java.sql.Timestamp way of storing NanoSeconds

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 b开发者_Python百科asically 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?

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));

// ...

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.

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.

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