协作对象之间的死锁

Question

While going through the Java Concurrency in Practice I came across the below code Deadlock , which demonstrated how a deadlock can occur between cooperating objects . If one thread calls getImage() and at the same time another calls setLocation() it can lead to deadlock due to lock ordering . To solve this we use Open calls No Deadlock below . My doubt is inside setLocation() in No Deadlock why are we doing synchronized(this) again inside the already synchronized method ? Isn't the synchronized method already synchronized on this ? How are we preventing the deadlock using the 2nd example?

DeadLock

class Taxi {
        @GuardedBy("this")
        private Point location, destination;
        private final Dispatcher dispatcher;

        public Taxi(Dispatcher dispatcher) {
            this.dispatcher = dispatcher;
        }

        public synchronized Point getLocation() {
            return location;
        }

        public synchronized void setLocation(Point location) {
            this.location = location;
            if (location.equals(destination))
                dispatcher.notifyAvailable(this);
        }
    }

    class Dispatcher {
        @GuardedBy("this")
        private final Set<Taxi> taxis;
        @GuardedBy("this")
        private final Set<Taxi> availableTaxis;

        public Dispatcher() {
            taxis = new HashSet<Taxi>();
            availableTaxis = new HashSet<Taxi>();
        }

        public synchronized void notifyAvailable(Taxi taxi) {
            availableTaxis.add(taxi);
        }

        public synchronized Image getImage() {
            Image image = new Image();
            for (Taxi t : taxis)
                image.drawMarker(t.getLocation());
            return image;
        }
    }

No Deadlock

class Taxi {
        @GuardedBy("this")
        private Point location, destination;
        private final Dispatcher dispatcher;

        public synchronized Point getLocation() {
            return location;
        }

        public synchronized void setLocation(Point location) {
            boolean reachedDestination;
            synchronized (this) {
                this.location = location;
                reachedDestination = location.equals(destination);
            }
            if (reachedDestination)
                dispatcher.notifyAvailable(this);
        }
    }

    class Dispatcher {
        @GuardedBy("this")
        private final Set<Taxi> taxis;
        @GuardedBy("this")
        private final Set<Taxi> availableTaxis;

        public synchronized void notifyAvailable(Taxi taxi) {
            availableTaxis.add(taxi);
        }

        public Image getImage() {
            Set<Taxi> copy;
            synchronized (this) {
                copy = new HashSet<Taxi>(taxis);
            }
            Image image = new Image();
            for (Taxi t : copy)
                image.drawMarker(t.getLocation());
            return image;
        }
    }

Answer 1

There is no deadlock in second because getImage not synchronized, so when calling getLocation no lock is held. You're right about the synchronized(this) inside setLocation, it serves no purpose.

Answer 2

The Taxi.setLocation method in the "No Deadlock" code listing shouldn't be synchronized .

The online errata page reads:

p.214 In Listing 10.6, Taxi.setLocation should not be a synchronized method. (The synchronized block in its body is correct, however.) (Fixed in 6th printing.)

In the deadlock-prone code, when a thread calls Taxi.setLocation , it first acquires the Taxi lock. And while holding the Taxi lock , it proceeds to get the Dispatcher lock.

Taxi.setLocation

┌----lock Taxi
|  ┌-lock Dispatcher
│  |
|  └-unlock Dispatcher
└----unlock Taxi

This pattern is similar to the LeftRightDeadlock example with nested lock acquisition.

// Warning: deadlock-prone!
public void leftRight() {
  synchronized (left) {
    synchronized (right) {
      doSomething()
    }
  }
}

In the revised version, note the Taxi lock is released before acquiring the Dispatcher lock .

Taxi.setLocation

┌----lock Taxi
└----unlock Taxi

┌----lock Dispatcher
└----unlock Dispatcher