Question

The javadoc for SimpleDateFormat states that SimpleDateFormat is not synchronized.

“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.”

But what is the best approach to using an instance of SimpleDateFormat in a multi threaded environment. Here are a few options I have thought of, I have used options 1 and 2 in the past but I am curious to know if there are any better alternatives or which of these options would offer the best performance and concurrency.

Option 1: Create local instances when required

public String formatDate(Date d) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
    return sdf.format(d);
}

Option 2: Create an instance of SimpleDateFormat as a class variable but synchronize access to it.

private SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
public String formatDate(Date d) {
    synchronized(sdf) {
        return sdf.format(d);
    }
}

Option 3: Create a ThreadLocal to store a different instance of SimpleDateFormat for each thread.

private ThreadLocal<SimpleDateFormat> tl = new ThreadLocal<SimpleDateFormat>();
public String formatDate(Date d) {
    SimpleDateFormat sdf = tl.get();
    if(sdf == null) {
        sdf = new SimpleDateFormat("yyyy-MM-hh");
        tl.set(sdf);
    }
    return sdf.format(d);
}

Answer 1

1. Creating SimpleDateFormat is expensive. Don’t use this unless it’s done seldom.

2. OK if you can live with a bit of blocking. Use if formatDate() is not used much.

3. Fastest option IF you reuse threads (thread pool). Uses more memory than 2. and has higher startup overhead.

For applications both 2. and 3. are viable options. Which is best for your case depends on your use case. Beware of premature optimization. Only do it if you believe this is an issue.

For libraries that would be used by 3rd party I’d use option 3.