How to Get a Java Program to Read a File

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

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

On the far left of the diagram are the utility methods readAllBytes, readAllLines, and the write methods, designed for simple, common cases. To the right of those are the methods used to iterate over a stream or lines of text, such every bit newBufferedReader, newBufferedWriter, and 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 right are the methods that use 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 prepare of initial attributes for the file. For instance, on a file system that supports the POSIX gear up of standards (such every bit UNIX), you lot can specify a file owner, grouping possessor, or file permissions at the time the file is created. The Managing Metadata page explains file attributes, and how to access and set them.

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This page has the following topics:

• The OpenOptions Parameter
• Ordinarily Used Methods for Small Files
• Buffered I/O Methods for Text Files
• Methods for Unbuffered Streams and Interoperable with coffee.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 section accept an optional OpenOptions parameter. This parameter is optional and the API tells y'all what the default behavior is for the method when none is specified.

The following StandardOpenOptions enums are supported:

• WRITE – Opens the file for write access.
• Suspend – Appends the new data to the terminate of the file. This option 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 it does non.
• DELETE_ON_CLOSE – Deletes the file when the stream is closed. This option is useful for temporary files.
• SPARSE – Hints that a newly created file volition exist sparse. This advanced option is honored on some file systems, such as NTFS, where large files with data "gaps" tin be stored in a more efficient manner where those empty gaps do 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-scale Files

Reading All Bytes or Lines from a File

If you have a small-ish file and you lot would like to read its entire contents in 1 pass, you tin can use the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods accept intendance of most of the work for you, such equally opening and closing the stream, merely are non intended for handling large files. The following code shows how to use the readAllBytes method:

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

Writing All Bytes or Lines to a File

You tin can utilise one 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 use 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 bundle supports channel I/O, which moves information in buffers, bypassing some of the layers that can clogging stream I/O.

Reading a File past 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 fashion.

The following code snippet shows how to use the newBufferedReader method to read from a file. The file is encoded in "The states-ASCII."

Charset charset = Charset.forName("United states-ASCII"); endeavour (BufferedReader reader = Files.newBufferedReader(file, charset)) {     Cord line = goose egg;     while ((line = reader.readLine()) != aught) {         System.out.println(line);     } } catch (IOException 10) {     System.err.format("IOException: %s%northward", x); }          

Writing a File by Using Buffered Stream I/O

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

The post-obit code snippet shows how to create a file encoded in "United states of america-ASCII" using this method:

Charset charset = Charset.forName("United states-ASCII"); String s = ...; try (BufferedWriter author = Files.newBufferedWriter(file, charset)) {     writer.write(s, 0, southward.length()); } catch (IOException x) {     System.err.format("IOException: %s%north", 10); }          

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Methods for Unbuffered Streams and Interoperable with java.io APIs

Reading a File past Using Stream I/O

To open a file for reading, you can apply 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))) {     String line = cypher;     while ((line = reader.readLine()) != nix) {         System.out.println(line);     } } take hold of (IOException x) {     System.err.println(10); }          

Creating and Writing a File by Using Stream I/O

You can create a file, append to a file, or write to a file by 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 exist, a new file is created. If the file exists, it is truncated. This option is equivalent to invoking the method with the CREATE and TRUNCATE_EXISTING options.

The following case opens a log file. If the file does not exist, it is created. If the file exists, it is opened for appending.

import static java.nio.file.StandardOpenOption.*; import coffee.nio.file.*; import java.io.*;  public class LogFileTest {    public static void main(Cord[] args) {      // Convert the string to a     // byte array.     String due south = "Howdy Earth! ";     byte data[] = s.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 ten) {       Arrangement.err.println(x);     }   } }          

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Methods for Channels and ByteBuffers

Reading and Writing Files by Using Channel I/O

While stream I/O reads a character at a time, channel 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 capability to move to unlike points in the file and then read from or write to that location makes random access of a file possible. Run into Random Access Files for more information.

In that location are ii methods for reading and writing aqueduct I/O.

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

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Annotation: The newByteChannel methods render an instance of a SeekableByteChannel. With a default file system, yous can cast this seekable byte channel to a FileChannel providing access to more avant-garde features such mapping a region of the file directly into retention for faster access, locking a region of the file so other processes cannot access it, or reading and writing bytes from an absolute position without affecting the aqueduct'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 add-on to i more option: READ is required because the SeekableByteChannel supports both reading and writing.

Specifying READ opens the aqueduct for reading. Specifying WRITE or APPEND opens the aqueduct for writing. If none of these options are specified, and so the aqueduct 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     try (SeekableByteChannel sbc = Files.newByteChannel(path)) {         last int BUFFER_CAPACITY = x;         ByteBuffer buf = ByteBuffer.allocate(BUFFER_CAPACITY);          // Read the bytes with the proper encoding for this platform. If         // you skip this footstep, you might run into foreign or illegible         // characters.         String encoding = Arrangement.getProperty("file.encoding");         while (sbc.read(buf) > 0) {             buf.flip();             System.out.impress(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 set 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 owner and read only permissions for group.

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

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Methods for Creating Regular and Temporary Files

Creating Files

You can create an empty file with an initial ready of attributes by using the createFile(Path, FileAttribute<?>) method. For example, if, at the time of cosmos, you want a file to take a particular set of file permissions, use the createFile method to do and then. If you lot do not specify any attributes, the file is created with default attributes. If the file already exists, createFile throws an exception.

In a single atomic operation, the createFile method checks for the beingness of the file and creates that file with the specified attributes, which makes the process more secure against malicious code.

The post-obit lawmaking snippet creates a file with default attributes:

Path file = ...; endeavor {     // Create the empty file with default permissions, etc.     Files.createFile(file); } catch (FileAlreadyExistsException 10) {     System.err.format("file named %s" +         " already exists%n", file); } catch (IOException 10) {     // Another sort of failure, such as permissions.     Organization.err.format("createFile fault: %due south%north", 10); }          

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

Y'all can besides create a new file by using the newOutputStream methods, every bit described in Creating and Writing a File using Stream I/O. If you open a new output stream and close it immediately, an empty file is created.

Creating Temporary Files

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

• createTempFile(Path, String, String, FileAttribute<?>)
• createTempFile(String, Cord, 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 permit yous to specify a suffix for the filename and the kickoff method allows you to also specify a prefix. The post-obit lawmaking snippet gives an example of the 2nd method:

endeavour {     Path tempFile = Files.createTempFile(nil, ".myapp");     Organisation.out.format("The temporary file" +         " has been created: %south%northward", tempFile) ; } catch (IOException 10) {     Organization.err.format("IOException: %s%n", ten); }          

The outcome of running this file would be something like 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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