Introduction

The org.apache.commons.io package contains utility code for file- and stream-based IO operation. There are three main types of class:

FilenameFilters

The Java API defines an interface {@link java.io.FilenameFilter}, which is used to filter directory listings. This is commonly used in the {@link java.io.File#list(java.io.FilenameFilter)} method, and in java.awt.FileDialog.

There are three "primitive" FilenameFilters:

And there are three "boolean" FilenameFilters:

These boolean FilenameFilters can be nested, to allow arbitrary expressions. For example, here is how one could print all non-directory files in the current directory, starting with "A", and ending in ".java" or ".class":

File dir = new File(".");
String[] files = dir.list( new AndFileFilter(
      new AndFileFilter( 
        new PrefixFileFilter("A"),
        new OrFileFilter(
          new ExtensionFileFilter(".class"),
          new ExtensionFileFilter(".java")
          )
        ),
      new InvertedFileFilter(
        new DirectoryFileFilter()
        )
      )
    );
for ( int i=0; i<files.length; i++ )
{
  System.out.println(files[i]);
}
    

The org.apache.commons.io.IOUtil class

The IOUtil class contains a comprehensive set of static methods for copying from:

• String
• byte[]
• InputStream
• Reader

• String
• byte[]
• OutputStream
• Writer

As an example, consider the task of reading bytes from a URL, and printing them. This would typically done like this:

import java.net.URL;
import java.io.*;

public class ManualCopy {
  public static void main(String args[]) throws IOException {
    InputStream in = new URL( "http://jakarta.apache.org" ).openStream();
    
      InputStreamReader inR = new InputStreamReader( in );
    BufferedReader buf = new BufferedReader( inR );
    String line;
    while ( ( line = buf.readLine() ) != null )
    {
      System.out.println( line );
    }
    
      in.close();
  }
}
   

With the IOUtil class, that could be done with:

import java.net.URL;
import java.io.*;
import org.apache.commons.io.IOUtil;

public class IOUtilCopy {
  public static void main(String args[]) throws IOException {
    InputStream in = new URL( "http://jakarta.apache.org" ).openStream();
    System.out.println( IOUtil.toString( in ) );
      in.close();
  }   
}
    

In certain application domains, such IO operations are common, and this class can save a great deal of time.

For utility code such as this, flexibility and speed are of primary importance. In IOUtil, each kind of copy method has a variant which allows the buffer size to be set. For methods that convert bytes to chars, the encoding method may also be set.

The org.apache.commons.io.FileUtil class

The FileUtil class contains methods for retrieving different components of a file path (directory name, file base name, file extension), methods for copying Files to other files and directories, and methods for deleting and cleaning directories. For more information, see the class description

The Endian classes

Different computer architectures adopt different conventions for byte ordering. In so-called "Little Endian" architectures (eg Intel), the low-order byte is stored in memory at the lowest address, and subsequent bytes at higher addresses. For "Big Endian" architectures (eg Motorola), the situation is reversed.

There are two classes in this package of relevance:

• The org.apache.commons.io.EndianUtil class contains static methods for swapping the Endian-ness of Java primitives and streams.
• The org.apache.avalon.excalibur.io.SwappedDataInputStream class is an implementation of the {@link java.io.DataInput} interface. With this, one can read data from files of non-native Endian-ness.

For more information, see http://www.cs.umass.edu/~verts/cs32/endian.html. @since 4.0