What it is
Apache Commons IO provides utilities for working with Java I/O, including file and directory manipulation, file filters, input/output stream handling, and file monitoring. It simplifies common I/O operations and reduces boilerplate code.
Commons IO provides utilities for file operations (copy, move, delete), directory traversal, filters, input/output streams, and monitoring. It helps avoid manual boilerplate code for common I/O tasks.
Installation
Add dependency in pom.xml:
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.15.0</version>
</dependency>Getting started
The smallest useful thing you can do with it, and what each part means.
import org.apache.commons.io.FileUtils;
import java.io.File;
File source = new File("source.txt");
File dest = new File("dest.txt");
FileUtils.copyFile(source, dest);String content = FileUtils.readFileToString(new File("example.txt"), "UTF-8");
System.out.println(content);Advanced usage
Where the library earns its place over a simpler alternative.
import org.apache.commons.io.FileUtils;
import java.io.File;
import java.util.Collection;
Collection<File> files = FileUtils.listFiles(new File("/path/to/dir"), new String[]{"txt", "java"}, true);
for (File file : files) {
System.out.println(file.getName());
}import org.apache.commons.io.monitor.FileAlterationMonitor;
import org.apache.commons.io.monitor.FileAlterationObserver;
FileAlterationObserver observer = new FileAlterationObserver("/path/to/dir");
observer.addListener(new MyFileListener());
FileAlterationMonitor monitor = new FileAlterationMonitor(5000, observer);
monitor.start();import org.apache.commons.io.IOUtils;
import java.io.FileInputStream;
try (FileInputStream fis = new FileInputStream("input.txt")) {
String content = IOUtils.toString(fis, "UTF-8");
System.out.println(content);
}FileUtils.deleteDirectory(new File("/path/to/delete"));Errors and fixes
The failures you are most likely to hit, and what actually resolves them.
- IOException
- Occurs when file or stream operations fail. Ensure paths exist, have correct permissions, and streams are properly handled.
- IllegalArgumentException
- Thrown when invalid arguments like null files are passed. Validate inputs before calling utility methods.
Best practices
- Use FileUtils and IOUtils to reduce boilerplate and improve readability.
- Always close streams or use try-with-resources to avoid resource leaks.
- Use FileAlterationMonitor for efficient directory monitoring instead of polling manually.
- Validate paths before performing delete or write operations to prevent accidental data loss.
- Combine Commons IO with NIO2 APIs for performance-critical applications.
Background
Why it exists, and what it was reacting to.
Commons IO was created to provide a robust set of tools for file, stream, and directory operations in Java. It includes classes to copy files, read/write content, monitor directories, apply filters, and work with streams efficiently. It is widely used in Java applications where I/O operations are frequent and complex, making code cleaner and safer.