Testing multi threads call for SimpleDateFormat

Question

I want to test a fix where SimpleDateFormat was used as a static class variable. I was getting

Previously my code was

public class Abc{
   private static SimpleDateFormat dateformatter;

   public static String method1(final Calendar calendar) {
      String thePattern = "ddMMM";
      dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something

   }

   public static String method2(final Calendar calendar) {
      String thePattern = "ddMMyyyy";
      dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something

   }
}

For this one, I was getting below exception

java.lang.ArrayIndexOutOfBoundsException: 965
            at sun.util.calendar.BaseCalendar.getCalendarDateFromFixedDate(BaseCalendar.java:454)
            at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2333)
            at java.util.GregorianCalendar.computeFields(GregorianCalendar.java:2248)
            at java.util.Calendar.setTimeInMillis(Calendar.java:1140)
            at java.util.Calendar.setTime(Calendar.java:1106)
            at java.text.SimpleDateFormat.format(SimpleDateFormat.java:955)
            at java.text.SimpleDateFormat.format(SimpleDateFormat.java:948)
            at java.text.DateFormat.format(DateFormat.java:336)

Now I have changed it to changed to :

public class Abc{  
   public static String method1(final Calendar calendar) {
      String thePattern = "ddMMM";
      SimpleDateFormat dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something
   }

   public static String method2(final Calendar calendar) {
      String thePattern = "ddMMyyyy";
      SimpleDateFormat dateformatter = new SimpleDateFormat(thePattern, Locale.US);
      sPars = dateformatter.format(calendar.getTime());
      //Something
   }
}

How can I test that the second one is a proper fix ? Is there any way in JUnits to test multithreading ?

See also https://stackoverflow.com/questions/12159/how-should-i-unit-test-threaded-code — Raedwald, Nov 12 '17 at 10:19

score 5 · Answer 1 · answered Jul 03 '13 at 10:11

SimpleDateFormat is not thread safe, as stated in the javadoc :

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.

But here you were even messing with references because of :

dateformatter = new SimpleDateFormat(thePattern, Locale.US);

Since dateformatter was static, its content was shared between threads.

Creating a new instance locally (like you did afterwards) is a good way to go. Local instances aren't shared between threads and won't mess with multithreading.

Just don't forget to remove the private static SimpleDateFormat dateformatter; to avoid confusion.

You can also use a ThreadLocal variable if you want to create only 1 instance per thread :

private static final ThreadLocal<SimpleDateFormat> dateformatter1 =
    new ThreadLocal<SimpleDateFormat>() {
         @Override protected SimpleDateFormat initialValue() {
             return new SimpleDateFormat("ddMMM");
     }
 };

private static final ThreadLocal<SimpleDateFormat> dateformatter2 =
    new ThreadLocal<SimpleDateFormat>() {
        @Override protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("ddMMyyyy");
    }
};

public static String method1(final Calendar calendar) {
    SimpleDateFormat dateformatter = dateformatter1.get();
    sPars = dateformatter.format(calendar.getTime());
    //Something
}

public static String method2(final Calendar calendar) {
    SimpleDateFormat dateformatter = dateformatter2.get();
    sPars = dateformatter.format(calendar.getTime());
    //Something
}

Testing multi threads call for SimpleDateFormat

1 Answers1