Java synchronized method around parameter value

Question

Consider the following method:

public void upsert(int customerId, int somethingElse) {
  // some code which is prone to race conditions    
}

Answer 1

The concept you're looking for is called segmented locking or striped locking. It is too wasteful to have a separate lock for each customer (locks are quite heavyweight). Instead you want to partition your customer ID space into a reasonable number of partitions, matching the desired degree of parallelism. Typically 8-16 would be enough, but this depends on the amount of work the method does.

This outlines a simple approach:

private final Object[] locks = new Object[8];

synchronized (locks[customerId % locks.length]) {
    ...implementation...
}