synchronized的功能拓展:重入锁(读书笔记)

Posted 2020-08-24 Darcy_wang

 重入锁可以完全代替synchronized关键字.在JDK5.0的早期版本中,重入锁的性能远远好于synchronized,但是从JDK6.0开始.JDK在synchronized上做了大量的优化.使得两者的性能差距不大,

public class ReenterLock implements Runnable {
    public static ReentrantLock lock = new ReentrantLock();
    public static int i = 0;


    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object‘s
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run() {
        for (int j = 0; j < 10000000; j++) {
            lock.lock();//加锁
            try {
                i++;
            } finally {
                lock.unlock();//释放锁
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ReenterLock t1 = new ReenterLock();//线程实例
        Thread th1 = new Thread(t1);
        Thread th2 = new Thread(t1);
        th1.start();
        th2.start();
        th1.join();
        th2.join();
        System.out.println("i = " + i);
    }
}
 

这里 我们使用重入锁保护临界区资源i 确保多线程对i操作的安全性,我们可以看出,与synchronized相比,重入锁有着显示的操作过程,开发人员必须手动指定何时加锁,何时释放锁,也正因为这样,重入锁对于逻辑控制的灵活性要远远好于synchronized,但是要必须释放锁,否则 其他线程就没有机会现在方法临界区资源了!另外重入锁允许一个线程连续几次获取同一把锁,但是释放锁的时候也要释放相同次数.

 

• 中断响应  与synchronized相比,如果一个线程在等待锁,那么结果只要两种情况,1 获得这把锁执行, 2 他保持等待状态,而使用重入锁,则提供了另外一种可能性,那就是线程可以被中断,也就是在等待过程中,程序可以根据需要取消对锁的请求.

 

下面代码产生了一个死锁,但是得益于锁的中断,我们可以很轻松的解决死锁.

public class IntLock implements Runnable {
    public static ReentrantLock lock1 = new ReentrantLock();
    public static ReentrantLock lock2 = new ReentrantLock();
    int lock;

    public IntLock(int lock) {
        this.lock = lock;
    }

    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object‘s
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run() {
        try {
            if (lock == 1) {
                lock1.lockInterruptibly();//锁1 这是一个可以对中断进行想要的锁申请动作!
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                lock2.lockInterruptibly();//锁2 加锁
            } else {
                lock2.lockInterruptibly();
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                lock1.lockInterruptibly();//锁1 加锁
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            if (lock1.isHeldByCurrentThread()) { //判断持有自己锁的线程是否是当前线程
                lock1.unlock();
            }
            if (lock2.isHeldByCurrentThread()) {
                lock2.unlock();
            }
            System.out.println(Thread.currentThread().getId() + ":线程退出");
        }
    }

    public static void main(String[] args) throws InterruptedException {
        IntLock r1 = new IntLock(1);//线程实例 1
        IntLock r2 = new IntLock(2);//线程实例 2
        Thread t1 = new Thread(r1);//线程1
        Thread t2 = new Thread(r2);//线程2
        t1.start();
        t2.start();
        Thread.sleep(1000);//Main线程 休眠1s

        t2.interrupt();//中断其中一个线程

    }
}

线程t1和t2启动后,t1占用rock1 在占用rock2  t2先占用rock2 然后请求rock1 因此很容易形成互相等待,  当我们让t2中断时,他放弃了申请lock1 然后释放了lock2 实际上是 t1线程 完成任务正常退出,而t2 是中断的,

• 锁申请等待限时  除了等待通知之外,要避免死锁还有另外一种方法,那就是限时等待, 就是规定一个时间,超出时间没有拿到锁 就退出 

public class TimeLock implements Runnable {
    public static ReentrantLock lock = new ReentrantLock();

    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object‘s
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run() {
        try {
            if (lock.tryLock(5, TimeUnit.SECONDS)) {//试图获取锁,等待5秒 如果超时那就false
                Thread.sleep(6000);
            } else {
                System.out.println("get lock failed");
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            if (lock.isHeldByCurrentThread()) {
                lock.unlock();
            }
        }
    }

    public static void main(String[] args) {
        TimeLock r1 = new TimeLock();
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r1);
        t1.start();
        t2.start();
    }
}

在本例中,由于占用锁的线程会持有锁长达6s,故另一个线程无法在5s的等待时间内获得锁,因此,请求锁会失败!

 

      ReentrantLock.tryLock()方法可以不带参数直接运行,在这种情况下,当前线程会尝试获得锁,如果锁并未被其他线程占用,则申请会成功!并立即返回true,如果锁被其他线程占用,则档期你先吃不会进行任何等待,而是立即返回false.

 

public class TryLock implements Runnable {
    public static ReentrantLock lock1 = new ReentrantLock();
    public static ReentrantLock lock2 = new ReentrantLock();
    int lock;

    public TryLock(int lock) {
        this.lock = lock;
    }

    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object‘s
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see Thread#run()
     */
    @Override
    public void run() {
        if (lock == 1) {
            while (true) {
                if (lock1.tryLock()) {
                    try {
                        try {
                            Thread.sleep(500);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        if (lock2.tryLock()) {
                            try {
                                System.out.println(Thread.currentThread().getId() + ":My Job done");
                                return;
                            } finally {
                                lock2.unlock();
                            }
                        }
                    } finally {
                        lock1.unlock();
                    }
                }
            }
        } else {
            while (true) {
                if (lock2.tryLock()) {
                    try {
                        try {
                            Thread.sleep(500);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        if (lock1.tryLock()) {
                            try {
                                System.out.println(Thread.currentThread().getId() + ":My Job done");
                                return;
                            } finally {
                                lock1.unlock();
                            }
                        }
                    } finally {
                        lock2.unlock();
                    }
                }
            }
        }
    }

    public static void main(String[] args) {
        TryLock r1 = new TryLock(1);
        TryLock r2 = new TryLock(2);
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
        t1.start();
        t2.start();
    }
}
 

 

上述代码中.采用了非常容易死锁的加锁顺序,在一般情况下,这会导致t1和t2互相等待. 但是使用tryLock()后,就会得到大大改善,线程不会傻傻的等待,而是不停的尝试.因此只要时间足够的长,线程总是会得到所有需要的资源.

• 公平锁 在大多情况下.锁的申请都是非公平的.两个线程同时申请锁a,谁先获得锁a呢 这是不一定的,系统只是会从这个锁的等待队列中随机挑选一个.而公平的锁,则不是这样的,他会按照时间的先后顺序,保证先到先得,后到后的,公平锁的一大特点就是,不会产生饥饿现象,只要你排队,最终还是可以得到资源的, 如果我们使用synchronized关键字进行锁控制,那么产生的锁就是非公平的,而重入锁循序我们对其公平性进行设置,下面是他的构造器

public ReentrantLock(boolean fair) {
    sync = fair ? new FairSync() : new NonfairSync();
}

 

 当参数fair为true时,表示锁是公平的,公平锁必须要求系统维护一个有序的队列,因此实现成本比较高,性能相对低下,因此默认锁是非公平的.