StampedLock

港控/mmm° 2022-10-11 01:08 167阅读 0赞

jdk中提供了ReadWriteLock来实现读写锁，从而实现了读读不互斥，读写互斥，但是说到底还是一种悲观锁的形式，因为读的时候不允许写。

jdk中还提供了一种乐观锁的读写锁StampedLock。乐观读不加锁，但是要有一种机制检测是否当前有写线程正在写入。

源码中示例:

*  <pre>{@code
     * 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);
     *     }
     *   }
     *
     *   double distanceFromOrigin() { // A read-only method
     *     long stamp = sl.tryOptimisticRead();
     *     double currentX = x, currentY = y;
     *     if (!sl.validate(stamp)) {
     *        stamp = sl.readLock();
     *        try {
     *          currentX = x;
     *          currentY = y;
     *        } finally {
     *           sl.unlockRead(stamp);
     *        }
     *     }
     *     return Math.sqrt(currentX * currentX + currentY * currentY);
     *   }
     *
     *   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) {
     *         long ws = sl.tryConvertToWriteLock(stamp);
     *         if (ws != 0L) {
     *           stamp = ws;
     *           x = newX;
     *           y = newY;
     *           break;
     *         }
     *         else {
     *           sl.unlockRead(stamp);
     *           stamp = sl.writeLock();
     *         }
     *       }
     *     } finally {
     *       sl.unlock(stamp);
     *     }
     *   }
     * }}</pre>