Formatting file sizes in Java/JSTL [duplicate]

Question

This question already has an answer here:

• How to convert byte size into human readable format in java? 22 answers

I was wondering if anyone knew of a good way to format files sizes in Java/JSP/JSTL pages.

Is there a util class that with do this?
I've searched commons but found nothing. Any custom tags?
Does a library already exist for this?

Ideally I'd like it to behave like the -h switch on Unix's ls command

34 -> 34
795 -> 795
2646 -> 2.6K
2705 -> 2.7K
4096 -> 4.0K
13588 -> 14K
28282471 -> 27M
28533748 -> 28M

Answer 1

A quick google search returned me this from Appache hadoop project. Copying from there: (Apache License, Version 2.0):

private static DecimalFormat oneDecimal = new DecimalFormat("0.0");

  /**
   * Given an integer, return a string that is in an approximate, but human 
   * readable format. 
   * It uses the bases 'k', 'm', and 'g' for 1024, 1024**2, and 1024**3.
   * @param number the number to format
   * @return a human readable form of the integer
   */
  public static String humanReadableInt(long number) {
    long absNumber = Math.abs(number);
    double result = number;
    String suffix = "";
    if (absNumber < 1024) {
      // nothing
    } else if (absNumber < 1024 * 1024) {
      result = number / 1024.0;
      suffix = "k";
    } else if (absNumber < 1024 * 1024 * 1024) {
      result = number / (1024.0 * 1024);
      suffix = "m";
    } else {
      result = number / (1024.0 * 1024 * 1024);
      suffix = "g";
    }
    return oneDecimal.format(result) + suffix;
  }

It uses 1K = 1024, but you can adapt this if you prefer. You also need to handle the <1024 case with a different DecimalFormat.

Answer 2

You can use the commons-io FileUtils.byteCountToDisplaySize methods. For a JSTL implementation you can add the following taglib function while having commons-io on your classpath:

<function>
  <name>fileSize</name>
  <function-class>org.apache.commons.io.FileUtils</function-class>
  <function-signature>String byteCountToDisplaySize(long)</function-signature>
</function>

Now in your JSP you can do:

<%@ taglib uri="/WEB-INF/FileSizeFormatter.tld" prefix="sz"%>
Some Size: ${sz:fileSize(1024)} <!-- 1 K -->
Some Size: ${sz:fileSize(10485760)} <!-- 10 MB -->