彻底搞懂Java多线程(一)

目录

• Java多线程
• 线程的创建
• 线程常用方法
• 线程的终止
• 1.自定义实现线程的终止
• 2.使用Thread的interrupted来中断
• 3.Thraed.interrupted()方法和Threaed.currentThread().interrupt()的区别
• 线程的状态
• 线程的优先级
• 守护线程
• 线程组
• 线程安全问题
• volatile关键字
• 总结

Java多线程

线程的创建

1.继承Thread

2.实现Runnable

3.实现Callable

使用继承Thread类来开发多线程的应用程序在设计上是有局限性的，因为Java是单继承。

继承Thread类

public class ThreadDemo1 {
    // 继承Thread类 写法1
    static class MyThread extends Thread{
        @Override
        public void run() {
            //要实现的业务代码
        }
    }
    // 写法2
    Thread thread = new Thread(){
        @Override
        public void run() {
            //要实现的业务代码
        }
    };
}

实现Runnable接口

//实现Runnable接口 写法1
class MyRunnable implements Runnable{
    @Override
    public void run() {
        //要实现的业务代码
    }
}
//实现Runnable接口 写法2 匿名内部类
class MyRunnable2 {
    public static void main(String[] args) {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                //要实现的业务代码
            }
        });
    }
}

实现Callable接口（Callable + FutureTask 创建带有返回值的线程）

package ThreadDeom;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 17:34;
 */
//创建有返回值的线程 Callable + Future
public class ThreadDemo2 {
    static class MyCallable implements Callable<Integer>{
        @Override
        public Integer call() throws Exception {
            return 0;
        }
    }
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        //创建Callable子对象
        MyCallable myCallable = new MyCallable();
        //使用FutureTask 接受 Callable
        FutureTask<Integer> futureTask = new FutureTask<>(myCallable);
        //创建线程并设置任务
        Thread thread = new Thread(futureTask);
        //启动线程
        thread.start();
        //得到线程的执行结果
        int num = futureTask.get();
    }
}

也可以使用lambda表达式

class ThreadDemo21{
    //lambda表达式
    Thread thread = new Thread(()-> {
        //要实现的业务代码
    });
}

Thread的构造方法

线程常用方法

获取当前线程的引用、线程的休眠

class Main{
    public static void main(String[] args) throws InterruptedException {
        Thread.sleep(1000);
        //休眠1000毫秒之后打印
        System.out.println(Thread.currentThread());
        System.out.println(Thread.currentThread().getName());
    }
}

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 18:38;
 */
public class ThreadDemo6 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("线程的ID：" + Thread.currentThread().getId());
                System.out.println("线程的名称：" + Thread.currentThread().getName());
                System.out.println("线程的状态：" + Thread.currentThread().getState());
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        },"线程一");
        thread.start();
        Thread.sleep(100);
        //打印线程的状态
        System.out.println("线程的状态："+thread.getState());
        System.out.println("线程的优先级："+thread.getPriority());
        System.out.println("线程是否存活："+thread.isAlive());
        System.out.println("线程是否是守护线程："+thread.isDaemon());
        System.out.println("线程是否被打断："+thread.isInterrupted());
    }
}

线程的等待

假设有一个坑位，thread1 和 thread2 都要上厕所。一次只能一个人上，thread2只能等待thread1使用完才能使用厕所。就可以使用join()方法，等待线程1执行完，thread2在去执行。👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 10:48;
 */
public class ThreadDemo13 {
    public static void main(String[] args) throws InterruptedException {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                System.out.println(Thread.currentThread().getName()+"🚾");
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName()+"出来了");
            }
        };
        Thread t1 = new Thread(runnable,"thread1");
        t1.start();
        //t1.join();
        Thread t2 = new Thread(runnable,"thread2");
        t2.start();
    }
}

没有join()显然是不行的。加上join()之后：

线程的终止

1.自定义实现线程的终止

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 9:59;
 */
public class ThreadDemo11 {
    private static boolean flag = false;
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                while (!flag){
                    System.out.println("我是 : " + Thread.currentThread().getName() + ",我还没有被interrupted呢");
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("我是 "+Thread.currentThread().getName()+",我被interrupted了");
            }
        },"thread");
        thread.start();
        Thread.sleep(300);
        flag = true;
    }
}

2.使用Thread的interrupted来中断

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 9:59;
 */
public class ThreadDemo11 {
//    private static boolean flag = false;
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                while (!Thread.interrupted()){
                    System.out.println("我是 : " + Thread.currentThread().getName() + ",我还没有被interrupted呢");
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
//                        e.printStackTrace();
                        break;
                    }
                }
                System.out.println("我是 "+Thread.currentThread().getName()+",我被interrupted了");
            }
        },"thread");
        thread.start();
        Thread.sleep(300);
        thread.interrupt();
//        flag = true;
    }
}

3.Thraed.interrupted()方法和Threaed.currentThread().interrupt()的区别

Thread.interrupted()方法第一次接收到终止的状态后，之后会将状态复位，Thread.interrupted()是静态的，是全局的。

Threaed.currentThread().interrupt()只是普通的方法。

Thraed.interrupted()方法

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 10:32;
 */
public class ThreadDemo12 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() ->{
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.interrupted());
            }
        });
        thread.start();
        thread.interrupt();
    }
}

Threaed.currentThread().interrupt()

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 10:32;
 */
public class ThreadDemo12 {
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(() ->{
            for (int i = 0; i < 10; i++) {
//                System.out.println(Thread.interrupted());
                System.out.println(Thread.currentThread().isInterrupted());
            }
        });
        thread.start();
        thread.interrupt();
    }
}

yield()方法

让出CPU的执行权

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 11:47;
 */
public class ThreadDemo15 {
    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            for (int i = 0; i < 100; i++) {
                Thread.yield();
                System.out.println("thread1");
            }
        });
        thread1.start();
        Thread thread2 = new Thread(() -> {
            for (int i = 0; i < 100; i++) {
                System.out.println("thread2");
            }
        });
        thread2.start();
    }
}

线程的状态

打印出线程的所有的状态，所有的线程的状态都在枚举中。👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 11:06;
 */
public class ThreadDemo14 {
    public static void main(String[] args) {
        for (Thread.State state: Thread.State.values()) {
            System.out.println(state);
        }
    }
}

• NEW 创建了线程但是还没有开始工作
• RUNNABLE 正在Java虚拟机中执行的线程
• BLOCKED 受到阻塞并且正在等待某个监视器的锁的时候所处的状态
• WAITTING 无限期的等待另一个线程执行某个特定操作的线程处于这个状态
• TIME_WAITTING 有具体等待时间的等待
• TERMINATED 已经退出的线程处于这种状态

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 11:06;
 */
class TestThreadDemo{
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        System.out.println(thread.getState());
        thread.start();
        System.out.println(thread.getState());
        Thread.sleep(100);
        System.out.println(thread.getState());
        thread.join();
        System.out.println(thread.getState());
    }
}

线程的优先级

在Java中线程 的优先级分为1 ~ 10 一共十个等级

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 21:22;
 */
public class ThreadDemo9 {
    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            Thread t1 = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("t1");
                }
            });
            //最大优先级
            t1.setPriority(10);
            t1.start();
            Thread t2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("t2");
                }
            });
            //最小优先级
            t2.setPriority(1);
            t2.start();
            Thread t3 = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("t3");
                }
            });
            t3.setPriority(1);
            t3.start();
        }
    }
}

线程的优先级不是绝对的，只是给程序的建议。

线程之间的优先级具有继承的特性，如果A线程启动了B线程，那么B的线程的优先级与A是一样的。👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 20:46;
 */
class ThreadA extends Thread{
    @Override
    public void run() {
        System.out.println("ThreadA优先级是：" + this.getPriority());
        ThreadB threadB = new ThreadB();
        threadB.start();
    }
}
class ThreadB extends ThreadA{
    @Override
    public void run() {
        System.out.println("ThreadB的优先级是：" + this.getPriority());
    }
}
public class ThreadDemo7 {
    public static void main(String[] args) {
        System.out.println("main线程开始的优先级是：" + Thread.currentThread().getPriority());   
        System.out.println("main线程结束的优先级是：" + Thread.currentThread().getPriority());
        ThreadA threadA = new ThreadA();
        threadA.start();
    }
}

再看👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 20:46;
 */
class ThreadA extends Thread{
    @Override
    public void run() {
        System.out.println("ThreadA优先级是：" + this.getPriority());
        ThreadB threadB = new ThreadB();
        threadB.start();
    }
}
class ThreadB extends ThreadA{
    @Override
    public void run() {
        System.out.println("ThreadB的优先级是：" + this.getPriority());
    }
}
public class ThreadDemo7 {
    public static void main(String[] args) {
        System.out.println("main线程开始的优先级是：" + Thread.currentThread().getPriority());
        Thread.currentThread().setPriority(9);
        System.out.println("main线程结束的优先级是：" + Thread.currentThread().getPriority());
        ThreadA threadA = new ThreadA();
        threadA.start();
    }
}

结果为👇

守护线程

Java中有两种线程：一种是用户线程，一种就是守护线程。

什么是守护线程？守护线程是一种特殊的线程，当进程中不存在用户线程的时候，守护线程就会自动销毁。典型的守护线程就是垃圾回收线程，当进程中没有了非守护线程，则垃圾回收线程也就没有存在的必要了。

Daemon线程的作用就是为其他线程的运行提供便利的。👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 21:06;
 */
public class ThreadDemo8 {
    static private int i = 0;
    public static void main(String[] args) throws InterruptedException {
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    i++;
                    System.out.println(i);
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        });
        //设置守护线程
        thread.setDaemon(true);
        thread.start();
        Thread.sleep(5000);
        System.out.println("我是守护线程thread 当用户线程执行完成后 我也就销毁了😭哭了");
    }
}

注意：守护线程的设置必须放在start（）之前，否则就会报错。

在守护线程中创建的线程默认也是守护线程。

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 9:35;
 */
public class ThreadDemo10 {
    public static void main(String[] args) {
        Thread thread1 = new Thread(()->{
            Thread thread2 = new Thread(() -> {
            },"thread2");
            System.out.println("thread2是守护线程吗？：" + thread2.isDaemon());
        },"thread1");
        System.out.println("thread1是守护线程吗？：" + thread1.isDaemon());
        //thread1.setDaemon(true);
        thread1.start();
       // System.out.println("thread1是守护线程吗？：" + thread1.isDaemon());
    }
}

再看👇

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 9:35;
 */
public class ThreadDemo10 {
    public static void main(String[] args) {
        Thread thread1 = new Thread(()->{
            Thread thread2 = new Thread(() -> {
            },"thread2");
            System.out.println("thread2是守护线程吗？：" + thread2.isDaemon());
        },"thread1");
        System.out.println("thread1是守护线程吗？：" + thread1.isDaemon());
        thread1.setDaemon(true);
        thread1.start();
        System.out.println("thread1是守护线程吗？：" + thread1.isDaemon());
    }
}

线程组

为了便于对某些具有相同功能的线程进行管理，可以把这些线程归属到同一个线程组中，线程组中既可以有线程对象，也可以有线程组，组中也可以有线程。使用线程模拟赛跑

public class ThreadDemo5 {
    //线程模拟赛跑（未使用线程分组）
    public static void main(String[] args) {
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "到达了终点");
            }
        }, "选手一");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName() + "到达了终点");
            }
        }, "选手二");
        t1.start();
        t2.start();
        System.out.println("所有选手到达了终点");
    }
}

运行结果：

不符合预期效果，就可以使用线程组来实现

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-11;
 * time: 18:24;
 */
class ThreadGroup1 {
    //线程分组模拟赛跑
    public static void main(String[] args) {
        ThreadGroup threadGroup = new ThreadGroup("Group");
        Thread t1 = new Thread(threadGroup, new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("选手一到达了终点");
            }
        });
        Thread t2 = new Thread(threadGroup, new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1200);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("选手二到达了终点");
            }
        });
        t2.start();
        t1.start();
        while (threadGroup.activeCount() != 0) {
        }
        System.out.println("所有选手到达了终点");
    }
}

线程组常用的方法

线程安全问题

来看单线程情况下让count分别自增和自减10000次

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 12:03;
 */
class Counter {
    private static int count = 0;
    public void increase(){
        for (int i = 0; i < 10000; i++) {
            count++;
        }
    }
    public void decrease(){
        for (int i = 0; i < 10000; i++) {
            count--;
        }
    }
    public int getCount(){
        return count;
    }
}
public class ThreadDemo16 {
    public static void main(String[] args) {
        //单线程
        Counter counter = new Counter();
        counter.increase();
        counter.decrease();
        System.out.println(counter.getCount());
    }
}

结果符合预期

如果想使程序的执行速度快，就可以使用多线程的方式来执行。在来看多线程情况下的问题

public class ThreadDemo16 {
    public static void main(String[] args) throws InterruptedException {
        //多线程情况下
        Counter counter = new Counter();
        Thread thread1 = new Thread(()->{
            counter.decrease();
        });
        Thread thread2 = new Thread(()->{
            counter.increase();
        });
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println(counter.getCount());
                /*
        //单线程
        Counter counter = new Counter();
        counter.increase();
        counter.decrease();
        System.out.println(counter.getCount());
         */
    }
}

执行结果：

每次的执行结果是不一样的。这就是多线程的不安全问题

预期的结果是0，但结果却不是。线程不安全问题的原因：

• 1.CPU的抢占式执行
• 2.多个线程共同操作一个变量
• 3.内存可见性
• 4.原子性问题
• 5.编译器优化（指令重排）

多个线程操作同一个变量

如果多个线程操作的不是一个变量，就不会发生线程的不安全问题，可以将上面的代码修改如下：👇

public class ThreadDemo16 {
    static int res1 = 0;
    static int res2 = 0;
    public static void main(String[] args) throws InterruptedException {
        Counter counter = new Counter();
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                res1 = counter.getCount();
            }
        });
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                res2 = counter.getCount();
            }
        });
        System.out.println(res1 + res2);
/*
        //多线程情况下
        Counter counter = new Counter();
        Thread thread1 = new Thread(()->{
            counter.decrease();
        });
        Thread thread2 = new Thread(()->{
            counter.increase();
        });
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println(counter.getCount());
        */
                /*
        //单线程
        Counter counter = new Counter();
        counter.increase();
        counter.decrease();
        System.out.println(counter.getCount());
         */
    }
}

这样就可以了：

内存不可见问题:看下面的代码，是不是到thread2执行的时候，就会改变num的值，从而终止了thread1呢？

package ThreadDeom;
import java.util.Scanner;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 13:03;
 */
public class ThreadDemo17 {
    private static int num = 0;
    public static void main(String[] args) {
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                while (num == 0){}
            }
        });
        thread1.start();
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                Scanner scanner = new Scanner(System.in);
                System.out.println("输入一个数字来终止线程thread1");
                num = scanner.nextInt();
            }
        });
        thread2.start();
    }
}

结果是不能的：

输入一个数字后回车，并没有让thread1的循环结束。这就是内存不可见的问题。

原子性的问题

上面的++和–操作其实是分三步来执行的

假设在第二部的时候，有另外一个线程也来修改值，那么就会出现脏数据的问题了。

所以就会发生线程的不安全问题

编译器优化编译器的优化会打乱原本程序的执行顺序，就有可能导致线程的不安全问题发生。在单线程不会发生线程的不安全问题，在多线程就可能会不安全。

volatile关键字

可以使用volatile关键字，这个关键字可以解决指令重排和内存不可见的问题。

加上volatile关键字之后的运行结果

但是volatile关键字不能解决原子性的问题👇：

package ThreadDeom;
/**
 * user：ypc；
 * date：2021-06-12;
 * time: 14:02;
 */
class Counter1 {
    private static volatile int count = 0;
    public void increase() {
        for (int i = 0; i < 10000; i++) {
            count++;
        }
    }
    public void decrease() {
        for (int i = 0; i < 10000; i++) {
            count--;
        }
    }
    public int getCount() {
        return count;
    }
}
public class ThreadDemo18 {
    public static void main(String[] args) throws InterruptedException {
        Counter1 counter1 = new Counter1();
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                counter1.decrease();
            }
        });
        Thread thread2 = new Thread(() -> {
            counter1.increase();
        });
        thread1.start();
        thread2.start();
        thread1.join();
        thread2.join();
        System.out.println(counter1.getCount());
    }
}

总结

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