Question

Instead of writing the following non-thread safe method.

private static final Calendar calendar = Calendar.getInstance();
public void fun() {
    // Going to call mutable methods in calendar.
}

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I change it to a thread safe version.

public void fun() {
    final Calendar calendar = Calendar.getInstance();
    // Going to call mutable methods in calendar.
}

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Instead of creating a new instance each time even for a same thread, I did the improvement by

public void fun() {
    final Calendar calendar = getCalendar();
    // Going to call mutable methods in calendar.
}

/**
 * Returns thread safe calendar.
 * @return thread safe calendar
 */
public Calendar getCalendar() {
    return calendar.get();
}

private static final ThreadLocal <Calendar> calendar = new ThreadLocal <Calendar>() {
    @Override protected Calendar initialValue() {
        return Calendar.getInstance();
     }
 };

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For my 3rd approach, is there any need to call the ThreadLocal.remove?

Answer 1

If your only intent is to make it threadsafe, then there is indeed no need to do so. But when the threads are maintained by a threadpool and your intent is more to give each freshly released thread from a threadpool its own initial value, then you should do so.