The question regards Java’s SimpleDateFormat class. I’ve read in the documentation that
Date formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.
Obviously, I’ve read that after being hit in the most obscure way by it.
Does anybody know why would it be necessary to have state information in class members accessed by the “format” method for example?
Is it a speed optimization of some sort? I couldn’t come up with a valid reason.
Your answer pretty much lies here:
Prior to 1.4 hotspot writing multi-threaded code in Java was primarily a homework assignment for graduate students. The language had been around 10 years before the serious arrival of web-application servers and rise of the container-driven highly concurrent systems that most of us (non-android anyway) spend the vast majority of our time working in.
In 1996, Garbage Collection was a very slow and painful process that generally made your UI pause and look locked up while it happened, like wise creating new objects was considered very expensive (creating contiguous memory space when you’re fighting with Windows 95 for a share of 4MB of physical memory with no L2 CPU cache takes some time…..).
So in an environment where multi-threading is extremely rare, and memory is at a premium (your average user is probably still on a 486 or Pentium 1 with 8MB or even 4MB of system memory….) it makes perfect sense to re-use a single instance of Calendar as much as possible, Calendar itself being something of a clunky beast.
We can scoff today at what a horrible practice it is for a class like that to be stateful, but it can also be easily defended as the right choice at the time.
Defending Sun’s obsession with 100% backward compatibility and never updating it is another matter of course!