Reputation: 5640

Current timestamp as filename in Java

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?

Upvotes: 65

Answers (10)

Basil Bourque

Reputation: 338835

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 );
}

Upvotes: 0

James Mudd

Reputation: 2373

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

Upvotes: 0

Alex

Reputation: 146

You can use this option

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

Upvotes: 1

Yash

Reputation: 9578

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

Upvotes: 4

Ram Ghadiyaram

Reputation: 29185

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

Upvotes: 4

Abdul Basit

Reputation: 189

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

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

Upvotes: 7

saigopi.me

Reputation: 14918

try this one

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

Upvotes: 9

Derek Springer

Reputation: 2716

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

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

Upvotes: 170

Java Drinker

Reputation: 3167

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

Upvotes: 2

NPE

Reputation: 500475

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