072-JAVA线程安全的重入锁方案【阻塞】

作者: XAbo | 来源:发表于2022-06-14 23:18 被阅读0次

重入锁（ReentrantLock）的引入就是为了解决同步锁存在活跃性的问题。

不同点：

可重入锁，同个线程可以对同一个锁多次获取。
可重入锁，可以被打断，需创建可打断的锁lockInterruptibly()。
可重入锁，ReentrantLock默认与同步锁一样是不公平锁，但是可以设置为公平锁，解决同步锁的饥饿，但是意义不大。
可以创建条件变量，解决同步锁的虚假唤醒。
可以由程序员加锁和释放锁。
可以设置锁时长tryLock()，带有时长的锁，也是可以被打断的。

相同点：

锁都可以被重入。

一、解决同步锁的活跃性

1.1 解决死锁

复现：哲学家就餐问题;synchronized会一致等待锁。

public class Lock {
    public static void main(String[] args) {
        Chopstick c1 = new Chopstick("1");
        Chopstick c2 = new Chopstick("2");
        Chopstick c3 = new Chopstick("3");
        Chopstick c4 = new Chopstick("4");
        Chopstick c5 = new Chopstick("5");
        new Philosopher("阿基米德", c5, c1).start();
        new Philosopher("赫拉克利特", c4, c5).start();
        new Philosopher("亚里士多德", c3, c4).start();
        new Philosopher("柏拉图", c2, c3).start();
        new Philosopher("苏格拉底", c1, c2).start();
    }

    static class Philosopher extends Thread {
        Chopstick right;
        Chopstick left;

        public Philosopher(String name, Chopstick right, Chopstick left) {
            super(name);
            this.left = left;
            this.right = right;
        }

        @Override
        public void run() {
            while (true) {
                synchronized (left) {
                    synchronized (right) {
                        try {
                            eat();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
        }
        private void eat() throws InterruptedException {
            System.out.println(currentThread().getName() + "吃");
        }
    }
    static class Chopstick {
        String name;
        public Chopstick(String name) {
            this.name = name;
        }
        @Override
        public String toString() {
            return "筷子{" +
                    "name='" + name + '\'' +
                    '}';
        }
    }
}

结果：死锁
使用tryLock解决死锁;结果：停不下来。

public class Lock {
    public static void main(String[] args) {
        Chopstick c1 = new Chopstick("1");
        Chopstick c2 = new Chopstick("2");
        Chopstick c3 = new Chopstick("3");
        Chopstick c4 = new Chopstick("4");
        Chopstick c5 = new Chopstick("5");
        new Philosopher("阿基米德", c5, c1).start();
        new Philosopher("赫拉克利特", c4, c5).start();
        new Philosopher("亚里士多德", c3, c4).start();
        new Philosopher("柏拉图", c2, c3).start();
        new Philosopher("苏格拉底", c1, c2).start();
    }

    static class Philosopher extends Thread {
        Chopstick right;
        Chopstick left;
        public Philosopher(String name, Chopstick right, Chopstick left) {
            super(name);
            this.left = left;
            this.right = right;
        }
        @Override
        public void run() {
            while (true) {

             if(left.tryLock()){
                try{
                    if(right.tryLock()){
                        try {
                            eat();
                        }finally {
                           right.unlock();
                        }
                    }
                }finally {
                    left.unlock();
                }
             }
            }
        }
        private void eat() {
            System.out.println(currentThread().getName() + "吃");
        }
    }
    static class Chopstick  extends ReentrantLock{
        String name;
        public Chopstick(String name) {
            this.name = name;
        }
        @Override
        public String toString() {
            return "筷子{" +
                    "name='" + name + '\'' +
                    '}';
        }
    }
}

1.2解决饥饿

公平锁一般不使用。常见公平锁对象：
new ReentrantLock(true)、new ReentrantReadWriteLock(true)等。

1.3 解决虚假唤醒

public class WaitTest {
    static boolean hasCigarette = false;
    static boolean hasTakeout = false;
    static ReentrantLock ROOM = new ReentrantLock();
    static Condition waitCigaretteSet =  ROOM.newCondition();
    static Condition waitTakeoutSet =  ROOM.newCondition();
    public static void main(String[] args) throws InterruptedException {
        new Thread(()->{
                ROOM.lock();
                try {
                    System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有烟吗？ " + hasCigarette);
                    while(!hasCigarette){
                        System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":没烟，歇会。" );
                        try {
                            waitCigaretteSet.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有烟吗？ " + hasCigarette);
                    if(hasCigarette){
                        System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有烟，干活。");
                    }else {
                        System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":接着睡。");
                    }
                }finally {
                    ROOM.unlock();
                }
        },"小南").start();
        new Thread(()->{
            ROOM.lock();
                try {
                    System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有饭吗？ " + hasTakeout);
                    while(!hasTakeout){
                        System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":没饭，歇会。" );
                        try {
                            waitTakeoutSet.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有饭吗？ " + hasTakeout);
                    if(hasTakeout){
                        System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":有饭，干活。");
                    }
                }finally {
                    ROOM.unlock();
                }
        },"大南").start();
        Thread.sleep(1000);
        new Thread(()->{
            ROOM.lock();
            try {
                System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":饭来。");
                hasTakeout = true;
                waitTakeoutSet.signal();
            }finally {
                ROOM.unlock();
            }
        },"小女").start();
        new Thread(()->{
            ROOM.lock();
            try {
                System.out.println(System.currentTimeMillis()+Thread.currentThread().getName()+":烟来。");
                hasCigarette = true;
                waitCigaretteSet.signal();
            }finally {
                ROOM.unlock();
            }
        },"小女女").start();
        
    }
}