Strange ArrayIndexOutOfBoundsException for Java SimpleDateFormat

Question

We run Java 1.4.

We have this method:

static SimpleDateFormat xmlFormatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");

public static Date fromXml(String xmlDateTime) {
    ParsePosition pp = new ParsePosition(0);
    return xmlFormatter.parse(xmlDateTime, pp);
}

Where xmlDateTime = 2013-08-22T16:03:00 for example. This has been working, but suddenly stopped!

We now get this exception:

java.lang.ArrayIndexOutOfBoundsException: -1
at java.text.DigitList.fitsIntoLong(DigitList.java:170)
at java.text.DecimalFormat.parse(DecimalFormat.java:1064)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1381)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1159)

I have tried to reproduce this in a Unit Test by using different date formats, ie:

2013-08-22T16:03:00
2013-08-22 16:03:00

But no luck! Any ideas?

Bohemian · Accepted Answer · 2013-08-22T15:56:59.610

48

It is a little known fact that SimpleDateFormat is not threadsafe!

It is not a bug: The javadoc documents this behaviour:

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.

Create an instance every time you need one, or if performance is a real issue, you could try using ThreadLocal to store an instance for each thread that needs one.

Don't feel bad: I fell for exactly this "optimization" (to reuse a single constant instance), and to my amazement, had to instantiate a new instance every time.

edited Aug 22 '13 at 15:56

answered Aug 22 '13 at 14:31

Bohemian

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When `joda-time` library is present then `DateTimeFormat`can be used. It's immutable and therefore threadsafe like everything in joda-time. btw +1 – Fabian Barney Aug 22 '13 at 14:45
I had the same issue on Android, but with formatting a date. How could it be that it's not thread safe? Seems to me that it should have a very generic algorithm inside, which doesn't need to assume anything about threads... – android developer Jan 08 '17 at 15:05
2

@android it's not threadsafe because the moron(s) who coded it decided to not make it threadsafe. Java 8's `DateTimeFormatter` et al *is* threadsafe however. – Bohemian Jan 08 '17 at 17:47
@Bohemian Sadly Java 8 is in very early stage on Android. I will just create a new instance per thread, using clone. – android developer Jan 08 '17 at 22:03

bowmore · Answer 2 · 2013-08-22T14:42:23.307

5

Looks like this bug report. The underlying reason was diagnosed to be that DecimalFormat simply isn't thread safe.

So you should not use the same SimpleDateFormat instance on different threads, since it, and DecimalFormat still aren't thread safe.

You could use ThreadLocal to have each thread use its own instance.

edited Aug 22 '13 at 14:42

answered Aug 22 '13 at 14:30

bowmore

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Looks like that bug only affected Java 3. – arshajii Aug 22 '13 at 14:36
The link is a bit misleading, because people reading your answer think that you're stating that this is a bug. And it is not a bug. I recommend deleting the sentence with the link. – Fabian Barney Aug 22 '13 at 14:39
The resolution was to make DecimalFormat thread safe, it still isn't in Java 7, so I guess the behavior can occur even on a modern JDK. – bowmore Aug 22 '13 at 14:40
@FabianBarney clarified the usefulness of the link. – bowmore Aug 22 '13 at 14:44

score 1 · Answer 3 · answered Mar 19 '14 at 18:34

1

Try using Commons Lang 3.x FastDateParser and FastDateFormat. These classes are thread safe and faster than SimpleDateFormat. They also support the same format/parse pattern specifications as SimpleDateFormat.

answered Mar 19 '14 at 18:34

Chas

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This helped me a lot today.. Thank you for this! – Ken Flake Mar 25 '20 at 03:12

score 1 · Answer 4 · answered Oct 17 '18 at 08:05

1

A simple way to create instance each time, a local/scope variable instead of global variable, it works for me

private void test {
    SimpleDateFormat DATE_FORMAT = new SimpleDateFormat(pattern, Locale.ENGLISH);
    // Do somethings
}

answered Oct 17 '18 at 08:05

Alen Lee

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Strange ArrayIndexOutOfBoundsException for Java SimpleDateFormat

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