多任务、多线程
在多任务场景下,两件事看上去同时在做,但实际上,你的大脑在同一时间只做一件事,间隔时间可能很少,但这似乎让你感觉这两件事是同时在做
考虑阻塞问题,引入多线程的场景,多线程并发场景
程序、进程、线程
程序=指令+数据(静态的)
在操作系统中运行的程序就是进程,一个进程可以有多个线程
比如,看视频时听声音,看图像,看弹幕等
学着看jdk文档
比如你要看Thread
你可以搜索,然后阅读
往下翻你会看到:
线程的创建
1.继承Thread类
//创建线程方式一:继承Thread类,重写run方法,调用start()方法开启线程 public class TestThread1 extends Thread{ @Override public void run() { //run()方法线程体 IntStream.range(0,20).forEach(i->{ System.out.println("我在看代码"+i); }); } public static void main(String[] args) { //创建一个线程对象 TestThread1 testThread1=new TestThread1(); //调用start()方法,启动线程,不一定立即执行,由cpu调度执行 testThread1.start(); //主方法 main方法 IntStream.range(0,20).forEach(i->{ System.out.println("我在学习多线程"+i); }); } }
一个小练习:
//练习thread实现对线程同步下载图片 public class TestThread2 extends Thread{ private String url; private String name; public TestThread2(String url, String name) { this.url = url; this.name = name; } @Override public void run() { WebDownload webDownload=new WebDownload(); webDownload.downloader(url,name); System.out.println("下载了文件名:"+name); } public static void main(String[] args) { TestThread2 t1=new TestThread2("https://profile.csdnimg.cn/B/D/2/3_sxh06","1.jpg"); TestThread2 t2=new TestThread2("https://profile.csdnimg.cn/B/D/2/3_sxh06","2.jpg"); TestThread2 t3=new TestThread2("https://profile.csdnimg.cn/B/D/2/3_sxh06","3.jpg"); t1.start(); t2.start(); t3.start(); } } //下载器 class WebDownload{ //下载方法 public void downloader(String url,String name) { try { FileUtils.copyURLToFile(new URL(url),new File(name)); } catch (IOException e) { e.printStackTrace(); System.out.println("IO异常,downloader方法出错"); } } }
2.实现Runable接口
//创建线程的方法2:实现Runable接口 public class TestThread3 implements Runnable{ @Override public void run() { //run()方法线程体 IntStream.range(0,20).forEach(i->{ System.out.println("我在看代码"+i); }); } public static void main(String[] args) { //创建一个线程对象 TestThread3 testThread3=new TestThread3(); //调用start()方法,启动线程,不一定立即执行,由cpu调度执行 // Thread thread=new Thread(testThread3); // thread.start(); //或者这样简写 new Thread(testThread3).start(); //主方法 main方法 IntStream.range(0,100).forEach(i->{ System.out.println("我在学习多线程"+i); }); } }
理解并发的场景
当多个线程使用同一个资源时,会出现问题,看看下面这个买火车票的例子:
public class TestThread4 implements Runnable{ //票数 private int ticketNums=10; @Override public void run() { while(true){ if (ticketNums<=0){ break; } //模拟延迟 try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName()+"-->拿到了第"+ticketNums--+"张票"); } } public static void main(String[] args) { TestThread4 ticket=new TestThread4(); new Thread(ticket,"小明").start(); new Thread(ticket,"张三").start(); new Thread(ticket,"李四").start(); } }
看看运行的结果:
可以看到案例中的线程不安全问题,同时数据也是不正确的
龟兔赛跑场景
/** * 模拟龟兔赛跑 */ public class Race implements Runnable{ //胜利者 private static String winner; @Override public void run() { for (int i=0;i<=100;i++){ //模拟兔子休息 if (Thread.currentThread().getName().equals("兔子")&&i%10==0){ try { Thread.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); } } boolean flag=gameOver(i); if (flag){ //判断比赛是否结束 break; } System.out.println(Thread.currentThread().getName()+"-->跑了"+i+"步"); } } /** * 判断比赛是否结束 */ private boolean gameOver(int steps){ //判断是否有胜利者 if (winner !=null){ //已经存在胜利者 return true; }else if (steps >= 100){ winner=Thread.currentThread().getName(); System.out.println("胜利者是:"+winner); return true; }else{ return false; } } public static void main(String[] args) { Race race=new Race(); new Thread(race,"兔子").start(); new Thread(race,"乌龟").start(); } }
实现callable接口
//线程创建方式3 public class TestCallable implements Callable<Boolean> { private String url; private String name; public TestCallable(String url, String name) { this.url = url; this.name = name; } @Override public Boolean call() { com.sxh.thread.WebDownload webDownload=new com.sxh.thread.WebDownload(); webDownload.downloader(url,name); System.out.println("下载了文件名:"+name); return true; } public static void main(String[] args) throws ExecutionException, InterruptedException { TestCallable t1=new TestCallable("https://profile.csdnimg.cn/B/D/2/3_sxh06","1.jpg"); TestCallable t2=new TestCallable("https://profile.csdnimg.cn/B/D/2/3_sxh06","2.jpg"); TestCallable t3=new TestCallable("https://profile.csdnimg.cn/B/D/2/3_sxh06","3.jpg"); //创建执行服务 ExecutorService ser= Executors.newFixedThreadPool(3); //提交执行 Future<Boolean> r1=ser.submit(t1); Future<Boolean> r2=ser.submit(t2); Future<Boolean> r3=ser.submit(t3); //获取结果 boolean rs1=r1.get(); boolean rs2=r2.get(); boolean rs3=r3.get(); //关闭服务 ser.shutdownNow(); } }
理解函数式接口
任何接口,只包含唯一一个抽象方法,就是函数式接口
/** * lambdab表达式的发展 */ public class TestLambda1 { //3.静态内部类 static class Like2 implements ILike{ @Override public void lambda() { System.out.println("i like lambda2"); } } public static void main(String[] args) { ILike like=new Like(); like.lambda(); like=new Like2(); like.lambda(); //4.局部内部类 class Like3 implements ILike{ @Override public void lambda() { System.out.println("i like lambda3"); } } like=new Like3(); like.lambda(); //5.匿名内部类 like=new ILike() { @Override public void lambda() { System.out.println("i like lambda4"); } }; like.lambda(); //6.用lambda简化 like=()->{ System.out.println("i like lambda5"); }; like.lambda(); } } //1.定义一个函数式接口 interface ILike{ void lambda(); } //2.实现类 class Like implements ILike{ @Override public void lambda() { System.out.println("i like lambda"); } }
理解线程的状态
线程停止
public class TestStop implements Runnable{ //1.设置一个标志位 private boolean flag=true; @Override public void run() { int i=0; while (flag){ System.out.println("run...thread.."+i++); } } //2.设置一个公开的方法停止线程,转换标志位 public void stop(){ this.flag=false; } public static void main(String[] args) { TestStop stop=new TestStop(); new Thread(stop).start(); for (int i = 0; i < 1000; i++) { System.out.println("main"+i); if (i==900){ //调用stop方法,让线程停止 stop.stop(); System.out.println("线程该停止了"); } } // IntStream.range(0,1000).forEach(i->{ // // }); } }
线程休眠sleep
每个对象都有一把锁,sleep不会释放锁
1.网路延迟
//模拟延迟 try { Thread.sleep(200); //ms } catch (InterruptedException e) { e.printStackTrace(); }
2.倒计时等
public static void main(String[] args) { try { tendown(); } catch (InterruptedException e) { e.printStackTrace(); } } public static void tendown() throws InterruptedException { int num=10; while (true){ Thread.sleep(1000); System.out.println(num--); if(num<=0) { break; } } }
public static void main(String[] args) { //打印系统当前时间 Date startTime=new Date(System.currentTimeMillis()); while (true){ try { Thread.sleep(1000); System.out.println(new SimpleDateFormat("HH:mm:ss").format(startTime)); startTime=new Date(System.currentTimeMillis());//更新时间 } catch (InterruptedException e) { e.printStackTrace(); } } }
线程礼让yield
//线程礼让 礼让不一定成功,由cpu重新调度 public class TestYield { public static void main(String[] args) { MyYield myYield=new MyYield(); new Thread(myYield,"a").start(); new Thread(myYield,"b").start(); } } class MyYield implements Runnable{ @Override public void run() { System.out.println(Thread.currentThread().getName()+"线程开始执行"); Thread.yield(); System.out.println(Thread.currentThread().getName()+"线程停止执行"); } }
线程强制执行
//测试join方法 想象为插队 public class TestJoin implements Runnable{ @Override public void run() { for (int i = 0; i < 100; i++) { System.out.println("线程vip来了"+i); } } public static void main(String[] args) throws InterruptedException { //启动线程 TestJoin testJoin=new TestJoin(); Thread thread=new Thread(testJoin); thread.start(); //主线程 for (int i = 0; i < 1000; i++) { if (i==200){ thread.join(); //插队 } System.out.println("main"+i); } } }
观察线程状态
public class TestState { public static void main(String[] args) throws InterruptedException { Thread thread=new Thread(()->{ for (int i = 0; i < 5; i++) { try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println("//"); }); //观察状态 Thread.State state=thread.getState(); System.out.println(state); //NEW //启动后 thread.start(); state=thread.getState(); System.out.println(state); //Run while (state != Thread.State.TERMINATED) { Thread.sleep(100); state=thread.getState();//更新线程状态 System.out.println(state); //Run } } }
线程的优先级
//测试线程的优先级 public class TestPriority { public static void main(String[] args) { //主线程默认优先级 System.out.println(Thread.currentThread().getName()+"--->"+Thread.currentThread().getPriority()); MyPriority myPriority=new MyPriority(); Thread t1=new Thread(myPriority); Thread t2=new Thread(myPriority); Thread t3=new Thread(myPriority); Thread t4=new Thread(myPriority); Thread t5=new Thread(myPriority); Thread t6=new Thread(myPriority); //先设置优先级,在启动 t1.start(); t2.setPriority(1); t2.start(); t3.setPriority(4); t3.start(); t4.setPriority(Thread.MAX_PRIORITY); t4.start(); t5.setPriority(-1); t5.start(); t6.setPriority(11); t6.start(); } } class MyPriority implements Runnable{ @Override public void run() { System.out.println(Thread.currentThread().getName()+"--->"+Thread.currentThread().getPriority()); } }
守护线程
线程分为用户线程和守护线程
//测试守护线程 public class TestDaemon { public static void main(String[] args) { God god=new God(); You you=new You(); Thread thread=new Thread(god); thread.setDaemon(true); //默认是false表示用户线程 thread.start(); new Thread(you).start(); } } class God implements Runnable{ @Override public void run() { while (true){ System.out.println("上帝保佑着你"); } } } class You implements Runnable{ @Override public void run() { for (int i = 0; i < 36000; i++) { System.out.println("你活着"+i); } System.out.println("goodbye!!"); } }
线程同步机制
解决安全性问题:队列+锁
1.synchronized 同步方法
默认锁的是this,如需锁其他的,使用下面的同步块
//synchronized 同步方法 private synchronized void buy(){ if (ticketNums<=0){ flag=false; return; } //模拟延迟 try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } //买票 System.out.println(Thread.currentThread().getName()+"-->拿到了第"+ticketNums--+"张票"); }
2.同步块synchronized(Obj){}
锁的对象是变化的量,需要增删改的对象
obj称之为同步监视器,即监视对象
public class UnsafeList { public static void main(String[] args) { List<String> list=new ArrayList<String>(); for (int i = 0; i < 10000; i++) { new Thread(()->{ synchronized (list){ list.add(Thread.currentThread().getName()); } }).start(); } try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(list.size()); } }
lock
class A{ //ReentrantLock 可重入锁 private final ReentrantLock lock=new ReentrantLock(); public void f(){ lock.lock();//加锁 try{ //..... } finally{ lock.unlock();//释放锁 } } }
synchronized与lock
- lock是显示锁需要手动开关,synchronized是隐式锁,出了作用域自动释放
- lock只有代码块锁,synchronized有代码块锁和方法锁
- JVM将花费更少的时间来调度线程,性能更好,更有扩展性
- 优先使用:Lock>同步代码块>同步方法
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原文链接:https://blog.csdn.net/sxh06/article/details/120261884