StampedLock e letture ottimistiche

Che cos'è StampedLock?

import java.util.concurrent.locks.*;

StampedLock sl = new StampedLock();

// Exclusive write:
long stamp = sl.writeLock();
try {
    // write data
} finally {
    sl.unlockWrite(stamp);
}

// Shared read:
stamp = sl.readLock();
try {
    // read data
} finally {
    sl.unlockRead(stamp);
}

Modalità di lettura ottimistica

StampedLock sl2 = new StampedLock();
double x = 0, y = 0;

// Try optimistic read first:
long stamp = sl2.tryOptimisticRead();
double localX = x, localY = y; // read values

if (!sl2.validate(stamp)) {
    // A write occurred — fall back to read lock
    stamp = sl2.readLock();
    try {
        localX = x;
        localY = y;
    } finally {
        sl2.unlockRead(stamp);
    }
}
// use localX and localY

Esempio completo con Point2D

class Point {
    private double x, y;
    private final StampedLock lock = new StampedLock();
    
    void move(double dx, double dy) {
        long stamp = lock.writeLock();
        try { x += dx; y += dy; }
        finally { lock.unlockWrite(stamp); }
    }
    
    double distanceFromOrigin() {
        long stamp = lock.tryOptimisticRead();
        double cx = x, cy = y;
        if (!lock.validate(stamp)) {
            stamp = lock.readLock();
            try { cx = x; cy = y; }
            finally { lock.unlockRead(stamp); }
        }
        return Math.sqrt(cx * cx + cy * cy);
    }
}

Conversione del lock

StampedLock sl3 = new StampedLock();

// Read → Write upgrade attempt:
long readStamp = sl3.readLock();
long writeStamp = sl3.tryConvertToWriteLock(readStamp);
if (writeStamp != 0) {
    // Successfully upgraded!
    // ... write ...
    sl3.unlockWrite(writeStamp);
} else {
    // Upgrade failed — release read and reacquire write
    sl3.unlockRead(readStamp);
    writeStamp = sl3.writeLock();
    // ... write ...
    sl3.unlockWrite(writeStamp);
}

Quando le letture ottimistiche sono più utili

StampedLock e ReadWriteLock a confronto

StampedLock NON è rientrante

StampedLock sl4 = new StampedLock();
long s1 = sl4.readLock();
// DO NOT: sl4.readLock() again — deadlocks with non-reentrant!

tryWriteLock e tryReadLock

StampedLock sl5 = new StampedLock();

long stamp = sl5.tryWriteLock();
if (stamp != 0) {
    try {
        System.out.println("Got write lock");
    } finally {
        sl5.unlockWrite(stamp);
    }
} else {
    System.out.println("Write lock unavailable");
}

Usarlo come ReadWriteLock

StampedLock sl6 = new StampedLock();
Lock readLock  = sl6.asReadLock();
Lock writeLock = sl6.asWriteLock();
ReadWriteLock rwView = sl6.asReadWriteLock();
// These provide standard Lock interface without optimistic mode

Indicazioni sul benchmark delle prestazioni

Invalidità dello stamp dopo il rilascio del lock

StampedLock sl7 = new StampedLock();
long stamp = sl7.writeLock();
sl7.unlockWrite(stamp);
// stamp is now invalid — don't use it:
// sl7.unlockWrite(stamp); // throws IllegalMonitorStateException or corrupts state

Verifica rapida

Riepilogo: StampedLock