java.sql.Timestamp way of storing NanoSeconds

Question

java.sql.Timestamp constructor go like this:

public Timestamp(long time) {
    super((time/1000)*1000);
    nanos = (int)((time%1000) * 1000000);
    if (nanos < 0) {
        nanos = 1000000000 + nanos;     
        super.setTime(((time/1000)-1)*1000);
    }
}

It basically accepts time in millisecond and then extracts the last 3 digits and makes it nanos. So for a millisecond value of 1304135631 421, I'm getting Timestamp.getnanos() as 421000000. This is plain calculation (adding 6 zeroes at the end)... does not seems to be optimum.

A better way could have been Timestamp constructor that accepts time in nanoseconds and then calculates the nanosecond value out of that.

If you run the below program, you'll see the difference between actual nanoseconds and the one returned by Timestamp way of calculating nanosecods.

long a = System.currentTimeMillis();
    for(;;){
        long b = System.currentTimeMillis();
        Timestamp tm = new Timestamp(System.currentTimeMillis());
        System.out.println(tm.getTime());
        System.out.println(tm.getNanos());
        System.out.println("This is actual nanos" + System.nanoTime()%1000000000);
        System.out.println("--------------------------");
        if(b-a >= 1)
            break;
    }

So all the discussion about Timestamp that says it stores time up to nanoseconds , does not seems to be so correct.. Isn't?

Mango

Answer 1

The time in millis does not represent the time in nanos. More precise it simply can't be. You're supposed to use Timestamp#setNanos() to set the real nanos.

long timeInMillis = System.currentTimeMillis();
long timeInNanos = System.nanoTime();

Timestamp timestamp = new Timestamp(timeInMillis);
timestamp.setNanos((int) (timeInNanos % 1000000000));

// ...

Rayburn

Yes.. This will certainly work. So to make Timestamp actually store nanos, it should be two step process. Bit awkward, instead a Constructor that accepts nanos (from Jan 1, 1970) would have been a better solution. – Mango

No this is not safe! It wraps! The problem is that currentTimeMillis and nanoTime may not agree on the larger resolution. e.g. nanoTime returns the values 12:06:30.9999, 12:06: 31 .1111 but currentTimeMillis returns 12:06:30.666 12:06: 30 .777, then using the above code to initialise the timestamp would result in timestamps 12:06:30.9999, 12:06: 30 .1111. Separately, re @Mango 's comment, how would you initialise a timestamp before 1970? – Liberal

It is true that setting nano seconds must be set separately, but using System#nanoTime() like this is not correct. The API documentation for System#nanoTime() says: "This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary time (perhaps in the future, so values may be negative)." (see download.oracle.com/javase/6/docs/api/java/lang/…) – Verify

Answer 2

Since the introduction of java.time.*, there is a new factory method in java.sql.Timestamp: Timestamp.from(Instant.now()) will do the job (with nanoseconds precision). There is also Timestamp.toInstant() to convert it the other way around.

Minorite

Java 8 offers LocalDateTime.now() ... Or if < 8, use System.nanoTime() to get current timestamp .... – Galleass

Answer 3

Although it's an old post, I would like to add that the docs of Timestamp does state that it "holds fractional seconds by allowing the specification of fractional seconds to a precision of nanaoseconds". The confusing part is "hold". This seems confusing at first but if understood correctly, it actually does not state that it holds nanaoseconds value.It says it "holds" fractional value and allows it to be a "precision" of nanoseconds. Precision should be understood in terms of representation of total number of digits. So it essentially means that the part is actually fractional (still milliseconds) but is multiplied by 1000000 to represent it as nanoseconds.

The accepted answer (by ever helpful BaluC) sums it up nicely.

Fleenor

Answer 4

I like OpenJPA's implementation of TimestampHelper. It use static initializers to keep track of elapsed nanoseconds between calls to make a timestamp.

Nasho