最普通的获取锁,最佳实践是finally中释放锁,保证发生异常的时候锁一定被释放
/** * 描述:Lock不会像syn一样,异常的时候自动释放锁 * 所以最佳实践是finally中释放锁,保证发生异常的时候锁一定被释放 */ private static Lock lock = new ReentrantLock(); public static void main(String[] args) { lock.lock(); try { //获取本锁保护的资源 System.out.println(Thread.currentThread().getName() + "开始执行任务"); } finally { lock.unlock(); } }
用来获取锁,如果当前锁没有被其它线程占用,则获取成功,则返回true,否则返回false,代表获取锁失败
/** * 描述:用TryLock避免死锁 */ static class TryLockDeadlock implements Runnable { int flag = 1; static Lock lock1 = new ReentrantLock(); static Lock lock2 = new ReentrantLock(); @Override public void run() { for (int i = 0; i < 100; i++) { if (flag == 1) { try { if (lock1.tryLock(800, TimeUnit.MILLISECONDS)) { try { System.out.println("线程1获取到了锁1"); Thread.sleep(new Random().nextInt(1000)); if (lock2.tryLock(800,TimeUnit.MILLISECONDS)){ try { System.out.println("线程1获取到了锁2"); System.out.println("线程1成功获取到了2把锁"); break; }finally { lock2.unlock(); } }else{ System.out.println("线程1获取锁2失败,已重试"); } } finally { lock1.unlock(); Thread.sleep(new Random().nextInt(1000)); } } else { System.out.println("线程1获取锁1失败,已重试"); } } catch (InterruptedException e) { e.printStackTrace(); } } if (flag == 0) { try { if (lock2.tryLock(3000, TimeUnit.MILLISECONDS)) { try { System.out.println("线程2获取到了锁2"); Thread.sleep(new Random().nextInt(1000)); if (lock1.tryLock(800,TimeUnit.MILLISECONDS)){ try { System.out.println("线程2获取到了锁1"); System.out.println("线程2成功获取到了2把锁"); break; }finally { lock1.unlock(); } }else{ System.out.println("线程2获取锁1失败,已重试"); } } finally { lock2.unlock(); Thread.sleep(new Random().nextInt(1000)); } } else { System.out.println("线程2获取锁2失败,已经重试"); } } catch (InterruptedException e) { e.printStackTrace(); } } } } public static void main(String[] args) { TryLockDeadlock r1 = new TryLockDeadlock(); TryLockDeadlock r2 = new TryLockDeadlock(); r1.flag = 1; r2.flag = 0; new Thread(r1).start(); new Thread(r2).start(); } } 执行结果: 线程1获取到了锁1 线程2获取到了锁2 线程1获取锁2失败,已重试 线程2获取到了锁1 线程2成功获取到了2把锁 线程1获取到了锁1 线程1获取到了锁2 线程1成功获取到了2把锁
相当于tryLock(long time,TimeUnit unit) 把超时时间设置为无限,在等待锁的过程中,线程可以被中断
/** * 描述:获取锁的过程中,中断了 */ static class LockInterruptibly implements Runnable { private Lock lock = new ReentrantLock(); @Override public void run() { System.out.println(Thread.currentThread().getName() + "尝试获取锁"); try { lock.lockInterruptibly(); try { System.out.println(Thread.currentThread().getName() + "获取到了锁"); Thread.sleep(5000); } catch (InterruptedException e) { System.out.println(Thread.currentThread().getName() + "睡眠中被中断了"); } finally { lock.unlock(); System.out.println(Thread.currentThread().getName() + "释放了锁"); } } catch (InterruptedException e) { System.out.println(Thread.currentThread().getName() + "等锁期间被中断了"); } } public static void main(String[] args) { LockInterruptibly lockInterruptibly = new LockInterruptibly(); Thread thread0 = new Thread(lockInterruptibly); Thread thread1 = new Thread(lockInterruptibly); thread0.start(); thread1.start(); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } thread0.interrupt(); } } 执行结果: Thread-0尝试获取锁 Thread-1尝试获取锁 Thread-0获取到了锁 Thread-0睡眠中被中断了 Thread-0释放了锁 Thread-1获取到了锁 Thread-1释放了锁
乐观锁:
比较乐观,认为自己在处理操作的时候,不会有其它线程来干扰,所以并不会锁住操作对象
劣势:
可能造成ABA问题,就是不知道是不是修改过
使用场景:
适合并发写入少的情况,大部分是读取的场景,不加锁的能让读取的性能大幅提高
悲观锁:
比较悲观,认为如果我不锁住这个资源,别人就会来争抢,就会造成数据结果错误,所以它会锁住操作对象,Java中悲观锁的实现就是syn和Lock相关类
劣势:
使用场景:
适合并发写入多的情况,适用于临界区持锁时间比较长的情况:
可重入就是说某个线程已经获得某个锁,可以再次获取锁而不会出现死锁
ReentrantLock 和 synchronized 都是可重入锁
// 递归调用演示可重入锁 static class RecursionDemo{ public static ReentrantLock lock = new ReentrantLock(); private static void accessResource(){ lock.lock(); try { System.out.println("已经对资源处理了"); if (lock.getHoldCount() < 5){ System.out.println("已经处理了"+lock.getHoldCount()+"次"); accessResource(); } }finally { lock.unlock(); } } public static void main(String[] args) { new RecursionDemo().accessResource(); } } 执行结果: 已经对资源处理了 已经处理了1次 已经对资源处理了 已经处理了2次 已经对资源处理了 已经处理了3次 已经对资源处理了 已经处理了4次 已经对资源处理了
ReentrantLock的其它方法
/** * 描述:演示公平锁和非公平锁 */ class FairLock{ public static void main(String[] args) { PrintQueue printQueue = new PrintQueue(); Thread[] thread = new Thread[10]; for (int i = 0; i < 10; i++) { thread[i] = new Thread(new Job(printQueue)); } for (int i = 0; i < 5; i++) { thread[i].start(); try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } } } } class Job implements Runnable{ PrintQueue printQueue; public Job(PrintQueue printQueue) { this.printQueue = printQueue; } @Override public void run() { System.out.println(Thread.currentThread().getName()+"开始打印"); printQueue.printJob(new Object()); System.out.println(Thread.currentThread().getName()+"打印完成"); } } class PrintQueue{ // true 公平,false是非公平 private Lock queueLock = new ReentrantLock(true); public void printJob(Object document){ queueLock.lock(); try { int duration = new Random().nextInt(10)+1; System.out.println(Thread.currentThread().getName()+"正在打印,需要"+duration+"秒"); Thread.sleep(duration * 1000); } catch (InterruptedException e) { e.printStackTrace(); } finally { queueLock.unlock(); } queueLock.lock(); try { int duration = new Random().nextInt(10)+1; System.out.println(Thread.currentThread().getName()+"正在打印,需要"+duration+"秒"); Thread.sleep(duration * 1000); } catch (InterruptedException e) { e.printStackTrace(); } finally { queueLock.unlock(); } } } 执行结果: Thread-0开始打印 Thread-0正在打印,需要10秒 Thread-1开始打印 Thread-2开始打印 Thread-3开始打印 Thread-4开始打印 Thread-1正在打印,需要2秒 Thread-2正在打印,需要2秒 Thread-3正在打印,需要2秒 Thread-4正在打印,需要4秒 Thread-0正在打印,需要2秒 Thread-0打印完成 Thread-1正在打印,需要7秒 Thread-1打印完成 Thread-2正在打印,需要8秒 Thread-2打印完成 Thread-3正在打印,需要3秒 Thread-3打印完成 Thread-4正在打印,需要8秒 Thread-4打印完成 true改为false演示非公平锁: Lock queueLock = new ReentrantLock(false); 执行结果: Thread-0正在打印,需要7秒 Thread-1开始打印 Thread-2开始打印 Thread-3开始打印 Thread-4开始打印 Thread-0正在打印,需要9秒 Thread-0打印完成 Thread-1正在打印,需要3秒 Thread-1正在打印,需要2秒 Thread-1打印完成 Thread-2正在打印,需要4秒 Thread-2正在打印,需要7秒 Thread-2打印完成 Thread-3正在打印,需要10秒 Thread-3正在打印,需要2秒 Thread-3打印完成 Thread-4正在打印,需要7秒 Thread-4正在打印,需要8秒 Thread-4打印完成
/** * 描述:演示可以多个一起读,只能一个写 */ class CinemaReadWrite{ private static ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock(); private static ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock(); private static ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock(); private static void read(){ readLock.lock(); try { System.out.println(Thread.currentThread().getName() + "得到了读锁,正在读取"); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } finally { System.out.println(Thread.currentThread().getName() + "释放了读锁"); readLock.unlock(); } } private static void write(){ writeLock.lock(); try { System.out.println(Thread.currentThread().getName() + "得到了写锁,正在写入"); Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } finally { System.out.println(Thread.currentThread().getName() + "释放了写锁"); writeLock.unlock(); } } public static void main(String[] args) { new Thread(()-> read(),"Thrad1").start(); new Thread(()-> read(),"Thrad2").start(); new Thread(()-> write(),"Thrad3").start(); new Thread(()-> write(),"Thrad4").start(); } } 执行结果: Thrad1得到了读锁,正在读取 Thrad2得到了读锁,正在读取 Thrad2释放了读锁 Thrad1释放了读锁 Thrad3得到了写锁,正在写入 Thrad3释放了写锁 Thrad4得到了写锁,正在写入 Thrad4释放了写锁
读锁和写锁的交互方式:
读锁插队策略:
自旋缺点:
原理:
/** * 描述:自旋锁演示 */ class SpinLock{ private AtomicReference<Thread> sign = new AtomicReference<>(); public void lock(){ Thread currentThread = Thread.currentThread(); while (!sign.compareAndSet(null,currentThread)){ System.out.println("自旋获取失败,再次尝试"); } } public void unLock(){ Thread currentThread = Thread.currentThread(); sign.compareAndSet(currentThread,null); } public static void main(String[] args) { SpinLock spinLock = new SpinLock(); Runnable runnable = new Runnable(){ @Override public void run(){ System.out.println(Thread.currentThread().getName()+"开始尝试自旋锁"); spinLock.lock(); System.out.println(Thread.currentThread().getName()+"获取到了自旋锁"); try { Thread.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }finally { spinLock.unLock(); System.out.println(Thread.currentThread().getName()+"释放了自旋锁"); } } }; Thread thread1 = new Thread(runnable); Thread thread2 = new Thread(runnable); thread1.start(); thread2.start(); } } 执行结果: Thread-0开始尝试自旋锁 Thread-0获取到了自旋锁 Thread-1开始尝试自旋锁 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 自旋获取失败,再次尝试 Thread-0释放了自旋锁 Thread-1获取到了自旋锁 Thread-1释放了自旋锁
使用场景: