CodeQL documentation

Thread-unsafe use of DateFormat

ID: java/thread-unsafe-dateformat
Kind: problem
Severity: warning
Precision: medium
Tags:
   - reliability
   - correctness
   - concurrency
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

Static fields of type java.text.DateFormat or its descendants should be avoided because the class DateFormat is not thread-safe.

Recommendation

Use instance fields instead and synchronize access where necessary.

Example

In the following example, DateFormattingThread declares a static field dateF of type DateFormat. When instances of DateFormattingThread are created and run by DateFormatThreadUnsafe, erroneous results are output because dateF is shared by all instances of DateFormattingThread.

class DateFormattingThread implements Runnable {
    private static DateFormat dateF = new SimpleDateFormat("yyyyMMdd");  // Static field declared

    public void run() {
        for(int i=0; i < 10; i++){
            try {
                Date d = dateF.parse("20121221");
                System.out.println(d);
            } catch (ParseException e) { }
        }
    }
}

public class DateFormatThreadUnsafe {
    
    public static void main(String[] args) {
        for(int i=0; i<100; i++){
            new Thread(new DateFormattingThread()).start();
        }
    }

}

In the following modification of the above example, DateFormattingThread declares an instance field dateF of type DateFormat. When instances of DateFormattingThread are created and run by DateFormatThreadUnsafeFix, correct results are output because there is a separate instance of dateF for each instance of DateFormattingThread.

class DateFormattingThread implements Runnable {
    private DateFormat dateF = new SimpleDateFormat("yyyyMMdd");  // Instance field declared

    public void run() {
        for(int i=0; i < 10; i++){
            try {
                Date d = dateF.parse("20121221");
                System.out.println(d);
            } catch (ParseException e) { }
        }
    }
}

public class DateFormatThreadUnsafeFix {
    
    public static void main(String[] args) {
        for(int i=0; i<100; i++){
            new Thread(new DateFormattingThread()).start();
        }
    }

}

References