Call to method of static java.text.DateFormat not

Question

I am receiving a Find Bugs error - call to method of static java.text.DateFormat and I don't know the reason why it's not good / advisable to be doing these things below.

private static final Date TODAY = Calendar.getInstance().getTime();
private static final DateFormat yymmdd = new SimpleDateFormat("yyMMdd"); 

private String fileName = "file_" + yymmdd.format(TODAY);

Answer 1

I'm not sure if FindBugs is complaining about this, but one problem I see with your code is that you're defining TODAY as a class level (static), constant (final) variable. This communicates the intent that you want TODAY to never change (I don't believe this is the case, since java.util.Dates are mutable, but that's another story).

Think about what happens if you application runs for multiple days? TODAY (unless you update it) will be referencing the day the application was started, not the current date. Are you sure this is what you meant?

This may not be a bug in your code at all, but the intent is not clear, and I believe that may be what FindBugs is complaining about.

Answer 2

An alternative that not has been mentioned is using ThreadLocal. See http://www.javacodegeeks.com/2010/07/java-best-practices-dateformat-in.html for more info + performance comparison between the 3 options:

• Creating an instance each time
• Synchronising access
• Using ThreadLocal

Example of using ThreadLocal:

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

Usage:

DATE_FORMAT.get().format( TODAY )

Answer 3

It's not thread-safe, for one thing.

Answer 4

You can get this to go away by wrapping all the references to the DateFormat in a synchronize block - just make sure all calls are wrapped in the same synchronize object!

Answer 5

I assume this is because format is not thread-safe?

(I have not seen what findbugs complain about, can you provide the warning text?)

Answer 6

Are you sure it's not

private static final DateFormat yymmdd = new SimpleDateFormat("yyMMdd"); 

? That's what the error message indicates.

I think what it aims at is the fact that DateFormat is not thread safe, so having an instance as a static field indicates potential race conditions.