Java Input/Output (I/O) Streams

This chapter is from the book

Core Java, Volume II--Advanced Features, 11th Edition

Learn More Buy

This chapter is from the book

This chapter is from the book 

Core Java, Volume II--Advanced Features, 11th Edition

Learn More Buy

2.4 Working with Files

You have learned how to read and write data from a file. However, there is more to file management than reading and writing. The Path interface and Files class encapsulate the functionality required to work with the file system on the user’s machine. For example, the Files class can be used to remove or rename a file, or to find out when a file was last modified. In other words, the input/output stream classes are concerned with the contents of files, whereas the classes that we discuss here are concerned with the storage of files on a disk.

The Path interface and Files class were added in Java 7. They are much more convenient to use than the File class which dates back all the way to JDK 1.0. We expect them to be very popular with Java programmers and discuss them in depth.

2.4.1 Paths

A Path is a sequence of directory names, optionally followed by a file name. The first component of a path may be a root component such as / or C:\. The permissible root components depend on the file system. A path that starts with a root component is absolute. Otherwise, it is relative. For example, here we construct an absolute and a relative path. For the absolute path, we assume a UNIX-like file system.

Path absolute = Paths.get("/home", "harry");
Path relative = Paths.get("myprog", "conf", "user.properties");

The static Paths.get method receives one or more strings, which it joins with the path separator of the default file system (/ for a UNIX-like file system, \ for Windows). It then parses the result, throwing an InvalidPathException if the result is not a valid path in the given file system. The result is a Path object.

The get method can get a single string containing multiple components. For example, you can read a path from a configuration file like this:

String baseDir = props.getProperty("base.dir");
   // May be a string such as /opt/myprog or c:\Program Files\myprog
Path basePath = Paths.get(baseDir); // OK that baseDir has separators

It is very common to combine or resolve paths. The call p.resolve(q) returns a path according to these rules:

• If q is absolute, then the result is q.

• Otherwise, the result is “p then q,” according to the rules of the file system.

For example, suppose your application needs to find its working directory relative to a given base directory that is read from a configuration file, as in the preceding example.

Path workRelative = Paths.get("work");
Path workPath = basePath.resolve(workRelative);

There is a shortcut for the resolve method that takes a string instead of a path:

Path workPath = basePath.resolve("work");

There is a convenience method resolveSibling that resolves against a path’s parent, yielding a sibling path. For example, if workPath is /opt/myapp/work, the call

Path tempPath = workPath.resolveSibling("temp");

creates /opt/myapp/temp.

The opposite of resolve is relativize. The call p.relativize(r) yields the path q which, when resolved with p, yields r. For example, relativizing /home/harry against /home/fred/input.txt yields ../fred/input.txt. Here, we assume that .. denotes the parent directory in the file system.

The normalize method removes any redundant . and .. components (or whatever the file system may deem redundant). For example, normalizing the path /home/harry/../fred/./input.txt yields /home/fred/input.txt.

The toAbsolutePath method yields the absolute path of a given path, starting at a root component, such as /home/fred/input.txt or c:\Users\fred\input.txt.

The Path interface has many useful methods for taking paths apart. This code sample shows some of the most useful ones:

Path p = Paths.get("/home", "fred", "myprog.properties");
Path parent = p.getParent(); // the path /home/fred
Path file = p.getFileName(); // the path myprog.properties
Path root = p.getRoot(); // the path /

As you have already seen in Volume I, you can construct a Scanner from a Path object:

var in = new Scanner(Paths.get("/home/fred/input.txt"));

2.4.2 Reading and Writing Files

The Files class makes quick work of common file operations. For example, you can easily read the entire contents of a file:

byte[] bytes = Files.readAllBytes(path);

If you want to read the file as a string, call readAllBytes followed by

var content = new String(bytes, charset);

But if you want the file as a sequence of lines, call

List<String> lines = Files.readAllLines(path, charset);

Conversely, if you want to write a string, call

Files.write(path, content.getBytes(charset));

To append to a given file, use

Files.write(path, content.getBytes(charset), StandardOpenOption.APPEND);

You can also write a collection of lines with

Files.write(path, lines);

These simple methods are intended for dealing with text files of moderate length. If your files are large or binary, you can still use the familiar input/output streams or readers/writers:

InputStream in = Files.newInputStream(path);
OutputStream out = Files.newOutputStream(path);
Reader in = Files.newBufferedReader(path, charset);
Writer out = Files.newBufferedWriter(path, charset);

These convenience methods save you from dealing with FileInputStream, FileOutputStream, BufferedReader, or BufferedWriter.

2.4.3 Creating Files and Directories

To create a new directory, call

Files.createDirectory(path);

All but the last component in the path must already exist. To create intermediate directories as well, use

Files.createDirectories(path);

You can create an empty file with

Files.createFile(path);

The call throws an exception if the file already exists. The check for existence and creation are atomic. If the file doesn’t exist, it is created before anyone else has a chance to do the same.

There are convenience methods for creating a temporary file or directory in a given or system-specific location.

Path newPath = Files.createTempFile(dir, prefix, suffix);
Path newPath = Files.createTempFile(prefix, suffix);
Path newPath = Files.createTempDirectory(dir, prefix);
Path newPath = Files.createTempDirectory(prefix);

Here, dir is a Path, and prefix/suffix are strings which may be null. For example, the call Files.createTempFile(null, ".txt") might return a path such as /tmp/1234405522364837194.txt.

When you create a file or directory, you can specify attributes, such as owners or permissions. However, the details depend on the file system, and we won’t cover them here.

2.4.4 Copying, Moving, and Deleting Files

To copy a file from one location to another, simply call

Files.copy(fromPath, toPath);

To move the file (that is, copy and delete the original), call

Files.move(fromPath, toPath);

The copy or move will fail if the target exists. If you want to overwrite an existing target, use the REPLACE_EXISTING option. If you want to copy all file attributes, use the COPY_ATTRIBUTES option. You can supply both like this:

Files.copy(fromPath, toPath, StandardCopyOption.REPLACE_EXISTING,
   StandardCopyOption.COPY_ATTRIBUTES);

You can specify that a move should be atomic. Then you are assured that either the move completed successfully, or the source continues to be present. Use the ATOMIC_MOVE option:

Files.move(fromPath, toPath, StandardCopyOption.ATOMIC_MOVE);

You can also copy an input stream to a Path, which just means saving the input stream to disk. Similarly, you can copy a Path to an output stream. Use the following calls:

Files.copy(inputStream, toPath);
Files.copy(fromPath, outputStream);

As with the other calls to copy, you can supply copy options as needed.

Finally, to delete a file, simply call

Files.delete(path);

This method throws an exception if the file doesn’t exist, so instead you may want to use

boolean deleted = Files.deleteIfExists(path);

The deletion methods can also be used to remove an empty directory.

See Table 2.3 for a summary of the options that are available for file operations.

Table 2.3 Standard Options for File Operations

2.4.5 Getting File Information

The following static methods return a boolean value to check a property of a path:

• exists

• isHidden

• isReadable, isWritable, isExecutable

• isRegularFile, isDirectory, isSymbolicLink

The size method returns the number of bytes in a file.

long fileSize = Files.size(path);

The getOwner method returns the owner of the file, as an instance of java.nio.file.attribute.UserPrincipal.

All file systems report a set of basic attributes, encapsulated by the BasicFileAttributes interface, which partially overlaps with that information. The basic file attributes are

• The times at which the file was created, last accessed, and last modified, as instances of the class java.nio.file.attribute.FileTime

• Whether the file is a regular file, a directory, a symbolic link, or none of these

• The file size

• The file key—an object of some class, specific to the file system, that may or may not uniquely identify a file

To get these attributes, call

BasicFileAttributes attributes = Files.readAttributes(path, BasicFileAttributes.class);

If you know that the user’s file system is POSIX-compliant, you can instead get an instance of PosixFileAttributes:

PosixFileAttributes attributes = Files.readAttributes(path, PosixFileAttributes.class);

Then you can find out the group owner and the owner, group, and world access permissions of the file. We won’t dwell on the details since so much of this information is not portable across operating systems.

2.4.6 Visiting Directory Entries

The static Files.list method returns a Stream<Path> that reads the entries of a directory. The directory is read lazily, making it possible to efficiently process directories with huge numbers of entries.

Since reading a directory involves a system resource that needs to be closed, you should use a try block:

try (Stream<Path> entries = Files.list(pathToDirectory))
{
   . . .
}

The list method does not enter subdirectories. To process all descendants of a directory, use the Files.walk method instead.

try (Stream<Path> entries = Files.walk(pathToRoot))
{
   // Contains all descendants, visited in depth-first order
}

Here is a sample traversal of the unzipped src.zip tree:

java
java/nio
java/nio/DirectCharBufferU.java
java/nio/ByteBufferAsShortBufferRL.java
java/nio/MappedByteBuffer.java
. . .
java/nio/ByteBufferAsDoubleBufferB.java
java/nio/charset
java/nio/charset/CoderMalfunctionError.java
java/nio/charset/CharsetDecoder.java
java/nio/charset/UnsupportedCharsetException.java
java/nio/charset/spi
java/nio/charset/spi/CharsetProvider.java
java/nio/charset/StandardCharsets.java
java/nio/charset/Charset.java
. . .
java/nio/charset/CoderResult.java
java/nio/HeapFloatBufferR.java
. . .

As you can see, whenever the traversal yields a directory, it is entered before continuing with its siblings.

You can limit the depth of the tree that you want to visit by calling Files.walk(pathToRoot, depth). Both walk methods have a varargs parameter of type FileVisitOption. . ., but there is only one option you can supply: FOLLOW_LINKS to follow symbolic links.

This code fragment uses the Files.walk method to copy one directory to another:

Files.walk(source).forEach(p ->
   {
      try
      {
         Path q = target.resolve(source.relativize(p));
         if (Files.isDirectory(p))
            Files.createDirectory(q);
         else
            Files.copy(p, q);
      }
      catch (IOException ex)
      {
         throw new UncheckedIOException(ex);
      }
   });

Unfortunately, you cannot easily use the Files.walk method to delete a tree of directories since you need to delete the children before deleting the parent. The next section shows you how to overcome that problem.

2.4.7 Using Directory Streams

As you saw in the preceding section, the Files.walk method produces a Stream<Path> that traverses the descendants of a directory. Sometimes, you need more fine-grained control over the traversal process. In that case, use the Files.newDirectoryStream object instead. It yields a DirectoryStream. Note that this is not a subinterface of java.util.stream.Stream but an interface that is specialized for directory traversal. It is a subinterface of Iterable so that you can use directory stream in an enhanced for loop. Here is the usage pattern:

try (DirectoryStream<Path> entries = Files.newDirectoryStream(dir))
{
   for (Path entry : entries)
      Process entries       
}

The try-with-resources block ensures that the directory stream is properly closed.

There is no specific order in which the directory entries are visited.

You can filter the files with a glob pattern:

try (DirectoryStream<Path> entries = Files.newDirectoryStream(dir, "*.java"))

Table 2.4 shows all glob patterns.

Table 2.4 Glob Patterns

If you want to visit all descendants of a directory, call the walkFileTree method instead and supply an object of type FileVisitor. That object gets notified

• When a file is encountered: FileVisitResult visitFile(T path, BasicFileAttributes attrs)

• Before a directory is processed: FileVisitResult preVisitDirectory(T dir, IOException ex)

• After a directory is processed: FileVisitResult postVisitDirectory(T dir, IOException ex)

• When an error occurred trying to visit a file or directory, such as trying to open a directory without the necessary permissions: FileVisitResult visitFileFailed(T path, IOException ex)

In each case, you can specify whether you want to

• Continue visiting the next file: FileVisitResult.CONTINUE

• Continue the walk, but without visiting the entries in this directory: FileVisitResult.SKIP_SUBTREE

• Continue the walk, but without visiting the siblings of this file: FileVisitResult.SKIP_SIBLINGS

• Terminate the walk: FileVisitResult.TERMINATE

If any of the methods throws an exception, the walk is also terminated, and that exception is thrown from the walkFileTree method.

A convenience class SimpleFileVisitor implements the FileVisitor interface. All methods except visitFileFailed do nothing and continue. The visitFileFailed method throws the exception that caused the failure, thereby terminating the visit.

For example, here is how to print out all subdirectories of a given directory:

Files.walkFileTree(Paths.get("/"), new SimpleFileVisitor<Path>()
   {
      public FileVisitResult preVisitDirectory(Path path, BasicFileAttributes attrs)
            throws IOException
      {
         System.out.println(path);
         return FileVisitResult.CONTINUE;
      }      public FileVisitResult postVisitDirectory(Path dir, IOException exc)
      {
         return FileVisitResult.CONTINUE;
      }      public FileVisitResult visitFileFailed(Path path, IOException exc)
            throws IOException
      {
         return FileVisitResult.SKIP_SUBTREE;
      }
   });

Note that we need to override postVisitDirectory and visitFileFailed. Otherwise, the visit would fail as soon as it encounters a directory that it’s not allowed to open or a file it’s not allowed to access.

Also note that the attributes of the path are passed as a parameter to the preVisitDirectory and visitFile methods. The visitor already had to make an OS call to get the attributes, since it needs to distinguish between files and directories. This way, you don’t need to make another call.

The other methods of the FileVisitor interface are useful if you need to do some work when entering or leaving a directory. For example, when you delete a directory tree, you need to remove the current directory after you have removed all of its files. Here is the complete code for deleting a directory tree:

// Delete the directory tree starting at root
Files.walkFileTree(root, new SimpleFileVisitor<Path>()
   {
      public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
            throws IOException
      {
         Files.delete(file);
         return FileVisitResult.CONTINUE;
      }
      public FileVisitResult postVisitDirectory(Path dir, IOException e) throws IOException
      {
         if (e != null) throw e;
         Files.delete(dir);
         return FileVisitResult.CONTINUE;
      }
   });

2.4.8 ZIP File Systems

The Paths class looks up paths in the default file system—the files on the user’s local disk. You can have other file systems. One of the more useful ones is a ZIP file system. If zipname is the name of a ZIP file, then the call

FileSystem fs = FileSystems.newFileSystem(Paths.get(zipname), null);

establishes a file system that contains all files in the ZIP archive. It’s an easy matter to copy a file out of that archive if you know its name:

Files.copy(fs.getPath(sourceName), targetPath);

Here, fs.getPath is the analog of Paths.get for an arbitrary file system.

To list all files in a ZIP archive, walk the file tree:

FileSystem fs = FileSystems.newFileSystem(Paths.get(zipname), null);
Files.walkFileTree(fs.getPath("/"), new SimpleFileVisitor<Path>()
   {
      public FileVisitResult visitFile(Path file, BasicFileAttributes attrs)
            throws IOException
      {               
         System.out.println(file);
         return FileVisitResult.CONTINUE;
      }
   });

That is nicer than the API described in Section 2.2.3, “ZIP Archives,” on p. 85 which required a set of new classes just to deal with ZIP archives.