java.text.SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");

long mnSeconds;
long mnNanoseconds;

public static String format(long mnSeconds, long mnNanoseconds) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.");
    return sdf.format(new Date(mnSeconds*1000))
           + String.format("%09d", mnNanoseconds);
}

2012-08-08 19:52:21.123456789

public static String format(long mnSeconds, long mnNanoseconds) {
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
    return sdf.format(new Date(mnSeconds*1000 + mnNanoseconds/1000000));
}

long epoch = System.currentTimeMillis();

System.out.println("Epoch : " + (epoch / 1000));

Instant                        // Represent a moment in UTC.
    .ofEpochSecond( mnSeconds )   // Determine a moment from a count of whole seconds since the Unix epoch of the first moment of 1970 in UTC (1970-01-01T00:00Z).
    .plusNanos( mnNanoseconds )    // Add on a fractional second as a count of nanoseconds. Returns another `Instant` object, per Immutable Objects pattern.
    .toString()                    // Generate text representing this `Instant` object in standard ISO 8601 format.
    .replace( "T" , " " )          // Replace the `T` in the middle with a SPACE.
    .replace "Z" , "" )            // Remove the `Z` on the end (indicating UTC).

long mnSeconds = … ;
long mnNanoseconds = … ;

Instant instant = Instant.ofEpochSecond( mnSeconds ).plusNanos( mnNanoseconds );

Instant instant = Instant.ofEpochSecond( mnSeconds , mnNanoseconds );

String output = instant.toString();

new java.util.Date(mnSeconds);