Interface UserDefinedFileAttributeView

All Superinterfaces:
AttributeView, FileAttributeView 
public interface UserDefinedFileAttributeView
extends FileAttributeView

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[]).

Since:
1.7

Methods

Modifier and Type Method Description
void delete​(String name)

Deletes a user-defined attribute.
List<String> list()

Returns a list containing the names of the user-defined attributes.
String name()

Returns the name of this attribute view.
int read​(String name, ByteBuffer dst)

Read the value of a user-defined attribute into a buffer.
int size​(String name)

Returns the size of the value of a user-defined attribute.
int write​(String name, ByteBuffer src)

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

Methods

name

String name()

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

Specified by:
name in interface AttributeView
Returns:
the name of the attribute view

list

List<String> list()
           throws IOException

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

Returns:
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.

size

int size(String name)
  throws IOException

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

Parameters:
name - The attribute name
Returns:
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.

read

int read(String name,
         ByteBuffer dst)
  throws IOException

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 - The attribute name
dst - The destination buffer
Returns:
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(java.lang.String)

write

int write(String name,
          ByteBuffer src)
   throws IOException

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 - The attribute name
src - The buffer containing the attribute value
Returns:
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.

delete

void delete(String name)
     throws IOException

Deletes a user-defined attribute.

Parameters:
name - 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.

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https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/nio/file/attribute/UserDefinedFileAttributeView.html