MyTextArea(150,0,Thread.MIN_PRIORITY));f.add(new

MyTextArea(0,0,Thread.MAX_PRIORITY));}}class

MyTextArea

extends

TextArea

implements

Runnable{MyTextArea(int

x,

int

y,

int

priority){setForeground(Color.BLUE);setSize(150,200);

setLocation(x,y);Thread

writer

=

new

Thread(this);writer.setPriority(priority);writer.start();}publicvoidrun(){for(int

i

=

1;i<=5000;i++){append(i+"\n");}}}线程的优先级别举例sleep方法可以调用Thread的静态方法：public

static

void

sleep(long

millis)throws

InterruptedException使得当前线程休眠（暂时停止执行millis毫秒）。Thread定义有方法：public

void

interrupt()interrupt()方法不会中断一个正在运行的线程。这一方法实际完成的是：在线程受到阻塞时抛出一个中断信号，这样线程就得以退出阻塞的状态。更确切的说，如果线程被Object.wait,Thread.join和Thread.sleep三种方法之一阻塞，那么，它将接收到一个中断异常（InterruptedException），从而提

地终结被阻塞状态。如果线程被上述几种方法阻塞，正确的停止线程方式是设置共享变量，并调用interrupt()（注意变量应该先设置）。如果线程没有被阻塞，这时调用interrupt()将不起作用；否则，线程就将得到异常，接着

逃离阻塞状态。sleep方法举例－1import

java.util.*;public

class

Test

{publicstatic

void

main(String[]

args)

{MyThread

thread

=

new

MyThread();thread.start();try

{Thread.sleep(10000);}catch

(InterruptedException

e)

{e.printStackTrace();}errupt();}}class

MyThread

extends

Thread

{public

void

run(){while(true){System.out.println("==="+new

Date()+"===");try

{sleep(1000);}

catch

(InterruptedException

e)

{return;}}}}sleep方法举例－2import

java.util.*;public

class

Test

{publicstatic

void

main(String[]

args)

{Thread

thread

=

new

Thread(new

Runnable()

{public

void

run()

{while

(true)

{System.out.println("==="+

new

Date()

+

"===");try

{Thread.sleep(1000);}catch

(InterruptedException

e)

{return;}}}});thread.start();try

{Thread.sleep(10000);}

catch

(InterruptedException

e)

{}errupt();}}import

java.io.*;

import

java.util.*;public

class

Test

implements

Runnable

{public

static

void

main(String[]

args)

{BufferedReader

br

=new

BufferedReader(new

InputStreamReader(System.in));Test

test

=

new

Test();

Thread

timer

=

new

Thread(test);Random

random

=

new

Random();timer.start();

int

correct

=

0;for(int

i

=

1;i<=10;i++){int

d1

=

random.nextInt(100),

d2

=

random.nextInt(100);System.out.println(d1+"+"+d2+"

=

?");

String

result

=

null;try

{while((result=br.readLine())!=null){try

{if(Integer.parseInt(result)==d1+d2){System.out.println("ok!!!");

correct++;

break;}}

catch

(NumberFormatException

e)

{break;}}}

catch

(IOException

e)

{e.printStackTrace();}}System.out.println("共答对"+correct+"道题目");

errupt();}线程举例－1…

…

…public

void

run()

{while

(true)

{String

temp

=

new

Date().toString();String

t

=

temp.substring(11,

temp.indexOf("C"));t

=

t.trim();String[]

time

=

t.split(":");if

(time.length

==

3)

{System.out.println(“现在是”+time[0]+“点”+time[1]+"分"+time[2]+"秒");}try

{Thread.sleep(5000);}

catch

(InterruptedException

e)

{return;}}}}线程举例－1（续）import

java.awt.*;public

class

Test{publicstatic

void

main(String[]

args)

{MyFrame

myFrame

=

new

MyFrame();}}class

MyFrame

extends

Frame

implements

Runnable{private

Thread

runner1,runner2;privatePointpoint1,point2;MyFrame(){super("runing...");setLayout(null);

setBounds(200,60,600,500);setBackground(Color.BLACK);setVisible(true);runner1

=

new

Thread(this);

point1

=

new

Point(0,0);runner2

=

new

Thread(this);

point2

=

new

Point(300,0);runner1.start();

runner2.start();}…

…

…线程举例－2public

void

paint(Graphics

g){g.setColor(Color.RED);g.fillOval(point1.x,point2.y,20,20);g.setColor(Color.YELLOW);g.fillOval(point2.x,point2.y,20,20);g.setColor(Color.GREEN);g.drawString(point1.x+","+point1.y,point1.x,point1.y);g.drawString(point2.x+","+point2.y,point2.x,point2.y);}public

void

run(){for(int

t=0;t<=95;t++){if(Thread.currentThread()==runner1){point1.x

=

20*t/10;

point1.y=(int)(0.5*9.8*t*t/100);}else

if(Thread.currentThread()==runner2){point2.y

=

(int)(0.5*9.8*t*t/100);}repai