【JUC】多线程手撕代码面试题

一、实现生产者消费者模型

1.使用sychronized和wait notify实现

public static void main(String[] args) throws InterruptedException {
        AtomicInteger a=new AtomicInteger(5);//原子类整型
        final int MAX_SIZE=10;//最大长度
        Thread pro = new Thread(new Runnable() {//消费者线程
            @Override
            public void run() {
                while(true){
                    synchronized (a){
                        while(a.get()==0){
                            try {
                                a.wait();//注意是锁住对象的wait 不能直接wait
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        System.out.println(Thread.currentThread().getName()+"消费"+a.get());
                        a.getAndDecrement();//消费者消费
                        a.notifyAll();//注意是锁住对象的notify
                    }



                }
            }
        }, "消费者");
        Thread cus = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    synchronized (a) {
                        while(a.get()==MAX_SIZE){
                            try {
                                a.wait();//锁住对象
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                        }
                        a.getAndIncrement();//生产者生产
                        System.out.println(Thread.currentThread().getName()+"生产"+a.get());
                        a.notifyAll();//锁住对象
                    }
                }

            }
        }, "生产者");
            pro.start();
            cus.start();
    }

2.lock和condition实现

public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();//锁
        Condition empty = lock.newCondition();//空情况
        Condition full = lock.newCondition();//满情况
        AtomicInteger num=new AtomicInteger(5);

        Thread cus = new Thread(new Runnable() {//消费者
            @Override
            public void run() {
                while (true) {
                    lock.lock();//要在try块外 上锁（确定一定上锁 try里上锁 遇到异常跳出就不会锁住了）
                    try {
                        while (num.get() == 0) {
                            empty.await();//此时为空 空情况等待
                        }
                        System.out.println(Thread.currentThread().getName() + "消费了" + num);
                        num.getAndDecrement();
                        full.signal();//此时已经消费了1个 所以把通知满情况等待的线程

                    } catch (InterruptedException e) {

                    } finally {
                        lock.unlock();//要在finally里 解锁
                    }
                }


            }
        }, "消费者");
        Thread pro=new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    lock.lock();
                    try {
                        while (num.get() == 10) {
                            full.await();//此时已满 满情况等待
                        }
                        System.out.println(Thread.currentThread().getName() + "生产了" + num);
                        num.getAndIncrement();
                        empty.signal();//生产了一个 通知空情况等待的线程

                    } catch (InterruptedException e) {

                    } finally {
                        lock.unlock();
                    }
                }
            }
        },"生产者");
        cus.start();
        pro.start();

    }

3.阻塞队列

 public static void main(String[] args) {
        ArrayBlockingQueue<Integer> blockingQueue = new ArrayBlockingQueue<Integer>(10);//阻塞队列 容量为10

        Thread cus=new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    try {
                        Integer take = blockingQueue.take();//拿走阻塞队列的数据
                        System.out.println("消费者 拿走了"+take);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }

            }
        },"消费者");
        Thread pro=new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    Random random = new Random();
                    int tmp=random.nextInt(10);
                    blockingQueue.offer(tmp);//提供数据
                    System.out.println("生产者生产了 +"+tmp);
                }

            }
        },"生产者");
        pro.start();
        cus.start();

    }

二、写一个死锁的例子

public static void main(String[] args) {
        Object a=new Object();
        Object b = new Object();

        Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (a) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "拿到了对象a");
                    synchronized (b) {
                        System.out.println("死锁 无法进入到这里");
                    }
                }
            }
        }, "A");
        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (b) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName() + "拿到了对象b");
                    synchronized (a) {
                        System.out.println("发生了死锁 无法进入这里");
                    }
                }
            }
        }, "B");
        A.start();
        B.start();

    }

三、双检锁单例模式

public class single {
    static volatile single instance;//使用volatile保证

    public static Object getSingle(){
        if(instance==null){
            synchronized (single.class){//锁类而不是锁对象
                if(instance==null){
                    instance=new single();

                }
            }
        }
        return  instance;
    }
    public static void main(String[] args) {
        System.out.println(getSingle());//测试1
        System.out.println(getSingle());//测试2
    }
}

 为什么使用volatile？或者不加volatile会出现什么问题？

当线程A在初始化instance，执行new single()方法时。线程B访问到了一个未实例化完整的instance 但是也是不为空 就会将不完整的返回。

原理：实例化对象三步骤：1内存开辟空间 2实例化对象 3指向内存空间。

由于2 3没有依赖关系 所以会被JMM重排序为 1 3 2。这时候instance是不完整的。

四、三个线程交替打印ABC

public class abc {
    static int count=0;//利用count来计数 和3取余 0=A 1=B 2=C
    public static void main(String[] args) {
        ReentrantLock lock = new ReentrantLock();//可重入锁
        Condition Acon = lock.newCondition();//等待/通知A
        Condition Bcon = lock.newCondition();//等待/通知B
        Condition Ccon = lock.newCondition();//等待/通知C


        Thread A = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){//如果要改成 输出10次 将这里的while改为for 10次就行
                    lock.lock();
                    try {
                        while(count%3!=0)
                            Acon.await();//当前取余值不为0 等待唤醒
                        System.out.println(Thread.currentThread().getName() + ":  A");
                        count++;
                        Bcon.signal();//唤醒B 该B执行了打印了
                    } catch (Exception e) {
                    } finally {
                        lock.unlock();
                    }
                }


            }
        }, "1");
        Thread B = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    lock.lock();
                    try {
                        while(count%3!=1)
                            Bcon.await();
                        System.out.println(Thread.currentThread().getName() + ":  B");
                        count++;
                        Ccon.signal();
                    } catch (Exception e) {
                    } finally {
                        lock.unlock();
                    }
                }


            }
        }, "2");
        Thread C = new Thread(new Runnable() {
            @Override
            public void run() {
                while (true){
                    lock.lock();
                    try {
                        while(count%3!=2)
                            Ccon.await();
                        System.out.println(Thread.currentThread().getName() + ":  C");
                        count++;
                        Acon.signal();
                    } catch (Exception e) {
                    } finally {
                        lock.unlock();
                    }
                }


            }
        }, "3");
        A.start();
        B.start();
        C.start();
    }
}

 五、多服务器抢票

/**
 * @Description:
 * @Author: cckong
 * @Date:
 */
public class ticket {
    private static int ticket=100;

    static Object server=new Object();
    static Runnable runnable = new Runnable() {
        @Override
        public void run() {
            synchronized (server){
                while (ticket>0){
                    server.notifyAll();
                    System.out.println(Thread.currentThread().getName()+" get "+ticket);
                    ticket--;
                    try {
                        server.wait();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                }
            }
            }


        };

    public static void main(String[] args) {


        new Thread(runnable,"a").start();
        new Thread(runnable,"b").start();
        new Thread(runnable,"c").start();

    }
}