Upgrade Java read lock to write lock for caching in Map

Question

I have a deadlock situation in the code below:

private static final ReadWriteLock opClassesLock = new ReentrantReadWriteLock();
private static final Map<Class<?>, ServiceClass> opClasses = new WeakHashMap<Class<?>, ServiceClass>();
public static ServiceClass get(Class<?> myClass) {
    opClassesLock.readLock().lock();
    try {
        ServiceClass op = opClasses.get(myClass);
        if (op == null) {
            opClassesLock.writeLock().lock(); // deadlock here
            try {
                op = new ServiceClass(myClass);
                opClasses.put(myClass, op);
            } finally {
                opClassesLock.writeLock().unlock();
            }
        }
        return op;
    } finally {
        opClassesLock.readLock().unlock();
    }
}

Had I checked the documentation for ReentrantReadWriteLock, I could have predicted this:

Reentrancy also allows downgrading from the write lock to a read lock, by acquiring the write lock, then the read lock and then releasing the write lock. However, upgrading from a read lock to the write lock is not possible.

Besides just using a single lock instead of a read/write-lock (which won't allow concurrent reads), is there any other way to solve this kind of problem?

maaartinus maaartinus · Accepted Answer · 2011-02-03T13:00:51

Use a well-tested solution like

new MapMaker().weakKeys().makeMap();

from Guava. You can even do things like

new MapMaker().weakKeys()
.concurrencyLevel(16)
.expireAfterAccess(5, TimeUnit.MINUTES) 
.maximumSize(1000)
.makeComputingMap(new Function<Class<?>, ServiceClass>() {
    @Override
    public ServiceClass apply(Class<?> myClass) {
        return new ServiceClass(myClass);
    }
});

which should solve your whole problem and offers a lot of possibilities to tune the caching.

The reason for the deadlock is acquiring the write lock while both thread are holding the read lock. Unlike downgrading the lock, upgrading may block. You'd need to release the read lock first.

When you get the write lock, you should test if another thread didn't do the work yet.

Upgrade Java read lock to write lock for caching in Map

3 Answers