【线程安全习题总结】· 累加count 顺序打印 死锁

目录

1.累加count

2.顺序打印 |

3.死锁代码

4.顺序打印 ||

---

1.累加count

使用两个线程来累加 count 的值

每个线程循环 1w 次，累加变量 count 的值，count 默认值为 0，注意线程安全问题。

private static int count = 0;
    public static void main(String[] args) throws InterruptedException {
        
        //创建一个锁对象
        Object locker = new Object();
        
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 10000; i++) {
                synchronized(locker) {
                    count++;
                }
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 10000; i++) {
                synchronized(locker) {
                    count++;
                }
            }
        });

        t1.start();
        t2.start();

        t1.join();
        t2.join();

        System.out.println(count);
        
    }

或者利用 join() 方法 注意写的顺序就不用加锁 线程也是安全的

private static int count = 0;
    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 10000; i++) {
                    count++;
            }
        });

        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 10000; i++) {
                    count++;
            }
        });

        //注意join的顺序及作用
        t1.start();
        t1.join();

        t2.start();
        t2.join();

        System.out.println(count);

    }

2.顺序打印 |

有三个线程，分别只能打印A，B和C

要求按顺序打印ABC，打印10次

输出示例：

ABC

ABC

ABC

ABC

ABC

ABC

ABC

ABC

ABC

ABC

public static void main(String[] args) throws InterruptedException {
    //创建三个锁对象
    Object locker1 = new Object();
    Object locker2 = new Object();
    Object locker3 = new Object();

    //线程一
    Thread t1 = new Thread(() -> {
        for (int i = 0; i < 10; i++) {
            synchronized (locker1) {
                try {
                    locker1.wait();
                    System.out.print("A");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            synchronized (locker2) {
                locker2.notify();
            }
        }

    });

    //线程二
    Thread t2 = new Thread(() -> {
        for (int i = 0; i < 10; i++) {
            synchronized (locker2) {
                try {
                    locker2.wait();
                    System.out.print("B");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            synchronized (locker3) {
                locker3.notify();
            }
        }
    });

    //线程三
    Thread t3 = new Thread(() -> {
        for (int i = 0; i < 10; i++) {
            synchronized (locker3) {
                try {
                    locker3.wait();
                    System.out.println("C");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }

            synchronized (locker1) {
                locker1.notify();
            }
        }
    });

    //启动线程
    t1.start();
    t2.start();
    t3.start();
    
    //让线程先睡眠1秒 主线程往下执行来唤醒锁locker1
    Thread.sleep(1000);
    synchronized (locker1) {
        locker1.notify();
    }

}

3.死锁代码

所谓的死锁就是线程一拥有锁1，线程二拥有锁2，双方在拥有自身锁的同时尝试获取对方的锁，最终两个线程就会进入无线等待的状态，这就是死锁。

public static void main(String[] args) {
        //创建两个锁对象
        Object locker1 = new Object();
        Object locker2 = new Object();

        //创建两个线程
        Thread t1 = new Thread(() -> {
            synchronized (locker1) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                synchronized (locker2) {
                    System.out.println(Thread.currentThread().getName());
                }

            }
        },"t1");

        Thread t2 = new Thread(() -> {
            synchronized (locker2) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
                synchronized (locker1) {
                    System.out.println(Thread.currentThread().getName());
                }
            }
        },"t2");

        t1.start();
        t2.start();

    }

4.顺序打印 ||

有三个线程，线程名称分别为：a，b，c。

每个线程打印自己的名称。

需要让他们同时启动，并按 c，b，a的顺序打印

public static void main(String[] args) throws InterruptedException {
    //创建三个锁对象
    Object locker1 = new Object();
    Object locker2 = new Object();
    Object locker3 = new Object();

    //线程一
    Thread c = new Thread(() -> {
        System.out.print(Thread.currentThread().getName()+" ");
    },"c");  //c写在这里 线程名

    //线程二
    Thread b = new Thread(() -> {
        try {
            //阻塞等待线程c执行完
            c.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.print(Thread.currentThread().getName()+" ");

    },"b"); //b写在这里 线程名

    //线程三
    Thread a = new Thread(() -> {
        try {
            //阻塞等待线程b执行完
            b.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.print(Thread.currentThread().getName()+" ");

    },"a"); //a写在这里 线程名

    //启动线程
    a.start();
    b.start();
    c.start();

}