Java threads locking on a specific object

Question

I have a web application and I am using Oracle database and I have a method basically like this:

public static void saveSomethingImportantToDataBase(Object t         


        

Answer 1

Here is an option adapted from And360's comment on Bohemian's answer, that tries to avoid race conditions, etc. Though I prefer my other answer to this question over this one, slightly:

import java.util.HashMap;
import java.util.concurrent.atomic.AtomicInteger;

// it is no advantage of using ConcurrentHashMap, since we synchronize access to it
// (we need to in order to "get" the lock and increment/decrement it safely)
// AtomicInteger is just a mutable int value holder
// we don't actually need it to be atomic
static final HashMap locks = new HashMap();

public static void saveSomethingImportantToDataBase(Object objectToSave) {
    AtomicInteger lock;
    synchronized (locks) {
        lock = locks.get(objectToSave.getId());
        if (lock == null) {
            lock = new AtomicInteger(1);
            locks.put(objectToSave.getId(), lock);
        }
        else 
          lock.incrementAndGet();
    }
    try {
        synchronized (lock) {
            // do synchronized work here (synchronized by objectToSave's id)
        }
    } finally {
        synchronized (locks) {
            lock.decrementAndGet();
            if (lock.get() == 0)  
              locks.remove(id);
        }
    }
}

You could split these out into helper methods "get lock object" and "release lock" or what not, as well, to cleanup the code. This way feels a little more kludgey than my other answer.