【Java并发编程】J.U.C下Lock的分类及特点详解

苡仁 2020年04月12日 25次浏览

1. Lock接口

1.1 Lock接口间接

  • Locksynchronized,是Java中最常见的锁,他们都可以达到线程安全的目的,Lock主要用于丰富加锁的形式,以及处理的方法

1.2 为什么需要Lock?

  • 主要是因为synchronized不够用,有如下问题:
    1. 效率低
    2. 不够灵活
    3. 无法知道是否成功获取到锁

1.3 Lock主要方法

Lock中声明了四个方法来获取锁:lock()tryLock()tryLock(long time, TimeUnit unit)lockInterruptibly()

  1. lock()就是最普通的获取锁,如果锁已被其他线程获取,则等待;Lock不会像synchronized一样在异常时自动释放锁,因此我们需要手动释放锁,最佳实践:在finally中释放锁,以保证发生异常时锁一定被释放。此外lock()方法不能被中断,这会有很大隐患,一旦陷入死锁,lock()就会陷入永久等待。
/**
 * Lock最佳实践 Lock不像synchronized主动释放锁,需要调用unlock
 * @author yiren
 */
public class LockInterface {
    private static Lock lock = new ReentrantLock();

    public static void main(String[] args) {
        lock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + " do some work!");
        }finally {
            lock.unlock();
        }
    }
}
  1. tryLock()用来尝试获取锁,如果当前所没有被其他线程占用,则获取成功返回true,锁获取失败返回false;相比于lock(),这样的方法显然功能更强大了,我们可以根据是否能获取到锁来决定后续的程序行为;且此方法会立即返回;
  2. tryLock(long time, TimeUnit unit)tryLock()使用类似,不过它本身可以阻塞等待一段时间锁,超时过后再放弃。
    • 在我死锁的文章中有个案例,就是利用tryLock来解决死锁问题,代码如下
/**
 * 使用tryLock来避免死锁
 *
 * @author yiren
 */
public class DeadlockTryLock {
    private static Lock lock1 = new ReentrantLock();
    private static Lock lock2 = new ReentrantLock();

    public static void main(String[] args) {
        Thread thread1 = new Thread(() -> {
            for (int i = 0; i < 100; i++) {
                try {
                    if (lock1.tryLock(1, TimeUnit.SECONDS)) {
                        System.out.println(Thread.currentThread().getName() + " got lock 1");
                        TimeUnit.MILLISECONDS.sleep(new Random().nextInt(10));
                        if (lock2.tryLock(1, TimeUnit.SECONDS)) {
                            System.out.println(Thread.currentThread().getName() + " got lock1 and lock2 successfully.");
                            lock2.unlock();
                            lock1.unlock();
                            break;
                        } else {
                            System.out.println(Thread.currentThread().getName() + " fail to get lock2");
                            lock1.unlock();
                        }
                        TimeUnit.MILLISECONDS.sleep(new Random().nextInt(10));
                    } else {
                        System.out.println(Thread.currentThread().getName() + " fail to get lock1");
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        Thread thread2 = new Thread(() -> {
            for (int i = 0; i < 100; i++) {
                try {
                    if (lock2.tryLock(1, TimeUnit.SECONDS)) {
                        System.out.println(Thread.currentThread().getName() + " got lock 2");
                        TimeUnit.MILLISECONDS.sleep(new Random().nextInt(10));
                        if (lock1.tryLock(1, TimeUnit.SECONDS)) {
                            System.out.println(Thread.currentThread().getName() + " got lock2 and lock1 successfully.");
                            lock1.unlock();
                            lock2.unlock();
                            break;
                        } else {
                            System.out.println(Thread.currentThread().getName() + " fail to get lock1");
                            lock2.unlock();
                        }
                        TimeUnit.MILLISECONDS.sleep(new Random().nextInt(10));
                    } else {
                        System.out.println(Thread.currentThread().getName() + " fail to get lock2");
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
        thread1.start();
        thread2.start();
    }
}
Thread-0 got lock 1
Thread-1 got lock 2
Thread-1 fail to get lock1
Thread-0 fail to get lock2
Thread-0 got lock 1
Thread-1 got lock 2
Thread-1 fail to get lock1
Thread-0 got lock1 and lock2 successfully.
Thread-1 got lock 2
Thread-1 got lock2 and lock1 successfully.

Process finished with exit code 0
  1. lockInterruptibly()相当于tryLock(long time, TimeUnit unit)把超时时间设置为无线。并且在等待锁的过程中,线程可以被中断。
/**
 * @author yiren
 */
public class LockInterruptibly {
    private static Lock lock = new ReentrantLock();

    public static void main(String[] args) throws InterruptedException {
        Runnable runnable = () -> {
            try {
                System.out.println(Thread.currentThread().getName() + " try to get lock");
                lock.lockInterruptibly();
                try {
                    System.out.println(Thread.currentThread().getName() + " got lock");
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    System.out.println(Thread.currentThread().getName() + " sleep ");
                } finally {
                    lock.unlock();
                    System.out.println(Thread.currentThread().getName() + " unlock");
                }
            } catch (InterruptedException e) {
                System.out.println(Thread.currentThread().getName() + " lockInterruptibly ");
            }
        };
        Thread thread1 = new Thread(runnable);
        Thread thread2 = new Thread(runnable);
        thread1.start();
        thread2.start();
        Thread.sleep(2000);
        thread2.interrupt();
    }
}
Thread-0 try to get lock
Thread-0 got lock
Thread-1 try to get lock
Thread-1 lockInterruptibly 
Thread-0 unlock

Process finished with exit code 0

1.4 可见性保证

  • Lock的加解锁和synchronized有同样的内存语义,也就是说下一个线程加锁后可以看到所有前一个线程解锁前发生的所有操作。拥有happens-before。

2. 锁的分类

2.1 乐观锁和悲观锁

  1. 悲观锁(互斥同步锁)的劣势
  • 阻塞和唤醒带来的性能劣势,悲观锁,锁住过后就是独占的。
  • 可能永久阻塞:如果尺有所的线程被永久阻塞,如遇到了死循环、死锁等活跃性问题,这时等待线程释放的锁的线程将永远得不到执行。
  • 优先级错乱:如果优先级低的线程获取到锁了,优先级高的也必须等待优先级低的锁释放。
  1. 什么是乐观锁和悲观锁
  • 乐观锁:总认为没人抢资源,所以通常先不加锁,等到出了问题了再处理。如果在更新的时候,去对比在我修改期间数据有没有被其他人修改过,如果没被修改过,那就说明真的只有自己操作,就去更新数据。那么如果被修改过,那就说明被人改了,此时就会选择放弃、报错、重试等策略。
    • 典型案例:乐观锁的实现一般都是利用CAS算法来实现,如:Atomic类、并发容器等
    • 典型案例:数据库中,可以添加一个version版本号,更新的时候先查询,然后更新的时候用update一条一句对版本进行判断并更新
    • 开销:虽然乐观锁一开始的开销比悲观锁校,但是如果自旋的事件很长或者不断重试,那么消费的资源也会越来越多。
    • 使用场景:乐观锁适用于:并发写入少,大部分是读取场景,不加锁的能让读取性能大幅度提高
  • 悲观锁:认为资源总是在竞争,如果不锁住就会造成数据错误,所以悲观锁为了保证正确性,会在每次获取并修改数据时,把数据锁住,让别人无法访问该数据,这样就可以确保数据内容万无一失;
    • 典型案例:Java中悲观锁典型的就是synchronizedLock相关类
    • 典型案例:数据库中select for update就是悲观锁
    • 开销:悲观锁的原始开销要高于乐观锁,但是一劳永逸,临界区尺有所时间就算越来越差,也不会对互斥锁的开销造成影响
    • 使用场景:悲观锁适用于临界区有IO操作,代码复杂或者循环量大,竞争非常激烈的情况,以避免大量的无用自旋等的性能消耗

2.2 可重入锁与非可重入锁

  • 以ReentrantLock为例,synchronized也支持
  1. 什么是可重入锁?
  • 可重入就是说某个线程已经获得某个锁,可以再次获取这个锁而不会出现死锁。
/**
* @author yiren
*/
public class ReentrantLockDemo {
   public static void main(String[] args) {
       Lock lock = new ReentrantLock();
       lock.lock();
       try {
           System.out.println("in 1");
           lock.lock();
           try {
               System.out.println("in 2");
           }finally {
               lock.unlock();
               System.out.println("out 2");
           }
       }finally {
           lock.unlock();
           System.out.println("out 1");
       }
   }
}
in 1
in 2
out 2
out 1

Process finished with exit code 0
  1. 可重入的好处
  • 避免死锁:如果一个方法已经获取到了锁,调用另外一个方法也要使用这个锁,那就会第二次加锁,如果不能成功获取锁,就会发生死锁。
  1. 代码演示
/**
* @author yiren
*/
public class ReentrantLockDemo {
   public static void main(String[] args) {
       ReentrantLock lock = new ReentrantLock();
       lock.lock();
       try {
           System.out.println("HoldCount:" + lock.getHoldCount() + " in 1");
           lock.lock();
           try {
               System.out.println("HoldCount:" + lock.getHoldCount() + " in 2");
               lock.lock();
               try {
                   System.out.println("HoldCount:" + lock.getHoldCount() + " in 3");
               }finally {
                   lock.unlock();
                   System.out.println("out 3");
               }
           }finally {
               lock.unlock();
               System.out.println("out 2");
           }
       }finally {
           lock.unlock();
           System.out.println("out 1");
       }
   }
}
HoldCount:1 in 1
HoldCount:2 in 2
HoldCount:3 in 3
out 3
out 2
out 1

Process finished with exit code 0
  1. 源码分析
  • ReentrantLock中默认是使用的NonfairSync,而NonfairSync继承自Sync,加锁和释放锁主要涉及里面下面两个方法,另外FairSync里面的关于重入锁部分也差不多。

        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

  • 加锁时在nonfairTryAcquire中else-if会判断如果当前线程就是已经占有锁的线程,则status就会加一,并返回true。
  • 释放锁时在tryRelease中也是先判断当前线程是否是已经占有锁的线程,然后在判断status,如果status等于0了,才真正释放锁。
  1. ReentrantLock其他方法介绍
    • isHeldByCurrentThread()可以查看出锁是否被当前线程锁持有
    • getQueueLength可以返回当前正在等待这把锁的队列有多长

2.3 公平锁与非公平锁

  1. 什么是公平与非公平锁
  • 公平:按照线程请求的顺序来分配锁
  • 非公平:不完全按照请求的顺序,在一定情况下可以插队;不过非公平锁,同样不提倡插队,它只在合适的时机插队,而不是盲目乱插队
  1. 为什么需要非公平锁
  • 注意:在ReentrantLock中,如果不指定,默认的实现就是非公平锁。如果在创建ReentrantLock是,传入参数true,此时就会变成公平锁
  • 使用非公平锁的原因是为了提高效率,避免唤醒带来的空档期
    • 比如:有三个线程,A现在持有锁,按照公平当A释放锁后,B就会唤醒执行,但是当A释放锁的时候,唤醒B,B没有及时响应还在唤醒中,线程C此时就可以立马执行,就会交给线程C执行,以此来避免B唤醒期间的资源浪费。
  1. 案例演示
    • 模拟打印工作,公平和非公平只需要修改printQueue里面ReentrantLock的参数
/**
 * @author yiren
 */
public class FairLock {

    public static void main(String[] args) throws InterruptedException {
        PrintQueue queue = new PrintQueue();
        ExecutorService executorService = Executors.newFixedThreadPool(4);
        for (int i = 0; i < 4; i++) {
            executorService.execute(()->{
                System.out.println(Thread.currentThread().getName()+ " start to print");
                queue.printJob(new Object());
                System.out.println(Thread.currentThread().getName()+ " finished print ");
            });
            TimeUnit.MILLISECONDS.sleep(100);
        }

    }

    private static class PrintQueue {
        private Lock lock = new ReentrantLock(true);

        private void printJob(Object document) {
            lock.lock();
            try {
                Integer duration = (int) (Math.random() * 3 + 1);
                System.out.println(Thread.currentThread().getName() + " print 1 need " + duration + " s");
                Thread.sleep(duration * 1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }

            lock.lock();
            try {
                Integer duration = (int) (Math.random() * 3 + 1);
                System.out.println(Thread.currentThread().getName() + " print 2 need " + duration + " s");
                Thread.sleep(duration * 1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                lock.unlock();
            }
        }
    }
}
  • 如果参数为true,为公平锁,结果如下
pool-1-thread-1 start to print
pool-1-thread-1 print 1 need 1 s
pool-1-thread-2 start to print
pool-1-thread-3 start to print
pool-1-thread-4 start to print
pool-1-thread-2 print 1 need 3 s
pool-1-thread-3 print 1 need 1 s
pool-1-thread-4 print 1 need 2 s
pool-1-thread-1 print 2 need 3 s
pool-1-thread-1 finished print 
pool-1-thread-2 print 2 need 2 s
pool-1-thread-2 finished print 
pool-1-thread-3 print 2 need 3 s
pool-1-thread-3 finished print 
pool-1-thread-4 print 2 need 3 s
pool-1-thread-4 finished print 

  • 我们可以通过结果看出,线程按照执行的先后顺序,来打印。不会出现插队的情况,先打印第一次,然后打印第二次,且多个线程依次执行。
  • 如果不给参数,就为非公平锁,结果如下:
pool-1-thread-1 start to print
pool-1-thread-1 print 1 need 3 s
pool-1-thread-2 start to print
pool-1-thread-3 start to print
pool-1-thread-4 start to print
pool-1-thread-1 print 2 need 2 s
pool-1-thread-1 finished print 
pool-1-thread-2 print 1 need 3 s
pool-1-thread-2 print 2 need 1 s
pool-1-thread-2 finished print 
pool-1-thread-3 print 1 need 3 s
pool-1-thread-3 print 2 need 2 s
pool-1-thread-3 finished print 
pool-1-thread-4 print 1 need 3 s
pool-1-thread-4 print 2 need 1 s
pool-1-thread-4 finished print 

  • 非公平状态下,我们可以看到,打印完第一次,如果按照排队顺序应该是线程2,但是打印的实际是线程1的第二次。
  1. 特例
  • tryLock()它不遵守设定的公平规则。也就是说:当有线程执行tryLock的时候,一旦有线程释放了锁,即使他之前已经有其他在等待队列里的线程,这个正在tryLock的线程依旧能获取到锁。
  1. 优缺点分析
  • 公平锁:
    • 优点:各个线程公平,每个线程在等待一段时间后,总有执行机会。
    • 缺点:更慢,吞吐量更小
  • 非公平锁:
    • 优点:更快,吞吐量更大
    • 缺点:有可能某些线程会产生饥饿,线程长时间,始终得不到执行
  1. 源码分析
  • 公平锁
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }
  • 非公平锁:

        /**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
  • 两者在获取锁的代码中,最主要的区别就是公平锁有一个!hasQueuedPredecessors(),它会判断是否有现成在队列前面已经排队了,如果没有才去获取锁。

2.4 共享锁和排他锁

  • Java中ReentrantReadWriteLock为代表
  1. 什么是共享锁和排他锁
  • 共享锁:又称读锁,获取共享锁过后,可以查看但是无法修改和删除,其他线程可以同时获取到共享锁

  • 排他锁:又称独占锁、独占锁,获取了排他锁后既可以读又可以写,但是其他线程无法再次获取。

  1. 读写锁的作用
  • 如果我们不适用读写锁,那么我们多个线程读的操作,并不能同时进行,只能排队,虽然没有线程安全问题,但是性能会变差。

  • 如果我们在读的地方用读锁,写的地方用写锁,可以提高效率。

  1. 读写锁的规则
  • 多个线程读锁可以重复获取
  • 但是如果有线程以及获取了读锁,那么其他线程就不可以获取写锁
  • 但是如果有线程以及获取了写锁,那么其他线程就不可以获取写锁
  • 总结:读写互斥、写写互斥。
  1. ReentrantReadWriteLock用法
/**
 * @author yiren
 */
public class ReadWriteLock {
    private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    private static ReentrantReadWriteLock.ReadLock readLock = readWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock = readWriteLock.writeLock();

    private static void read() {
        readLock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + " start to read, got read lock");
            TimeUnit.SECONDS.sleep(1);
            System.out.println(Thread.currentThread().getName() + " read finished, release read lock");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            readLock.unlock();
        }
    }

    private static void write() {
        writeLock.lock();
        try {
            System.out.println(Thread.currentThread().getName() + " start to write, got write lock");
            TimeUnit.SECONDS.sleep(1);
            System.out.println(Thread.currentThread().getName() + " read finished, release write lock");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        for (int i = 0; i < 2; i++) {
            executorService.execute(ReadWriteLock::write);
        }
        for (int i = 0; i < 5; i++) {
            executorService.execute(ReadWriteLock::read);
        }

    }
}
pool-1-thread-1 start to write, got write lock
pool-1-thread-1 read finished, release write lock
pool-1-thread-2 start to write, got write lock
pool-1-thread-2 read finished, release write lock
pool-1-thread-3 start to read, got read lock
pool-1-thread-2 start to read, got read lock
pool-1-thread-1 start to read, got read lock
pool-1-thread-3 read finished, release read lock
pool-1-thread-1 read finished, release read lock
pool-1-thread-2 read finished, release read lock
pool-1-thread-1 start to read, got read lock
pool-1-thread-3 start to read, got read lock
pool-1-thread-3 read finished, release read lock
pool-1-thread-1 read finished, release read lock

  • 我们可以看到,读可以同时进行,而写的时候则是需要等持有写锁的线程的完成了,再进入到另一个写锁,并且我们可以看到,当写锁持有的时候,读锁也立即获取到,而是等待写锁完成后,再获取到读锁
  1. 读锁插队策略
  • 按照上面所说,如果先进入读任务,那么来了按顺序再来一个写锁,然后再来一个读锁,我们可以试想,读锁,不需要排队,可以直接进入。此时会有一个问题,如果后面继续再来读锁,写锁是不是一直获取不了。就会造成饥饿。
  • ReentrantReadWriteLock(非公平锁时,公平情况下都得排队)并不是这样做的,它的策略是,如果读任务正在进行,此时先来一个写锁排在队头部,然后再来一个读锁它发现队列头部是写锁任务,此时进来的读任务就不会插队,会进入队列排在写锁之后,以保证写锁可以得到执行。宁可降低一点性能,也要避免写线程饥饿。
  • 看下非公平锁是否插队判断的源码:
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -8159625535654395037L;
        final boolean writerShouldBlock() {
            return false; // writers can always barge
        }
        final boolean readerShouldBlock() {
            /* As a heuristic to avoid indefinite writer starvation,
             * block if the thread that momentarily appears to be head
             * of queue, if one exists, is a waiting writer.  This is
             * only a probabilistic effect since a new reader will not
             * block if there is a waiting writer behind other enabled
             * readers that have not yet drained from the queue.
             */
            return apparentlyFirstQueuedIsExclusive();
        }
    }
  • 上面注释就说明了,写的人总是可以插队
  • 但是读者调用了apparentlyFirstQueuedIsExclusive队列头结点是不是排他锁(写锁)如果是就不允许插队了。
  • 我们可以对上面读写锁的案例进行修改一下main方法
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(5);
        executorService.execute(ReadWriteLock::write);
        executorService.execute(ReadWriteLock::read);
        executorService.execute(ReadWriteLock::read);
        executorService.execute(ReadWriteLock::write);
        executorService.execute(ReadWriteLock::read);

    }
pool-1-thread-1 start to write, got write lock
pool-1-thread-1 read finished, release write lock
pool-1-thread-2 start to read, got read lock
pool-1-thread-3 start to read, got read lock
pool-1-thread-2 read finished, release read lock
pool-1-thread-3 read finished, release read lock
pool-1-thread-4 start to write, got write lock
pool-1-thread-4 read finished, release write lock
pool-1-thread-5 start to read, got read lock
pool-1-thread-5 read finished, release read lock
  • 此时我们就可以看到,线程5读线程,并没有插队执行,而是等待了线程4完成了,再执行。

  • 额外提醒:读锁在队列头部不是写锁的时候,是可以插队的。

    • 如现在的队列是这样的:Reader->Reader->Writer->Reader,这最后一个读锁,就有可能和前两个一起执行。我们修改一下上面的代码,把线程数改成4
    
        public static void main(String[] args) {
            ExecutorService executorService = Executors.newFixedThreadPool(4);
            executorService.execute(ReadWriteLock::write);
            executorService.execute(ReadWriteLock::read);
            executorService.execute(ReadWriteLock::read);
            executorService.execute(ReadWriteLock::write);
            executorService.execute(ReadWriteLock::read);
        }
    
    pool-1-thread-1 start to write, got write lock
    pool-1-thread-1 read finished, release write lock
    pool-1-thread-2 start to read, got read lock
    pool-1-thread-3 start to read, got read lock
    pool-1-thread-1 start to read, got read lock
    pool-1-thread-2 read finished, release read lock
    pool-1-thread-3 read finished, release read lock
    pool-1-thread-1 read finished, release read lock
    pool-1-thread-4 start to write, got write lock
    pool-1-thread-4 read finished, release write lock
    
    • 可以看此时的执行的就是三个读锁先执行了,然后再执行写锁!
  1. 读写锁的升降级
  • 支持锁的降级,但是不支持升级
  • 代码演示:
/**
 * @author yiren
 */
public class ReadWriteLockLevel {
    public static void main(String[] args) {
        ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
        Thread thread = new Thread(() -> {
            readWriteLock.writeLock().lock();
            try {
                System.out.println("writer task!");
                Thread.sleep(1000);
                readWriteLock.readLock().lock();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                readWriteLock.writeLock().unlock();
            }
            try {
                System.out.println("reader task!");
                Thread.sleep(1000);
                System.out.println("reader task! end");
            } catch (InterruptedException e) {
                e.printStackTrace();
            } finally {
                readWriteLock.readLock().unlock();
            }
        });

        Thread thread1 = new Thread(() -> {
            readWriteLock.readLock().lock();
            try {
                System.out.println("other reader task!");
            }finally {
                readWriteLock.readLock().unlock();
            }

        });

        thread.start();
        thread1.start();
    }
}
writer task!
reader task!
other reader task!
reader task! end

Process finished with exit code 0
  • 我们可以看到,锁降级过后,读锁就可以再次获取
  • 而读锁是不能升级成写锁的,上面就说过,读锁和写锁不会同时存在!

2.5 自旋锁和阻塞锁

  1. 什么是自旋锁和阻塞锁?
  • 阻塞或唤醒一个线程需要操作系统切换CPU状态来完成,这种状态转换需要耗费处理器时间
  • 如果同步代码块中的内容过于简单,状态装换消耗的事件可能比用户代码执行的时间还要长
  • 在许多场景中,同步资源锁定时间很短,为了这一小段时间去切换线程,线程挂起和恢复现场的花费可能会让系统得不偿失
  • 如果物理机器有多个处理器,能够让两个或者以上的线程同时并行执行,我们就可以让后面那个请求锁的线程不放弃CPU的执行时间,看看持有锁的线程是否很快就会释放锁。
  • 为了让当前线程等一下,我们就让当前线程进行自旋,如果在自旋完成后,前面锁定同步资源的线程以及释放了锁,那么当前线程就可以不必阻塞而是直接获取同步资源,从而避免切换线程的开销。这就是自旋锁
  • 相反阻塞锁就是如果线程没有拿到锁,就会直接把线程阻塞,知道被唤醒。
  1. 自旋锁的缺点:如果锁的占用时间过长,那么自旋的线程就会白白浪费处理器资源,浪费资源随时间线性增长

  2. 原理和源码分析

    • 在J.U.C下atomic包下的类基本都是自旋锁试下
    • 如:AtomicInteger:自旋锁实现是CAS,AtomicInteger中调用了unsafe进行自增操作的源码中的do-while循环就是一个自旋操作,如果修改过程中遇到其他线程竞争导致没修改成功,就在while里面疯狂循环,直到修改成功
        // AtomicInteger
        public final int getAndIncrement() {
            return unsafe.getAndAddInt(this, valueOffset, 1);
        }
    
        // Unsafe
        public final int getAndAddInt(Object var1, long var2, int var4) {
            int var5;
            do {
                var5 = this.getIntVolatile(var1, var2);
            } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
    
            return var5;
        }
    
    
  3. 自己实现一个简单的自旋锁:

/**
 * @author yiren
 */
public class SpinLock {
    private static AtomicReference<Thread> sign = new AtomicReference<>();

    private static void lock() {
        Thread current = Thread.currentThread();
        while (!sign.compareAndSet(null, current)) {
            System.out.println("fail to set!");
        }
    }

    private static void unlock() {
        Thread thread = Thread.currentThread();
        sign.compareAndSet(thread, null);
    }

    public static void main(String[] args) {
        Runnable runnable = () -> {
            System.out.println("start to get lock");
            SpinLock.lock();
            System.out.println("got lock successfully!");
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                SpinLock.unlock();
            }
        };

        Thread thread = new Thread(runnable);
        Thread thread1 = new Thread(runnable);

        thread.start();
        thread1.start();
    }
}
  1. 自旋锁使用场景:
    • 自旋锁一般用于多核的服务器,在并发度不高的情况下,比阻塞锁效率高
    • 用于临界区比较短小的情况下,否则线程一旦拿到锁,很久才释放,就会造成性能浪费了。

2.5 可中断锁与不可中断锁

  • 在java中,synchronized就是不可中断锁,而Lock是可中断锁,可通过tryLock(time)lockInterruptibly来实现响应中断

  • 上面Lock接口案例演示中已经演示过,可看第一部分的LockInterruptibly

3. 锁优化

3.1 JVM对锁的优化

  • 自旋锁和自适应:比如自旋多少过后,它会把锁编程阻塞锁
  • 锁消除:有些场景下,不需要加锁,JVM会分析出来,然后直接消除它
  • 锁粗化:如果一系列操作都是对一个对象反复加锁,也会带来性能开销,所以JVM会把它们合成一次加解锁。

3.2 编码优化

  • 缩小同步代码块,只锁需要锁的

  • 尽量不要锁住方法

  • 减少锁的请求次数,减少频繁获取锁的开销。

  • 避免人为制造“热点”,比如一个集合你每次用大小都去遍历一遍计数

  • 锁里面尽量不要包含锁

  • 选择合适的锁的类型或者合适的工具类


  • 文章内容来源:
  • 《Java并发编程艺术》、JDK1.8版本源码、慕课网悟空JUC课程