多任务:同一时刻运行多个程序的能力。每一个任务称为一个线程。可以同时运行一个以上线程的程序称为多线程程序。
Java编写程序都运行在在Java虚拟机(JVM)中,在JVM的内部,程序的多任务是通过线程来实现的。每用java命令启动一个java应用程序,就会启动一个JVM进程。在同一个JVM进程中,有且只有一个进程,就是它自己。在这个JVM环境中,所有程序代码的运行都是以线程来运行。
一般常见的Java应用程序都是单线程的。比如,用java命令运行一个最简单的HelloWorld的Java应用程序时,就启动了一个JVM进程,JVM找到程序程序的入口点main(),然后运行main()方法,这样就产生了一个线程,这个线程称之为主线程。当main方法结束后,主线程运行完成。JVM进程也随即退出 。
对于一个进程中的多个线程来说,多个线程共享进程的内存块,当有新的线程产生的时候,操作系统不分配新的内存,而是让新线程共享原有的进程块的内存。因此,线程间的通信很容易,速度也很快。不同的进程因为处于不同的内存块,因此进程之间的通信相对困难。
进程是指一个内存中运行的应用程序,每个进程都有自己独立的一块内存空间,一个进程中可以启动多个线程。比如在Windows系统中,一个运行的exe就是一个进程。
线程是指进程中的一个执行流程,一个进程可以运行多个线程。比如java.exe进程可以运行很多线程。线程总是输入某个进程,进程中的多个线程共享进程的内存。
1 package Thread; 2 import java.awt.*; 3 import java.awt.event.*; 4 import javax.swing.*; 5 public class BounceThread { 6 public static void main(String[] args){ 7 EventQueue.invokeLater(new Runnable(){ 8 public void run(){ 9 JFrame frame=new BounceFrame();10 frame.setTitle("BounceFrame");11 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);12 frame.setVisible(true);13 }14 });15 }16 }17 /*class BallRunnable implements Runnable{18 private Ball ball;19 private Component component;20 public static final int STEPS=1000;21 public static final int DELAY=5;22 public BallRunnable(Ball aBall,Component aComponent){23 ball=aBall;24 component=aComponent;25 }26 public void run(){27 try{28 for(int i=1;i<=STEPS;i++){29 ball.move(component.getBounds());30 component.repaint();31 Thread.sleep(DELAY);32 }33 }34 catch(InterruptedException e){}35 }36 }*/37 class BounceFrame extends JFrame{38 private BallComponent comp;39 public static final int STEPS=1000;40 public static final int DELAY=100;41 public BounceFrame(){42 comp=new BallComponent();43 add(comp,BorderLayout.CENTER);44 JPanel buttonPanel=new JPanel();45 addButton(buttonPanel,"Start",new ActionListener(){46 public void actionPerformed(ActionEvent event){47 addBall();48 }49 });50 addButton(buttonPanel,"Close",new ActionListener(){51 public void actionPerformed(ActionEvent event){52 System.exit(0);53 }54 });55 add(buttonPanel,BorderLayout.SOUTH);56 pack();57 }58 public void addButton(Container c,String title,ActionListener listener){59 JButton button=new JButton(title);60 c.add(button);61 button.addActionListener(listener);62 }63 /*public void addBall(){64 Ball b=new Ball();65 comp.add(b);66 Runnable r=new BallRunnable(b,comp);67 Thread t=new Thread(r);68 t.start();69 }*/70 public void addBall(){71 try{72 Ball ball=new Ball();73 comp.add(ball);74 for(int i=1;i<=STEPS;i++){75 ball.move(comp.getBounds());76 comp.paint(comp.getGraphics());77 Thread.sleep(DELAY);78 }79 }80 catch(InterruptedException e){}81 }82 }
BollComponent.java
1 package Thread; 2 import java.awt.*; 3 4 import java.util.*; 5 import javax.swing.*; 6 public class BallComponent extends JPanel{ 7 private static final int DEFAULT_WIDTH=450; 8 private static final int DEFAULT_HEIGHT=350; 9 private java.util.List<Ball>balls=new ArrayList<>();10 public void add(Ball b){11 balls.add(b);12 }13 public void paintComponent(Graphics g){14 super.paintComponent(g);15 Graphics2D g2=(Graphics2D)g;16 for(Ball b:balls){17 g2.fill(b.getShape());18 }19 }20 public Dimension getPreferredSize(){21 return new Dimension(DEFAULT_WIDTH,DEFAULT_HEIGHT);22 }23 }
Ball.java
1 package Thread; 2 import java.awt.geom.*; 3 import java.awt.geom.Ellipse2D.Double; 4 public class Ball { 5 private static final int XSIZE=15; 6 private static final int YSIZE=15; 7 private double x=0; 8 private double y=0; 9 private double dx=1;10 private double dy=1;11 public void move(Rectangle2D bounds){12 x+=dx;13 y+=dy;14 if(x<bounds.getMinX()){15 x=bounds.getMinX();16 dx=-dx;17 }18 if(x+XSIZE>=bounds.getMaxX()){19 x=bounds.getMaxX()-XSIZE;20 dx=-dx;21 }22 if(y<bounds.getMinY()){23 y=bounds.getMinY();24 dy=-dy;25 }26 if(y+YSIZE>=bounds.getMaxY()){27 y=bounds.getMaxY()-YSIZE;28 dy=-dy;29 }30 }31 public Ellipse2D getShape(){32 return new Ellipse2D.Double(x,y,XSIZE,YSIZE);33 }34 }
针对上述的情况,下面的代码是改进后的,当点击close时,就会退出当前线程。而且不论何时点击Start按钮,addBall都会启动一个新线程.
实现多个线程的方法:将移动球的代码放置在一个独立的线程中,点击开始就会重新启动一个线程。简单过程如下:
1、将任务代码放在实现了Runnable接口的类的run方法中。
1 class MyRunnable implements Runnable{2 public void run(){3 task code4 } 5 }
2、创建一个类对象。Runnable r=new MyRunnable();
3、由Runnable创建一个Thread对象。Thread t=new Thread();
4、启动线程:t.start();
BounceThread.java
1 package Thread; 2 import java.awt.*; 3 import java.awt.event.*; 4 import javax.swing.*; 5 public class BounceThread { 6 public static void main(String[] args){ 7 EventQueue.invokeLater(new Runnable(){ 8 public void run(){ 9 JFrame frame=new BounceFrame();10 frame.setTitle("BounceFrame");11 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);12 frame.setVisible(true);13 }14 });15 }16 }17 class BallRunnable implements Runnable{18 private Ball ball;19 private Component component;20 public static final int STEPS=1000;21 public static final int DELAY=5;22 public BallRunnable(Ball aBall,Component aComponent){23 ball=aBall;24 component=aComponent;25 }26 public void run(){27 try{28 for(int i=1;i<=STEPS;i++){29 ball.move(component.getBounds());30 component.repaint();31 Thread.sleep(DELAY);32 }33 }34 catch(InterruptedException e){}35 }36 }37 class BounceFrame extends JFrame{38 private BallComponent comp;39 //public static final int STEPS=1000;40 //public static final int DELAY=100;41 public BounceFrame(){42 comp=new BallComponent();43 add(comp,BorderLayout.CENTER);44 JPanel buttonPanel=new JPanel();45 addButton(buttonPanel,"Start",new ActionListener(){46 public void actionPerformed(ActionEvent event){47 addBall();48 }49 });50 addButton(buttonPanel,"Close",new ActionListener(){51 public void actionPerformed(ActionEvent event){52 System.exit(0);53 }54 });55 add(buttonPanel,BorderLayout.SOUTH);56 pack();57 }58 public void addButton(Container c,String title,ActionListener listener){59 JButton button=new JButton(title);60 c.add(button);61 button.addActionListener(listener);62 }63 public void addBall(){64 Ball b=new Ball();65 comp.add(b);66 Runnable r=new BallRunnable(b,comp);67 Thread t=new Thread(r);68 t.start();69 }70 /*public void addBall(){71 try{72 Ball ball=new Ball();73 comp.add(ball);74 for(int i=1;i<=STEPS;i++){75 ball.move(comp.getBounds());76 comp.paint(comp.getGraphics());77 Thread.sleep(DELAY);78 }79 }80 catch(InterruptedException e){}81 }*/82 }
BollComponent.java
1 package Thread; 2 import java.awt.*; 3 4 import java.util.*; 5 import javax.swing.*; 6 public class BallComponent extends JPanel{ 7 private static final int DEFAULT_WIDTH=450; 8 private static final int DEFAULT_HEIGHT=350; 9 private java.util.List<Ball>balls=new ArrayList<>();10 public void add(Ball b){11 balls.add(b);12 }13 public void paintComponent(Graphics g){14 super.paintComponent(g);15 Graphics2D g2=(Graphics2D)g;16 for(Ball b:balls){17 g2.fill(b.getShape());18 }19 }20 public Dimension getPreferredSize(){21 return new Dimension(DEFAULT_WIDTH,DEFAULT_HEIGHT);22 }23 }
Ball.java
1 package Thread; 2 import java.awt.geom.*; 3 import java.awt.geom.Ellipse2D.Double; 4 public class Ball { 5 private static final int XSIZE=15; 6 private static final int YSIZE=15; 7 private double x=0; 8 private double y=0; 9 private double dx=1;10 private double dy=1;11 public void move(Rectangle2D bounds){12 x+=dx;13 y+=dy;14 if(x<bounds.getMinX()){15 x=bounds.getMinX();16 dx=-dx;17 }18 if(x+XSIZE>=bounds.getMaxX()){19 x=bounds.getMaxX()-XSIZE;20 dx=-dx;21 }22 if(y<bounds.getMinY()){23 y=bounds.getMinY();24 dy=-dy;25 }26 if(y+YSIZE>=bounds.getMaxY()){27 y=bounds.getMaxY()-YSIZE;28 dy=-dy;29 }30 }31 public Ellipse2D getShape(){32 return new Ellipse2D.Double(x,y,XSIZE,YSIZE);33 }34 }
运行结果如下:
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