JDK提供了写锁接口ReadWriteLock和它的实现ReentrantReadWriteLock。要实现一个读写锁,需要考虑很多细节,其中之一就是锁升级和锁降级的问题。什么是升级和降级呢?ReadWriteLock的javadoc有一段话:
Can the write lock be downgraded to a read lock without allowing an intervening writer? Can a read lock be upgraded to a write lock,in preference to other waiting readers or writers?
锁降级:从写锁变成读锁;锁升级:从读锁变成写锁。读锁是可以被多线程共享的,写锁是单线程独占的。也就是说写锁的并发限制比读锁高,这可能就是升级/降级名称的来源。
如下代码会产生死锁,因为同一个线程中,在没有释放读锁的情况下,就去申请写锁,这属于锁升级,ReentrantReadWriteLock是不支持的。
ReadWriteLock rtLock = new ReentrantReadWriteLock(); rtLock.readLock().lock(); System.out.println("get readLock."); rtLock.writeLock().lock(); System.out.println("blocking");
ReentrantReadWriteLock支持锁降级,如下代码不会产生死锁。
ReadWriteLock rtLock = new ReentrantReadWriteLock(); rtLock.writeLock().lock(); System.out.println("writeLock"); rtLock.readLock().lock(); System.out.println("get read lock");
这段代码虽然不会导致死锁,但没有正确的释放锁。从写锁降级成读锁,并不会自动释放当前线程获取的写锁,仍然需要显示的释放,否则别的线程永远也获取不到写锁。锁的释放和获取可以看下:可重入锁的获取和释放需要注意的一点儿事
final ReadWriteLock readWriteLock = new ReentrantReadWriteLock(); Thread wt = new Thread(new Runnable() { @Override public void run() { readWriteLock.writeLock().lock(); System.out.println("writeLock"); readWriteLock.readLock().lock(); System.out.println("readLock"); readWriteLock.readLock().unlock(); System.out.println("block"); } }); wt.start(); Thread.sleep(100); System.out.println("main blocking."); readWriteLock.readLock().lock();
JDK读写锁ReadWriteLock的升级和降级问题
时间: 2024-10-12 02:22:39