Today at work we found a subtle issue that will sometimes break your code in very difficult to find ways. Read this if you don’t like days of bug hunting for mysterious issues only occurring on high-load production machines.
Ever wrote code like this?
private static final DateFormat FORMAT = new SimpleDateFormat("yyyy-MM-dd");
Most of us did. It’s the most intuitive way to use a DateFormat to format some Date object as a human-readable String.
Unfortunately, it’s wrong.
When used in a multi-threaded context (e.g. in a Servlet), this will end up breaking. Sometimes. When you least expect it.
