Question

Checkstyle reports this code as ‘The double-checked locking idiom is broken’, but I don’t think that my code actually is affected by the problems with double-checked locking.

The code is supposed to create a row in a database if a row with that id doesn’t exist. It runs in a multi-threaded environment and I want to avoid the primary-key-exists SQL-exceptions.

The pseudo-code:

private void createRow(int id) {   Row row = dao().fetch(id);   if (row == null) {      synchronized (TestClass.class) {         row = dao().fetch(id);         if (row == null) {            dao().create(id);         }      }   } } 

I can agree that it looks like double-checked locking, but I am not using static variables and the code in fetch() and create() is probably too complex to be inlined and put out of order.

Am I wrong or checkstyle? 🙂

Answer 1

Assuming you want that innermost line to read:

row = dao().create(id); 

It’s not a classic double-checked lock problem assuming dao().fetch is properly mutexed from the create method.

Edit: (code was updated)

The classic problem of a double-checked lock is having a value assigned before initialization occurs where two threads are accessing the same value.

Assuming the DAO is properly synchronized and will not return a partially initialized value, this doesn’t suffer from the flaws of the double-checked lock idiom.