Unexpected side effects when parsing dates in Android

Question

In various Android projects, I use the following static function to parse dates such as 1900-12-31. Of course, this function should be deterministic - but it turns out it is not. Why?

Normally, it parses the date 2010-10-30, for example, to the correct Date instance holding that value. But I have noticed that when I have an IntentService running at the same time and parsing some dates, this function parses the same date as above to 1983-01-20, which is one of the dates parsed in the IntentService. How can this happen?

public static Date dateFromString(String dateStr) {
    SimpleDateFormat mDateFormat = new SimpleDateFormat("yyyy-MM-dd", Locale.getDefault());
    SimpleDateFormat mDateTimeFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ", Locale.getDefault());
    Date dateOut = null;
    try {
        if (dateStr != null) {
            if (dateStr.length() == 7) {
                if (dateStr.startsWith("--")) {
                    dateStr = "0000"+dateStr.substring(1);
                }
            }
            else if (dateStr.length() == 6) {
                if (dateStr.startsWith("-")) {
                    dateStr = "0000"+dateStr;
                }
            }
            else if (dateStr.length() == 5) {
                dateStr = "0000-"+dateStr;
            }
            else if (dateStr.matches("[0-9]{2}\\.[0-9]{2}\\.[0-9]{4}")) {
                dateStr = dateStr.substring(6, 10)+"-"+dateStr.substring(3, 5)+"-"+dateStr.substring(0, 2);
            }
            else if (dateStr.matches("[0-9]{2}\\/[0-9]{2}\\/[0-9]{4}")) {
                dateStr = dateStr.substring(6, 10)+"-"+dateStr.substring(3, 5)+"-"+dateStr.substring(0, 2);
            }
            else if (dateStr.matches("[0-9]{8}")) {
                dateStr = dateStr.substring(0, 4)+"-"+dateStr.substring(4, 6)+"-"+dateStr.substring(6, 8);
            }
            if (dateStr.length() >= 20) {
                String dateTimeStr = dateStr.trim();
                if (dateTimeStr.endsWith("Z")) {
                    dateTimeStr = dateStr.substring(0, dateTimeStr.length()-1)+"+0000";
                }
                if (dateStr.charAt(10) == ' ') {
                    dateTimeStr = dateStr.substring(0, 10)+"T"+dateStr.substring(11);
                }
                try {
                    dateOut = mDateTimeFormat.parse(dateTimeStr);
                }
                catch (Exception e2) {
                    dateOut = mDateFormat.parse(dateStr);
                }
            }
            else {
                dateOut = mDateFormat.parse(dateStr);
            }
        }
    }
    catch (Exception e) {
        dateOut = null;
    }
    return dateOut;
}

Edit: I do the parsing in my Activity's onCreate() where I start an AsyncTask that does the job. In the Activity's onStop(), a background service is started which does the same job. When I close the app (onStop()) and quickly restart it (onCreate()), both seem to be running simultaneously and the error occurrs.

Just as an experiment, what happens if you make dateFromString synchronized? — Diego Basch
– Diego Basch, Commented Jan 14, 2013 at 0:08
Firstly, why is the method static and secondly which class is it in? — Squonk
– Squonk, Commented Jan 14, 2013 at 0:10
@DiegoBasch Thanks, I will try! But actually, synchronized should by no means be necessary, as it only prevents interference on objects - which are not used here (static). — caw
– caw, Commented Jan 14, 2013 at 0:44
@Squonk Actually, it used to be non-static before. But as it does not operate on any objects, I made it static. The problem also occurred for the non-static method. It is located in MyApp.java which extends Application. — caw
– caw, Commented Jan 14, 2013 at 0:47
@MarcoW. : I think the answer from Dheeraj V.S. probably explains it. The fact SimpleDateFormat is not thread safe is most likely to be the cause of the problem. — Squonk
– Squonk, Commented Jan 14, 2013 at 3:44

Dheeraj Vepakomma · Accepted Answer · 2013-01-14 04:19:11Z

7

The documentation of SimpleDateFormat says:

SimpleDateFormat is not thread-safe. Users should create a separate instance for each thread.

There you go. Just create the SimpleDateFormat object separately in each thread and pass it to the method.

edited Jan 14, 2013 at 4:19

answered Jan 14, 2013 at 3:35

Dheeraj Vepakomma

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2 Comments

iagreen Over a year ago

He does create a separate instance in each thread, in fact he creates two new instances in every call.

caw Over a year ago

Thank you! I didn't expect the parse() method to have side effects and change some of the class's fields - but in fact the parse() method (which is inherited from the Date class) writes to Date's field calendar. Thus, it's logical that concurrent use of parse() causes problems.

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