Reading a File in Java From a Package

This page discusses the details of reading, writing, creating, and opening files. There are a wide array of file I/O methods to choose from. To assistance make sense of the API, the following diagram arranges the file I/O methods by complication.

File I/O Methods Arranged from Less Circuitous to More Circuitous

On the far left of the diagram are the utility methods readAllBytes, readAllLines, and the write methods, designed for unproblematic, mutual cases. To the right of those are the methods used to iterate over a stream or lines of text, such every bit newBufferedReader, newBufferedWriter, then newInputStream and newOutputStream. These methods are interoperable with the java.io package. To the right of those are the methods for dealing with ByteChannels, SeekableByteChannels, and ByteBuffers, such as the newByteChannel method. Finally, on the far correct are the methods that employ FileChannel for advanced applications needing file locking or memory-mapped I/O.

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Annotation: The methods for creating a new file enable you to specify an optional gear up of initial attributes for the file. For case, on a file system that supports the POSIX set of standards (such as UNIX), you can specify a file owner, group owner, or file permissions at the time the file is created. The Managing Metadata folio explains file attributes, and how to access and gear up them.

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This page has the following topics:

• The OpenOptions Parameter
• Normally Used Methods for Small-scale Files
• Buffered I/O Methods for Text Files
• Methods for Unbuffered Streams and Interoperable with java.io APIs
• Methods for Channels and ByteBuffers
• Methods for Creating Regular and Temporary Files

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The OpenOptions Parameter

Several of the methods in this department take an optional OpenOptions parameter. This parameter is optional and the API tells you what the default beliefs is for the method when none is specified.

The following StandardOpenOptions enums are supported:

• WRITE – Opens the file for write access.
• APPEND – Appends the new data to the end of the file. This selection is used with the WRITE or CREATE options.
• TRUNCATE_EXISTING – Truncates the file to zero bytes. This option is 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 information technology does not.
• DELETE_ON_CLOSE – Deletes the file when the stream is airtight. This option is useful for temporary files.
• SPARSE – Hints that a newly created file will be sparse. This avant-garde option is honored on some file systems, such as NTFS, where large files with data "gaps" tin be stored in a more efficient way where those empty gaps practise not swallow disk space.
• 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.

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Commonly Used Methods for Small Files

Reading All Bytes or Lines from a File

If yous accept a small-ish file and you would like to read its entire contents in 1 pass, you can utilise the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods take intendance of most of the work for you, such as opening and closing the stream, simply are not intended for handling large files. The following code shows how to apply the readAllBytes method:

Path file = ...; byte[] fileArray; fileArray = Files.readAllBytes(file);          

Writing All Bytes or Lines to a File

You can use ane of the write methods to write bytes, or lines, to a file.

• write(Path, byte[], OpenOption...)
• write(Path, Iterable< extends CharSequence>, Charset, OpenOption...)

The following lawmaking snippet shows how to utilize a write method.

Path file = ...; byte[] buf = ...; Files.write(file, buf);          

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Buffered I/O Methods for Text Files

The java.nio.file package supports channel I/O, which moves data in buffers, bypassing some of the layers that can bottleneck stream I/O.

Reading a File by Using Buffered Stream I/O

The newBufferedReader(Path, Charset) method opens a file for reading, returning a BufferedReader that tin can be used to read text from a file in an efficient manner.

The post-obit code snippet shows how to utilize the newBufferedReader method to read from a file. The file is encoded in "U.s.a.-ASCII."

Charset charset = Charset.forName("Us-ASCII"); try (BufferedReader reader = Files.newBufferedReader(file, charset)) {     String line = null;     while ((line = reader.readLine()) != null) {         System.out.println(line);     } } catch (IOException x) {     System.err.format("IOException: %due south%north", 10); }          

Writing a File by Using Buffered Stream I/O

You tin can employ the newBufferedWriter(Path, Charset, OpenOption...) method to write to a file using a BufferedWriter.

The following code snippet shows how to create a file encoded in "US-ASCII" using this method:

Charset charset = Charset.forName("US-ASCII"); String s = ...; try (BufferedWriter writer = Files.newBufferedWriter(file, charset)) {     writer.write(s, 0, s.length()); } grab (IOException x) {     System.err.format("IOException: %southward%due north", x); }          

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Methods for Unbuffered Streams and Interoperable with java.io APIs

Reading a File by Using Stream I/O

To open up a file for reading, you tin utilize the newInputStream(Path, OpenOption...) method. This method returns an unbuffered input stream for reading bytes from the file.

Path file = ...; try (InputStream in = Files.newInputStream(file);     BufferedReader reader =       new BufferedReader(new InputStreamReader(in))) {     Cord line = zippo;     while ((line = reader.readLine()) != naught) {         System.out.println(line);     } } catch (IOException x) {     System.err.println(10); }          

Creating and Writing a File by Using Stream I/O

Yous can create a file, append to a file, or write to a file past using the newOutputStream(Path, OpenOption...) method. This method opens or creates a file for writing bytes and returns an unbuffered output stream.

The method takes an optional OpenOption parameter. If no open options are specified, and the file does not be, a new file is created. If the file exists, it is truncated. This choice is equivalent to invoking the method with the CREATE and TRUNCATE_EXISTING options.

The following example opens a log file. If the file does not exist, information technology is created. If the file exists, information technology is opened for appending.

import static java.nio.file.StandardOpenOption.*; import coffee.nio.file.*; import java.io.*;  public form LogFileTest {    public static void main(String[] args) {      // Convert the string to a     // byte assortment.     String s = "How-do-you-do World! ";     byte data[] = southward.getBytes();     Path p = Paths.get("./logfile.txt");      try (OutputStream out = new BufferedOutputStream(       Files.newOutputStream(p, CREATE, APPEND))) {       out.write(information, 0, data.length);     } grab (IOException x) {       Arrangement.err.println(x);     }   } }          

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Methods for Channels and ByteBuffers

Reading and Writing Files past Using Aqueduct I/O

While stream I/O reads a character at a time, aqueduct I/O reads a buffer at a time. The ByteChannel interface provides basic read and write functionality. A SeekableByteChannel is a ByteChannel that has the capability to maintain a position in the channel and to change that position. A SeekableByteChannel also supports truncating the file associated with the channel and querying the file for its size.

The adequacy to move to unlike points in the file and then read from or write to that location makes random access of a file possible. See Random Access Files for more information.

There are two methods for reading and writing channel I/O.

• newByteChannel(Path, OpenOption...)
• newByteChannel(Path, Set up<? extends OpenOption>, FileAttribute<?>...)

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Note: The newByteChannel methods render an example of a SeekableByteChannel. With a default file organisation, you can cast this seekable byte channel to a FileChannel providing access to more than advanced features such mapping a region of the file directly into retention for faster admission, locking a region of the file so other processes cannot access information technology, or reading and writing bytes from an absolute position without affecting the channel'southward current position.

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Both newByteChannel methods enable you to specify a list of OpenOption options. The aforementioned open options used by the newOutputStream methods are supported, in improver to one more than option: READ is required considering the SeekableByteChannel supports both reading and writing.

Specifying READ opens the channel for reading. Specifying WRITE or APPEND opens the channel for writing. If none of these options are specified, then the channel is opened for reading.

The following code snippet reads a file and prints it to standard output:

public static void readFile(Path path) throws IOException {      // Files.newByteChannel() defaults to StandardOpenOption.READ     attempt (SeekableByteChannel sbc = Files.newByteChannel(path)) {         last int BUFFER_CAPACITY = ten;         ByteBuffer buf = ByteBuffer.allocate(BUFFER_CAPACITY);          // Read the bytes with the proper encoding for this platform. If         // you skip this step, you might encounter foreign or illegible         // characters.         String encoding = Arrangement.getProperty("file.encoding");         while (sbc.read(buf) > 0) {             buf.flip();             System.out.print(Charset.forName(encoding).decode(buf));             buf.clear();         }     }     }          

The following example, written for UNIX and other POSIX file systems, creates a log file with a specific gear up of file permissions. This code creates a log file or appends to the log file if it already exists. The log file is created with read/write permissions for possessor and read just permissions for grouping.

import static java.nio.file.StandardOpenOption.*; import coffee.nio.*; import java.nio.channels.*; import coffee.nio.file.*; import java.nio.file.attribute.*; import java.io.*; import java.util.*;  public form LogFilePermissionsTest {    public static void main(Cord[] args) {        // Create the set of options for appending to the file.     Set<OpenOption> options = new HashSet<OpenOption>();     options.add(APPEND);     options.add(CREATE);      // Create the custom permissions attribute.     Set<PosixFilePermission> perms =       PosixFilePermissions.fromString("rw-r-----");     FileAttribute<Set<PosixFilePermission>> attr =       PosixFilePermissions.asFileAttribute(perms);      // Catechumen the string to a ByteBuffer.     String southward = "Hello World! ";     byte information[] = southward.getBytes();     ByteBuffer bb = ByteBuffer.wrap(data);          Path file = Paths.get("./permissions.log");      try (SeekableByteChannel sbc =       Files.newByteChannel(file, options, attr)) {       sbc.write(bb);     } catch (IOException x) {       Organisation.out.println("Exception thrown: " + x);     }   } }          

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Methods for Creating Regular and Temporary Files

Creating Files

You lot tin can create an empty file with an initial set of attributes past using the createFile(Path, FileAttribute<?>) method. For instance, if, at the fourth dimension of creation, yous desire a file to have a particular set up of file permissions, apply the createFile method to practice so. If you exercise not specify any attributes, the file is created with default attributes. If the file already exists, createFile throws an exception.

In a unmarried diminutive operation, the createFile method checks for the being of the file and creates that file with the specified attributes, which makes the procedure more secure against malicious lawmaking.

The following code snippet creates a file with default attributes:

Path file = ...; endeavour {     // Create the empty file with default permissions, etc.     Files.createFile(file); } take hold of (FileAlreadyExistsException x) {     Organisation.err.format("file named %southward" +         " already exists%north", file); } grab (IOException 10) {     // Some other sort of failure, such as permissions.     System.err.format("createFile error: %s%n", x); }          

POSIX File Permissions has an instance that uses createFile(Path, FileAttribute<?>) to create a file with pre-set permissions.

You tin as well create a new file past using the newOutputStream methods, equally described in Creating and Writing a File using Stream I/O. If y'all open a new output stream and close information technology immediately, an empty file is created.

Creating Temporary Files

You can create a temporary file using one of the following createTempFile methods:

• createTempFile(Path, String, String, FileAttribute<?>)
• createTempFile(String, String, FileAttribute<?>)

The first method allows the code to specify a directory for the temporary file and the second method creates a new file in the default temporary-file directory. Both methods let you to specify a suffix for the filename and the commencement method allows yous to likewise specify a prefix. The following code snippet gives an example of the second method:

attempt {     Path tempFile = Files.createTempFile(null, ".myapp");     Organization.out.format("The temporary file" +         " has been created: %s%n", tempFile) ; } grab (IOException x) {     System.err.format("IOException: %s%n", x); }          

The event of running this file would be something similar the following:

The temporary file has been created: /tmp/509668702974537184.myapp          

The specific format of the temporary file proper name is platform specific.

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Source: https://docs.oracle.com/javase/tutorial/essential/io/file.html

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