c_9_3_understanding_file_attributes

🧠 9.3 Understanding File Attributes

• File attributes are metadata of files and directories - which is not stored in the contents of the file but rather the file system
• Files class has methods for accessing file attributes

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🟥 9.3.1 Discovering Basic File Attributes

• We shall discuss methods DIRECTLY WITHIN the Files class for reading file attributes
• These methods can be used in ANY operating system but may have limited information in some systems.

⭐ Reading Common Attributes with isDirectory(), isRegularFile(), and isSymbolicLink() ⭐

• These methods all take a Path and return a boolean
• Files.isRegularFile(Path) is true if path points to a regular file (any non-directory, non-symbolic link, non-resource or any non-regular file)
• Files.isDirectory(Path) is true if path points to directory
• Files.isSymbolicLink(Path) is true if path is a symbolic link

Path directory = Paths
    .get("src//chapter_9");
Files.isRegularFile(directory); // false
Files.isDirectory(directory); // true
Files.isSymbolicLink(directory); // false

Path file = Paths
    .get("src//chapter_9//README.md");
Files.isRegularFile(file); // true
Files.isDirectory(file); // false
Files.isSymbolicLink(file); // false

Path symbolicLink = Paths
    .get("src//symbolic-link"); // just a placeholder
Files.isRegularFile(symbolicLink);
// ^ COULD be true if points to regular file
Files.isDirectory(symbolicLink);
// ^ COULD be true if points to directory
Files.isSymbolicLink(symbolicLink); // true

⭐ Checking File Visibility with isHidden() ⭐

• Method signature:

boolean isHidden(Path) throws IOException

• This method can throw a NoSuchFileException for example

• In linux based systems, hidden files are denoted by the filename - starting with a . makes it hidden

• In windows systems, you have to set a hidden attribute.

• I create a hidden file:

Path hiddenFile = Paths.get("src//"
    + "chapter_9//"
    + "c_9_3_understanding_file_attributes//"
    + "java//"
    + "c_9_3_1//"
    + "hidden-file.txt");
try {
    boolean x = Files.isHidden(hiddenFile);
    System.out.println(x); // prints true
} catch (IOException e) { }

⭐ Testing File Accessibility with isReadable() and isExecutable() ⭐

• These methods do not throw a checked exception!
• A file could be visibile to a user but not actually readable/executable
• A file's extension does not determine if it is executable, a PNG or txt can both be executable

Path file = Paths
        .get("src//"
            + "chapter_9//"
            + "c_9_3_understanding_file_attributes//"
            + "java//"
            + "c_9_3_1//"
            + "hidden-file.txt");
Files.isReadable(file); // true
Files.isExecutable(file); // true

⭐ Reading File Length with size() ⭐

• Signature:

long size(Path) throws IOException

• This method will throw a IOException if the file does not exist or the file information can not be accessed.

Path file = Paths
    .get("src//"
        + "chapter_9//"
        + "c_9_3_understanding_file_attributes//"
        + "java//"
        + "c_9_3_1//"
        + "ReadingFileLength.java");
try {
    long size = Files.size(file);
    System.out.println(size); // 845
} catch (IOException e) { }

⭐ Managing File Modifications with getLastModifiedTime() and setLastModifiedTime() ⭐

• Method signatures:

FileTime getLastModifiedTime(Path) throws IOException
Path setLastModifiedTime(Path, FileTime) throws IOException

• Majority of OS's support the ability to track the last modified date of a file
• This can be used by applications to distinguish when a file needs processing when its contents have been modified - this is more efficient than loading the entire file!
• Example:

Path file = Paths
        .get("src//"
            + "chapter_9//"
            + "c_9_3_understanding_file_attributes//"
            + "java//"
            + "c_9_3_1//"
            + "ReadingFileLength.java");
try {
    FileTime fileTime = Files.getLastModifiedTime(file);
    // ^ 2024-02-18T14:49:56.173496Z
    long epochTime = fileTime.toMillis(); 
    // ^ 1708267796173
} catch (IOException e) { }

try {
    FileTime fileTimeNow =
        FileTime.fromMillis(System.currentTimeMillis());
    // ^ 2024-02-18T15:01:47.891Z
    Files.setLastModifiedTime(file, fileTimeNow);
    Files.getLastModifiedTime(file); // 2024-02-18T15:01:47.891Z
} catch (IOException e) { }

⭐ Managing Ownership with getOwner() and setOwner() ⭐

• Method signatures:

UserPrincipal getOwner(Path) throws IOException
Path setOwner(Path,UserPrincipal) throws IOException

• The UserPrincipalLookupService can be used to find a UserPrincipal record in the filesystem. This service can be obtained statically as shown:

try {
    UserPrincipalLookupService lookupService
        = FileSystems.getDefault()
            .getUserPrincipalLookupService();
    UserPrincipal me = lookupService
        .lookupPrincipalByName("Shiv");
    // ^ DESKTOP-RSM8H8J\Shiv (User)
}

• Example of getting the PrincipalUser:

Path file = Paths.get("src//"
    + "chapter_9//"
    + "c_9_3_understanding_file_attributes//"
    + "java//"
    + "c_9_3_1//"
    + "hidden-file.txt");
try {
    UserPrincipal owner = Files.getOwner(file);
    // ^ DESKTOP-RSM8H8J\Shiv (User)
}

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🟥 9.3.2 Improving Access With Views

• So far we have been obtain accessing individual file attributes with single method calls
• We can obtain all metadata efficiently with a single call
• Also attributes are file system dependent and can be easily generalised for all file systems.

• NIO.2 API addresses both of these concerns by enabling you to construct views for different file systems with a single method call
• A View is a group of related attributes for a specific file system. A file can support multiple views
• The performance advantage for using views to read multiple attributes is substantial

⭐ Understanding Views ⭐

• To obtain a view, you must provide a Path and a class object which instructs the type of view which is needed

• The Files class has two methods for accessing view information:

  1. Files.readAttributes() returns a read-only view of the file attributes
  2. Files.getFileAttributeView() returns a view which provides direct access to file attributes, enabling modification

• Both of these methods throw IOException, e.g. for if the view is not supported by the OS.

• Below are the attributes and view classes: | Attribute Class | View Class | Description | | -------------- | ------------- | -------------------- | | BasicFileAttributes | BasicFileAttributeView | Basic set of attributes supported by all file systems. | | DosFileAttributes | DosFileAttributeView|Attributes supported by DOS/windows based systems| | PosixFileAttributes|PosixFileAttributeView|Attributes supported by POSIX systems (Linux/Mac)|

• Only the first row is covered by the exam!

⭐ Reading Attributes ⭐

• NIO.2 API provides a Files.readAttributes(Path,Class<A>) method which returns a read-only view of a file view

🌱 BasicFileAttributes 🌱

• This class has attributes common to all file systems, including many of the single-line methods from the Files class

Path file = Paths.get("src//"
    + "chapter_9//"
    + "README.md");
BasicFileAttributes attributeData 
    = Files.readAttributes(file, BasicFileAttributes.class);
attributeData.isRegularFile(); // true
attributeData.isDirectory(); // false
attributeData.isSymbolicLink(); // false
attributeData.isOther(); // false
// ^ isOther() can be used to check if file is 
// a resource or a device

attributeData.size(); // 343

FileTime x = attributeDate.creationTime();
// 2024-01-06T08:42:35.6047179Z
FileTime y = attributeData.lastModifiedTime();
// 2024-01-06T08:46:37.0737933Z
FileTime z = attributeData.lastAccessTime();
// 2024-02-10T11:35:04.9077723Z
Object w = attributeData.fileKey(); // null
// returns unique identifier in OS
// null if not supported

⭐ Modifying Attributes ⭐

• In order to modify attributes of a file we need to obtain a view
• We have the Files.getAttributeView() method which lets us do this:

<V extends FileAttributeView> getFileAttributeView(Path,Class<V>)

• We can use this method to obtain a BasicFileAttributeView!
• We can then call setTimes(lastModified,lastAccess,creation) to update FileTime data:

Path file = Paths.get("src//"
    + "chapter_9//"
    + "README.md");
BasicFileAttributeView view
    = Files.getFileAttributeView(file, BasicFileAttributeView.class);
try {
    BasicFileAttributes data = view.readAttributes();
    FileTime lastModified = data.lastModifiedTime();
    // 2024-01-09T07:52:15.2344042Z

    FileTime newLastModified = FileTime.fromMillis(lastModified.toMillis()+10_000);
    view.setTimes(newLastModified, null, null); // null => do not update
} catch (IOException e) {}