@Slf4j public class LockExample2 { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; public static int count = 0; private final static Lock lock = new ReentrantLock(); public static void main(String[] args) throws Exception { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal ; i++) { executorService.execute(() -> { try { semaphore.acquire(); add(); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("count:{}", count); } private static void add() { lock.lock(); try { count++; } finally { lock.unlock(); } } }
我们首先使用 private final static Lock lock = new ReentrantLock()声明一个所得实例,然后使用
lock.lock(); try { count++; } finally { lock.unlock(); }
进行加锁和解锁操作。
我们在通过一个例子来看看这个ReentrantReadWriteLock怎么用。
@Slf4j public class LockExample3 { private final Map<String, Data> map = new TreeMap<>(); private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(); private final Lock readLock = lock.readLock(); private final Lock writeLock = lock.writeLock(); public Data get(String key) { readLock.lock(); try { return map.get(key); } finally { readLock.unlock(); } } public Set<String> getAllKeys() { readLock.lock(); try { return map.keySet(); } finally { readLock.unlock(); } } public Data put(String key, Data value) { writeLock.lock(); try { return map.put(key, value); } finally { readLock.unlock(); } } class Data { } }
通过 private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock()声明一个ReentrantReadWriteLock,然后再分别获取 private final Lock readLock = lock.readLock() private final Lock writeLock = lock.writeLock()读锁和写锁。
我们在这个map读的时候加上读锁在写的时候加上写锁,但是这里有问题就是这个锁是悲观锁,也就是说在执行写锁的时候一定不能有读锁,当读操作特 特别多的时候很有可能会让写锁一直无法执行。
我们看一下官方的例子学习一下,StampedLock
import java.util.concurrent.locks.StampedLock; public class LockExample4 { class Point { private double x, y; private final StampedLock sl = new StampedLock(); void move(double deltaX, double deltaY) { // an exclusively locked method long stamp = sl.writeLock(); try { x += deltaX; y += deltaY; } finally { sl.unlockWrite(stamp); } } //下面看看乐观读锁案例 double distanceFromOrigin() { // A read-only method long stamp = sl.tryOptimisticRead(); //获得一个乐观读锁 double currentX = x, currentY = y; //将两个字段读入本地局部变量 if (!sl.validate(stamp)) { //检查发出乐观读锁后同时是否有其他写锁发生? stamp = sl.readLock(); //如果没有,我们再次获得一个读悲观锁 try { currentX = x; // 将两个字段读入本地局部变量 currentY = y; // 将两个字段读入本地局部变量 } finally { sl.unlockRead(stamp); } } return Math.sqrt(currentX * currentX + currentY * currentY); } //下面是悲观读锁案例 void moveIfAtOrigin(double newX, double newY) { // upgrade // Could instead start with optimistic, not read mode long stamp = sl.readLock(); try { while (x == 0.0 && y == 0.0) { //循环,检查当前状态是否符合 long ws = sl.tryConvertToWriteLock(stamp); //将读锁转为写锁 if (ws != 0L) { //这是确认转为写锁是否成功 stamp = ws; //如果成功 替换票据 x = newX; //进行状态改变 y = newY; //进行状态改变 break; } else { //如果不能成功转换为写锁 sl.unlockRead(stamp); //我们显式释放读锁 stamp = sl.writeLock(); //显式直接进行写锁 然后再通过循环再试 } } } finally { sl.unlock(stamp); //释放读锁或写锁 } } } }
我们再将前面的里改成StampedLock
@Slf4j public class LockExample5 { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; public static int count = 0; private final static StampedLock lock = new StampedLock(); public static void main(String[] args) throws Exception { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i < clientTotal ; i++) { executorService.execute(() -> { try { semaphore.acquire(); add(); semaphore.release(); } catch (Exception e) { log.error("exception", e); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("count:{}", count); } private static void add() { long stamp = lock.writeLock(); try { count++; } finally { lock.unlock(stamp); } } }
这里和之前的不一样的地方就是
long stamp = lock.writeLock(); try { count++; } finally { lock.unlock(stamp); }
在加锁后会返回一个值,解锁的时候需要传入这个值。