Let’s try it out.
Here is a program in which multiple threads use a shared SimpleDateFormat
.
Program:
public static void main(String[] args) throws Exception {
final DateFormat format = new SimpleDateFormat("yyyyMMdd");
Callable<Date> task = new Callable<Date>(){
public Date call() throws Exception {
return format.parse("20101022");
}
};
//pool with 5 threads
ExecutorService exec = Executors.newFixedThreadPool(5);
List<Future<Date>> results = new ArrayList<Future<Date>>();
//perform 10 date conversions
for(int i = 0 ; i < 10 ; i++){
results.add(exec.submit(task));
}
exec.shutdown();
//look at the results
for(Future<Date> result : results){
System.out.println(result.get());
}
}
Run this a few times and you will see:
Exceptions:
Here are a few examples:
1.
Caused by: java.lang.NumberFormatException: For input string: ""
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Long.parseLong(Long.java:431)
at java.lang.Long.parseLong(Long.java:468)
at java.text.DigitList.getLong(DigitList.java:177)
at java.text.DecimalFormat.parse(DecimalFormat.java:1298)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
2.
Caused by: java.lang.NumberFormatException: For input string: ".10201E.102014E4"
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1224)
at java.lang.Double.parseDouble(Double.java:510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1589)
3.
Caused by: java.lang.NumberFormatException: multiple points
at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1084)
at java.lang.Double.parseDouble(Double.java:510)
at java.text.DigitList.getDouble(DigitList.java:151)
at java.text.DecimalFormat.parse(DecimalFormat.java:1303)
at java.text.SimpleDateFormat.subParse(SimpleDateFormat.java:1936)
at java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1312)
Incorrect Results:
Sat Oct 22 00:00:00 BST 2011
Thu Jan 22 00:00:00 GMT 1970
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Thu Oct 22 00:00:00 GMT 1970
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Correct Results:
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Fri Oct 22 00:00:00 BST 2010
Another approach to safely use DateFormats in a multi-threaded environment is to use a ThreadLocal
variable to hold the DateFormat
object, which means that each thread will have its own copy and doesn’t need to wait for other threads to release it. This is how:
public class DateFormatTest {
private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>(){
@Override
protected DateFormat initialValue() {
return new SimpleDateFormat("yyyyMMdd");
}
};
public Date convert(String source) throws ParseException{
Date d = df.get().parse(source);
return d;
}
}
Here is a good post with more details.