synchronized block - lock more than one object

java multithreading locking synchronized
speendo picture speendo · Jan 5, 2011 · Viewed 48.5k times · Source

I'm modelling a game where multiple players (threads) move at the same time. The information of where a player is located at the moment is stored twice: the player has a variable "hostField" that references to a field on the board and every field has an ArrayList storing the players that are currently located at this field.

I'm not very happy with the fact that I have redundant information, but I found no way avoiding that without looping through a big dataset.

However, when a player moves from one field to another, I'd like to make sure (1) the redundant information stays linked (2) nobody else is manipulating the field at the moment.

Therefore I need to do something like

synchronized(player, field) {
    // code
}

Which is not possible, right?

What should I do? :)

Answer

Péter Török picture Péter Török · Jan 5, 2011

A trivial solution would be

synchronized(player) {
    synchronized(field) {
        // code
    }
}

However, make sure that you always lock the resources in the same order to avoid deadlocks.

Note that in practice, the bottleneck is the field, so a single lock on the field (or on a dedicated, common lock object, as @ripper234 rightly pointed out) may suffice (unless you are concurrently manipulating players in other, conflicting ways).

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