Saturday, May 24, 2014

Java File I/O Basics

Java 7 introduced the java.nio.file package to provide comprehensive support for file I/O. Besides a lot of other functionality this package includes the Files class (if you already use this class you can stop reading here). Files contains a lot of static methods that can be used to accomplish common tasks when working with files. Unfortunately it looks to me that still a lot of newer (Java 7+) code is written using old (pre Java 7) ways of working with files. This does not have to be bad, but it can make things more complex than needed. A possible reason for this might be that a lot of articles and high rated Stackoverflow answers were written before the release of Java 7.

In the rest of this post I will provide some code samples that show how you can accomplish common file related tasks with Java 7 or newer.

Working with files
// Create directories
// This will create the "bar" directory in "/foo"
// If "/foo" does not exist, it will be created first
Files.createDirectories(Paths.get("/foo/bar"));

// Copy a file
// This copies the file "/foo/bar.txt" to "/foo/baz.txt"
Files.copy(Paths.get("/foo/bar.txt"), Paths.get("/foo/baz.txt"));

// Move a file
// This moves the file "/foo/bar.txt" to "/foo/baz.txt"
Files.move(Paths.get("/foo/bar.txt"), Paths.get("/foo/baz.txt"));

// Delete a file
Files.delete(Paths.get("/foo/bar.txt"));

// Delete a file but do not fail if the file does not exist
Files.deleteIfExists(Paths.get("/foo/bar.txt"));

// Check if a file exists
boolean exists = Files.exists(Paths.get("/foo/bar.txt"));
Most methods of Files take one or more arguments of type Path. Path instances represent a path to a file or directory and can be obtained using Paths.get(). Note that most methods shown here also have an additional varargs parameter that can be used to pass additional options.

For example:
Files.copy(Paths.get("/foo.txt"), Paths.get("/bar.txt"), StandardCopyOption.REPLACE_EXISTING);

Iterating through all files within a directory
Files.walkFileTree(Paths.get("/foo"), new SimpleFileVisitor<Path>() {
  @Override
  public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
    System.out.println("file: " + file);
    return FileVisitResult.CONTINUE;
  }
});
Here the visitFile() method will be called for every file within the /foo directory. You can override additional methods of SimpleFileVisitor if you want to track directories too.

Writing and reading files
// Write lines to file
List<String> lines = Arrays.asList("first", "second", "third");
Files.write(Paths.get("/foo/bar.txt"), lines, Charset.forName("UTF-8"));

// Read lines
List<String> lines = Files.readAllLines(Paths.get("/foo/bar.txt"), Charset.forName("UTF-8"));
The shown methods work with characters. Similar methods are available if you need to work with bytes.

Conclusion
If you didn't know about java.nio.file.Files you should definitely have a look at the Javadoc method summary. There is a lot of useful stuff inside.

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