Wednesday, January 6, 2010

A Note on the Thread (Un)safety of Format Classes

I recently got a note on another blog post asking this question:

I have a general question on the thread safety and this is not directly related with your blog. I would appreciate if you could post it on your blog. I have a class that has only one static method that accepts two string parameters and does some operation locally (as shown below). Do I need to synchronize this method?

public class A {
  public static boolean getDate(String str, String str2){
    SimpleDateFormat formatter =
      new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    boolean isBefore = false;
    Date date1;
    Date date2;
    try {
      date1 = formatter.parse(str);
      date2 = formatter.parse(str);
      isBefore = date1.after(date2);
    } catch (Exception e) {
    return isBefore;

Since it had nothing to do with the blog post in question, I'll answer it here.

The questioner is clearly asking about the well-documented lack of thread safety of the Format classes. This is a long-standing bug at Sun. If you create a Format object (or a MessageFormat, NumberFormat, DecimalFormat, ChoiceFormat, DateFormat or SimpleDateFormat object), it cannot be shared among threads. The above code does not share its SimpleDateFormat object among threads, so it is safe.

If the formatter field had been a static field of the class, the method would not be thread-safe. However, this way, you have to create a (relatively) expensive SimpleDateFormat object every time you invoke the method. There are many ways around this. One answer (if you want to avoid locking) is to use a ThreadLocal:

private static final ThreadLocal formatters =
new ThreadLocal() {
@Override public SimpleDateFormat initialValue() {
return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
public static boolean getDate(String str, String str2) {
SimpleDateFormat formatter = formatters.get();
The even better answer is to use the thread-safe Joda time libraries, instead. I hate to recommend using a non-standard library when there is an equivalent standard one, but this Format behavior is pretty broken.


CARFIELD said...

for just this example, the formatter created everytime this method executed. Are we really need to put it to threadlocal??

Of course, making formatter global and put in threadlocal run a lot faster.

A.A.A said...

That just bit my ass yesterday. i was making piece of the code working with multi threading, and i started to get sporadic "array Index out of bound exceptions" from Date formatter class. What a piece of turd IBM injected to the JDK.

Silly me, i thought i would be ok just making the calls synchronized. Now i switched to Joda time, problem solved.
i so wish that Jsr-310 can make it to JDk7 but my hopes are not so high.

Sumanth P Krishna said...

How do we know that a class is not Thread safe? Is immutability the only test or are there any other key factors to look for (I am assuming that the class documentation is not mentioning anything about thread safety).

Jeremy Manson said...

@Sumanth - in this case, you read the JavaDoc carefully. Ideally, that would be enough. If the JavaDoc doesn't make it clear, you probably have to go with the usual suspects: testing, reading the code, asking someone else who might know, and so on.

Sorry I can't be more helpful. In an ideal world, everyone would document API thread-safety.

sander hautvast said...

jeremy, your example, though correct is incomlpete. Check out . But I agree, joda-time is way simpler

Anonymous said...

I think the real issue is that Sun did not clearly explain that the Format classes were based on the Factory Pattern, so ARE mutable, thus NOT thread-safe!

I've been using ThreadLocal since I read THE Java Currency book, so get used to it!

A Professional Java Developer

IIT-Global-Jobs said...

Will threadlocal code still prove to be better performing as compared to the original posted code if one thread invokes new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS") only once in the lifetime. Also how to deal with threadlocal if instead of "yyyy-MM-dd HH:mm:ss.SSS" you want to pass it as a variable.

Jeremy Manson said...

@IIT: If you are only using SimpleDateFormat in a single thread, then constructing it in that thread is fine.

As for initializing with a variable, I can think of several solutions, but it really depends on your code.

Anonymous said...

Wouldn't it be much easier just to wrap SimpleDateFormat in a threadsafe enclosure:

new DateFormat() {
private SimpleDateFormat _df =..;
public synchronized String format(...) {
return _df.format(..);

If the cost of building/rebuilding is too high.. then maybe the cost of synchronizing isn't.

Of course you'll lose concurrency..

Jeremy Manson said...

@Anonymous - yes, that would be easier, but as you say, it is less concurrent. You really don't want to have a lock hidden away in your program that a lot of threads will be banging away on, if you can avoid it.

monn said...

For this particular case, isn't easier just to return str.compareTo(str2) < 0?

Jeremy Manson said...

@monn - That's not generally a great idea. What if someone else changed the date format slightly, so that that didn't work, but didn't notice the fact that you had used compareTo() to compare them?

When you have a typesafe way of doing something, relying on clever tricks isn't always the best way to go.

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