ThreadLocal Resource Leak and WeakReference

Question

My limited understanding of ThreadLocal is that it has resource leak issues. I gather this problem can be remedied through proper use of WeakReferences with ThreadLocal (although I may have misunderstood this point.) I would simply like a pattern or example for correctly using ThreadLocal with WeakReference, if one exists. For instance, in this code snippet where would the WeakReference be introduced?

static class DateTimeFormatter {
    private static final ThreadLocal<SimpleDateFormat> DATE_PARSER_THREAD_LOCAL = new ThreadLocal<SimpleDateFormat>() {
        protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("yyyy/MM/dd HH:mmz");
        }
    };
    public String format(final Date date) {
        return DATE_PARSER_THREAD_LOCAL.get().format(date);
    }
    public Date parse(final String date) throws ParseException
    {
      return DATE_PARSER_THREAD_LOCAL.get().parse(date);
    }
}

Answer 1

ThreadLocal uses a WeakReference internally. If the ThreadLocal is not strongly referenced, it will be garbage-collected, even though various threads have values stored via that ThreadLocal.

Additionally, ThreadLocal values are actually stored in the Thread; if a thread dies, all of the values associated with that thread through a ThreadLocal are collected.

If you have a ThreadLocal as a final class member, that's a strong reference, and it cannot be collected until the class is unloaded. But this is how any class member works, and isn't considered a memory leak.

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Update: The cited problem only comes into play when the value stored in a ThreadLocal strongly references that ThreadLocal—sort of a circular reference.

In this case, the value (a SimpleDateFormat), has no backwards reference to the ThreadLocal. There's no memory leak in this code.