Java IO - Part 3: File IO - 2

This article is a part of the Java Programming Language article series.

Contents

• Read, Write, and Create File
• Random Access Files
• Create and Read Directories
• Links, Symbolic or Otherwise
• Other Useful Methods
• Legacy File IO

Reading, Writing, and Creating Files

Methods By Complexity

The OpenOption Parameter

• An optional parameter
• The StandardOpenOption enum

StandardOpenOption

• READ: Open for read access
• WRITE: Opens the file for write access
• APPEND: Appends the new data to the end of the file, used with the WRITE or CREATE options
• TRUNCATE_EXISTING: Truncates the file to zero bytes, used with the WRITE option
• CREATE_NEW: Creates a new file and throws an exception if the file already exists
• CREATE: Opens the file if it exists or creates a new file if it does not
• DELETE_ON_CLOSE: Deletes the file when the stream is closed, useful for temporary files
• SPARSE: Hints that a newly created file will be sparse
• SYNC: Keeps the file (both content and metadata) synchronized with the underlying storage device
• DSYNC: Keeps the file content synchronized with the underlying storage device

For Small Files

Reading All Bytes

Writing All Bytes

Buffered IO Methods for Text Files

Reading a File

Writing a File

Unbuffered Streams and Interoperable with java.io APIs

Reading a File

Writing a File

Channels and ByteBuffers

• Reads a buffer at a time

Reading a File

Writing a File

Creating Files

The newOutputStream method is another option

Creating Temporary Files

• The format of the temporary file name is platform specific

Random Access Files

The SeekableByteChannel interface

• position: returns the channel’s current position
• position(long): sets the channel’s position
• read(ByteBuffer): reads bytes into the buffer from the channel
• write(ByteBuffer): writes bytes from the buffer to the channel
• truncate(long): truncates the file (or other entity) connected to the channel

Creating and Reading Directories

Listing a File System’s Root Directories

Creating a Directory

Creating a Temporary Directory

Listing a Directory’s Contents

Filtering a Directory Listing By Using Globbing

Glob

• A pattern specified as a string

Glob syntax

• An asterisk (*): matches any number of characters (including none)
• Two asterisks (**): works like * but crosses directory boundaries, used for matching complete paths
• A question mark (?): matches exactly one character
• Braces ({}): specify a collection of subpatterns
• Square brackets ([]): convey a set of single characters or, when the hyphen character (-) is used, a range of characters ([aeiou], [a-z,A-Z]), within the square brackets, *, ?, and \ match themselves
• To match *, ?, or the other special characters, use the backslash character (\\, \?)

Writing Your Own Directory Filter

Links, Symbolic or Otherwise

Creating a Symbolic Link

Creating a Hard Link

Detecting a Symbolic Link

Finding the Target of a Link

Other Useful Methods

Determining MIME Type

Default File System

• The getDefault method

Path String Separator

File System’s File Stores

• File store: mounted file system (UNIX), volume (Windows)

Legacy File IO

Drawbacks

• Many methods didn’t throw exceptions when they failed
• If a file deletion failed, the cause may not be determined: the file didn’t exist or the user didn’t have permissions or another problem
• The rename method didn’t work consistently across platforms
• There was no real support for symbolic links
• Accessing file metadata was inefficient
• Many of the File methods didn’t scale: Requesting a large directory listing over a server could result in a hang, also cause memory resource problems
• It was not possible to write reliable code that could recursively walk a file tree

Interoperability With Legacy Code

• The toPath method: converts java.io.File to java.nio.file.Path

• The toFile method: converts java.nio.file.Path to java.io.File

Mapping java.io.File Functionality to java.nio.file

Mapping for Legacy File IO to NIO.2