Current timestamp as filename in Java

Question

I want to name new files created by my Java application with the current timestamp.

I need help with this. How do I name the new files created with the current timestamp? Which classes should I include?

Ulyanovsk

What if multiple files are created at the same time? Is this an XY problem? What are you really trying to do? – Meantime

Answer 1

No need to get too complicated, try this one liner:

String fileName = new SimpleDateFormat("yyyyMMddHHmm'.txt'").format(new Date());

Stephanus

dude, I love one-liners :) +1 – Proem

Nice simple line – Dim

add "seconds" too if not all file will overwrite if they save in within same minute ("yyyyMMddHHmmss'.txt'") – Satirical

Update: The terribly flawed date-time classes such as java.util.Date, java.util.Calendar, and java.text.SimpleDateFormat are now legacy, supplanted by the java.time classes built into Java 8 and later. – Lamelliform

Answer 2

try this one

String fileSuffix = new SimpleDateFormat("yyyyMMddHHmmss").format(new Date());

Satirical

Answer 3

You can get the current timestamp appended with a file extension in the following way:

String fileName = new Date().getTime() + ".txt";

Gusto

Answer 4

You can use DateTime

import org.joda.time.DateTime

Option 1 : with yyyyMMddHHmmss

DateTime.now().toString("yyyyMMddHHmmss")

Will give 20190205214430

Option 2 : yyyy-dd-M--HH-mm-ss

   DateTime.now().toString("yyyy-dd-M--HH-mm-ss")

will give 2019-05-2--21-43-32

Ultracentrifuge

Answer 5

Improving the @Derek Springer post with fill length function:

public static String getFileWithDate(String fileName, String fileSaperator, String dateFormat) {
    String FileNamePrefix = fileName.substring(0, fileName.lastIndexOf(fileSaperator));
    String FileNameSuffix = fileName.substring(fileName.lastIndexOf(fileSaperator)+1, fileName.length());
    //System.out.println("File= Prefix~Suffix:"+FileNamePrefix +"~"+FileNameSuffix);
    
    String newFileName = new SimpleDateFormat("'"+FileNamePrefix+"_'"+dateFormat+"'"+fileSaperator+FileNameSuffix+"'").format(new Date());
    System.out.println("New File:"+newFileName);
    return newFileName;
}

Using the funciton and its Output:

String fileSaperator = ".", format = "yyyyMMMdd_HHmm";
getFileWithDate("Text1.txt", fileSaperator, format);
getFileWithDate("Text1.doc", fileSaperator, format);
getFileWithDate("Text1.txt.json", fileSaperator, format);

Output:

Old File:Text1.txt   New File:Text1_2020Nov11_1807.txt
Old File:Text1.doc   New File:Text1_2020Nov11_1807.doc
Old File:Text1.txt.json  New File:Text1.txt_2020Nov11_1807.json

Gerundive

Answer 6

Date, SimpleDateFormat and whatever classes are required on the I/O side of things (there are many possibilities).

Seam

Question is already answered by aix, but don't forget to ensure that you use only the chars allowed on all OSs you're serving. I mean use SimpleDateFormat. – Lemal

@aix: my application creates new files every 30 mins. Hence i need time+date as the file name. i tried this: 'String pattern = "mm/dd/yyyy"; SimpleDateFormat format = new SimpleDateFormat (pattern); str = format.format(new Date());' then i named my file as str.".txt", but it is giving me an error. [Error: 49\20\2011.txt (The system cannot find the path specified)]. Besides, I also need the current time. Please Help. – Ulyanovsk

Answer 7

Use SimpleDateFormat as aix suggested to format the current time into a string. You should use a format that does not include / characters etc. I would suggest something like yyyyMMddhhmm

Complimentary

Answer 8

You can use this option

String fileName = new SimpleDateFormat("yyyyMMddHHmmss'.txt'", Locale.getDefault()).format(new Date());

Cosette

Answer 9

A newer alternative using the Java 8 API

String filename = LocalDateTime.now().format(DateTimeFormatter.ofPattern("yyyy-MM-dd-HH-mm-ss")) + ".txt";

Using

import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

Gives an output like

2023-12-20-09-27-17.txt

Anglesite

LocalDateTime is inherently ambiguous as it lacks the context of a time zone or offset from UTC. So it does not meet the need stated in the Question: a timestamp. So your result is ambiguous as well. We have no idea if you mean 9:27 AM in Tokyo, 9:27 AM in Toulouse, or 9:27 AM in Toledo Ohio — all very different moments several hours apart. – Lamelliform

Answer 10

tl;dr

OffsetDateTime
.now( ZoneOffset.UTC )
.format( DateTimeFormatter.ofPattern( "uuuuMMdd'T'HHmmssX" ) ) 

20231220T192055Z

See code run at Ideone.com.

Avoid legacy date-time classes

Use only the modern java.time classes defined in JSR 310.

Avoid the terribly flawed date-time classes seen in the older Answers here. Do not use SimpleDateFormat, Date, Calendar, etc.

java.time.Instant

To capture the current moment as seen with an offset from the temporal meridian of UTC of zero hours-mintues-seconds, use java.time.Instant.

Instant now  = Instant.now() ;

Truncate to your desired resolution. I will assume you want whole seconds.

Instant now = Instant.now().truncateTo( ChronoUnit.SECONDS ) ;

File systems

Various file systems use various characters as delimiters in their file paths. In Unix including modern macOS file system, a slash. In legacy Mac file system, a colon. In Windows, a backslash.

So we must avoid those characters when producing a file name.

“Basic” ISO 8601

One way to avoid those characters is to use the basic variation of ISO 8601 formats. The basic variations avoid the use of punctuation marks. The T in the middle remains to separate the date portion from the time-of-day portion.

If our date-time value is in UTC (has an offset of zero), we want a Z to appear on the end. The Z is pronounced “Zulu”. The formatting code X will produce a Z if the offset is zero.

Use DateTimeFormatter class to define a formatting pattern.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "uuuuMMdd'T'HHmmssX" ) ;

To be more human readable, we could insert hyphen characters as separators. As far as I know, no file system uses hyphen as a path delimiter. But this custom format is not standard.

DateTimeFormatter f = DateTimeFormatter.ofPattern( "uuuu-MM-dd'T'HH-mm-ssX" ) ;

java.time.OffsetDateTime

To generate formatted text, we need to convert our Instant to the more flexible class OffsetDateTime. The Instant class is a basic building-block of the java.time framework, so it designed simply to represent a moment, with little other functionality.

OffsetDateTime odt = now.atOffset( ZoneOffset.UTC ) ;

Generate text.

String output = odt.format( f ) ;

20231220T192055Z

File path

Use NIO.2 to create a Path object for your new file. Calling Path.of is one way.

Path path = Path.of( "wherever" , "someFolder" , dateTimeString + ".txt" ) ;

Example code

String dateTime =
        OffsetDateTime
                .now ( ZoneOffset.UTC )
                .format ( DateTimeFormatter.ofPattern ( "uuuuMMdd'T'HHmmssX" ) );
Path path = Path.of ( "/Users/your_username" , dateTime + ".txt" );
try
        (
                BufferedWriter writer = Files.newBufferedWriter ( path ) ;
        )
{
    writer.write ( "At the tone the time will be: " + Instant.now ( ) );
} catch ( IOException e )
{
    throw new RuntimeException ( e );
}

Lamelliform