Java多线程学习笔记

时间:2022-06-01 21:12:58

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

Java多线程学习笔记

 

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

Java多线程学习笔记

往下翻你会看到:

Java多线程学习笔记

Java多线程学习笔记

Java多线程学习笔记

 

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);
      });
  }
}

Java多线程学习笔记

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:

Java多线程学习笔记

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

Java多线程学习笔记

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

  1. 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
  2. lock只有代码块锁,synchronized有代码块锁和方法锁
  3. JVM将花费更少的时间来调度线程,性能更好,更有扩展性
  4. 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