Question

Assuming this is a ‘singleton’ implementation: Am I guaranteed that this will only call productCatalogLoader.load() once, and also that no nullpointers can happen ? Any way to make this simpler ?

private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock(); private ProductCatalog productCatalog;  public ProductCatalog get() {         if (this.productCatalog == null) {             reload();          }         return this.productCatalog; }                                                           public void reload() {      lock.writeLock().lock();     try {         if (this.productCatalog != null) return;         this.productCatalog = productCatalogLoader.load();     } finally {         lock.writeLock().unlock();     } } 

}

Edit: This was a modestly successful attempt reduce much more complex code to a simple question sample. Several people caught on to the fact that my singleton was too complex 😉 (Actually there’s also a quartz timer calling reload, but the ‘reload’ implementation is a bit different IRL). I got the answer I needed anyway, broken double checked locking.

Answer 1

this looks like broken double checked locking. the problem is this.productCatalog can be assigned before the object is fully contructed.

say we have

var foo = new ProductCatalog(); foo.blah = 'blah blah'; this.productCatalog = foo; 

the can be reordered as

var foo = new ProductCatalog(); this.productCatalog = foo; foo.blah = 'blah blah';