Static method, static variable can it get mutated by multiple threads in web app - jsp
java, jsp, methods, multithreading, static
Solution
You create a new `Calendar` per thread, which is good, but you're using the `SimpleDateFormat` across multiple threads. This would be ok if that class were thread-safe, but it's not.
`SimpleDateFormat` is notorious for being thread-unsafe. Simply create a new one for each invocation (i.e. in the method), or, better still, use Joda-Time to avoid thread issues completely.
On the subject of creating one formatter class as an optimisation, this is a classic example of premature optimisation and inadvertent results. Given that this is within a JSP, the HTTP request/response time will dwarf the instatiation time (and resource requirements) of a new formatter. Make your classes thread-safe and immutable and worry about optimisation when it becomes an issue.
Problem
I am working on this project. There is a class DefaultsHelper which has : ``` public class DefaultsHelper { static SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd") ; public static String getDate(int days) { GregorianCalendar c = new GregorianCalendar() ; c.setTime(new Date()) ; c.add(Calendar.DAY_OF_MONTH, days); return df.format(c.getTime()) ; } } ``` In a web app - if two users where viewing a jsp whereby the getDate function was called at the exact same time - is it possible that the df object could get inconsistent and thereby not return expected values? I think the DefaultsHelper was meant to be a utility class to stop having to instantiate new df objects