Java Threads, Second Edition

Preventing Deadlock

Deadlock between threads competing for the same set of locks is the hardest problem to solve in any threaded program. It’s a hard enough problem, in fact, that we will not solve it—or even attempt to solve it. Instead, we’ll try to offer a good understanding of deadlock and some guidelines on how to prevent it. Preventing deadlock is completely the responsibility of the Java developer—the Java virtual machine will not do deadlock prevention or deadlock detection on your behalf.

We’ll look at deadlock in conjunction with the following code, which emulates how a kitchen might operate. When a cook wants to make cookies, she grabs the measuring cup to measure ingredients into the bowl; when a cook wants to make an omelette, he grabs a bowl, beats some eggs, and then measures out the eggs for each individual omelette. This is the order a typical cook uses to make these items, and as long as we have only one cook, everything is fine with these procedures. If we have two cooks, however, and one wants to make cookies while the other wants to make omelettes, we have a deadlock situation: the omelette maker needs the measuring cup to measure out the eggs that are in the mixing bowl; the cookie maker needs the bowl to put in the flour that is in the measuring cup:^[11]

public class Kitchen {
    static MeasuringCup theCup;
    static Bowl theBowl;

    public void makeCookie() { synchronized(theCup) { theCup.measureOut(1, theFlour); synchronized(theBowl) { theBowl.putIngredients(theCup); ...