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added in API level 26

UserDefinedFileAttributeView

public interface UserDefinedFileAttributeView
implements FileAttributeView

java.nio.file.attribute.UserDefinedFileAttributeView


A file attribute view that provides a view of a file's user-defined attributes, sometimes known as extended attributes. User-defined file attributes are used to store metadata with a file that is not meaningful to the file system. It is primarily intended for file system implementations that support such a capability directly but may be emulated. The details of such emulation are highly implementation specific and therefore not specified.

This FileAttributeView provides a view of a file's user-defined attributes as a set of name/value pairs, where the attribute name is represented by a String. An implementation may require to encode and decode from the platform or file system representation when accessing the attribute. The value has opaque content. This attribute view defines the read and write methods to read the value into or write from a ByteBuffer. This FileAttributeView is not intended for use where the size of an attribute value is larger than Integer.MAX_VALUE.

User-defined attributes may be used in some implementations to store security related attributes so consequently, in the case of the default provider at least, all methods that access user-defined attributes require the RuntimePermission("accessUserDefinedAttributes") permission when a security manager is installed.

The supportsFileAttributeView method may be used to test if a specific FileStore supports the storage of user-defined attributes.

Where dynamic access to file attributes is required, the getAttribute method may be used to read the attribute value. The attribute value is returned as a byte array (byte[]). The setAttribute method may be used to write the value of a user-defined attribute from a buffer (as if by invoking the write method), or byte array (byte[]).

Summary

Public methods

abstract void delete(String name)

Deletes a user-defined attribute.

abstract List<String> list()

Returns a list containing the names of the user-defined attributes.

abstract String name()

Returns the name of this attribute view.

abstract int read(String name, ByteBuffer dst)

Read the value of a user-defined attribute into a buffer.

abstract int size(String name)

Returns the size of the value of a user-defined attribute.

abstract int write(String name, ByteBuffer src)

Writes the value of a user-defined attribute from a buffer.

Inherited methods

Public methods

delete

added in API level 26
public abstract void delete (String name)

Deletes a user-defined attribute.

Parameters
name String: The attribute name

Throws
IOException If an I/O error occurs or the attribute does not exist
SecurityException In the case of the default provider, a security manager is installed, and it denies RuntimePermission("accessUserDefinedAttributes") or its checkWrite method denies write access to the file.

list

added in API level 26
public abstract List<String> list ()

Returns a list containing the names of the user-defined attributes.

Returns
List<String> An unmodifiable list containing the names of the file's user-defined

Throws
IOException If an I/O error occurs
SecurityException In the case of the default provider, a security manager is installed, and it denies RuntimePermission("accessUserDefinedAttributes") or its checkRead method denies read access to the file.

name

added in API level 26
public abstract String name ()

Returns the name of this attribute view. Attribute views of this type have the name "user".

Returns
String the name of the attribute view

read

added in API level 26
public abstract int read (String name, 
                ByteBuffer dst)

Read the value of a user-defined attribute into a buffer.

This method reads the value of the attribute into the given buffer as a sequence of bytes, failing if the number of bytes remaining in the buffer is insufficient to read the complete attribute value. The number of bytes transferred into the buffer is n, where n is the size of the attribute value. The first byte in the sequence is at index p and the last byte is at index p + n - 1, where p is the buffer's position. Upon return the buffer's position will be equal to p + n; its limit will not have changed.

Usage Example: Suppose we want to read a file's MIME type that is stored as a user-defined attribute with the name "user.mimetype".

    UserDefinedFileAttributeView view =
        Files.getFileAttributeView(path, UserDefinedFileAttributeView.class);
    String name = "user.mimetype";
    ByteBuffer buf = ByteBuffer.allocate(view.size(name));
    view.read(name, buf);
    buf.flip();
    String value = Charset.defaultCharset().decode(buf).toString();
 

Parameters
name String: The attribute name

dst ByteBuffer: The destination buffer

Returns
int The number of bytes read, possibly zero

Throws
IllegalArgumentException If the destination buffer is read-only
IOException If an I/O error occurs or there is insufficient space in the destination buffer for the attribute value
SecurityException In the case of the default provider, a security manager is installed, and it denies RuntimePermission("accessUserDefinedAttributes") or its checkRead method denies read access to the file.

See also:

size

added in API level 26
public abstract int size (String name)

Returns the size of the value of a user-defined attribute.

Parameters
name String: The attribute name

Returns
int The size of the attribute value, in bytes.

Throws
ArithmeticException If the size of the attribute is larger than Integer.MAX_VALUE
IOException If an I/O error occurs
SecurityException In the case of the default provider, a security manager is installed, and it denies RuntimePermission("accessUserDefinedAttributes") or its checkRead method denies read access to the file.

write

added in API level 26
public abstract int write (String name, 
                ByteBuffer src)

Writes the value of a user-defined attribute from a buffer.

This method writes the value of the attribute from a given buffer as a sequence of bytes. The size of the value to transfer is r, where r is the number of bytes remaining in the buffer, that is src.remaining(). The sequence of bytes is transferred from the buffer starting at index p, where p is the buffer's position. Upon return, the buffer's position will be equal to p + n, where n is the number of bytes transferred; its limit will not have changed.

If an attribute of the given name already exists then its value is replaced. If the attribute does not exist then it is created. If it implementation specific if a test to check for the existence of the attribute and the creation of attribute are atomic with respect to other file system activities.

Where there is insufficient space to store the attribute, or the attribute name or value exceed an implementation specific maximum size then an IOException is thrown.

Usage Example: Suppose we want to write a file's MIME type as a user-defined attribute:

    UserDefinedFileAttributeView view =
        FIles.getFileAttributeView(path, UserDefinedFileAttributeView.class);
    view.write("user.mimetype", Charset.defaultCharset().encode("text/html"));
 

Parameters
name String: The attribute name

src ByteBuffer: The buffer containing the attribute value

Returns
int The number of bytes written, possibly zero

Throws
IOException If an I/O error occurs
SecurityException In the case of the default provider, a security manager is installed, and it denies RuntimePermission("accessUserDefinedAttributes") or its checkWrite method denies write access to the file.