Reading, Writing, and Creating Files
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 help make 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 as 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 right are the methods that use FileChannel for advanced applications needing file locking or memory-mapped I/O.
This page has the following topics:
The OpenOptions Parameter
Several of the methods in this section take an optional OpenOptions parameter. This parameter is optional and the API tells you 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.
- APPEND – Appends the new data to the end 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 not.
- 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 will be sparse. This advanced option is honored on some file systems, such as NTFS, where large files with data "gaps" can be stored in a more efficient manner where those empty gaps do not consume 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.
Commonly Used Methods for Small Files
Reading All Bytes or Lines from a File
If you have a small-ish file and you would like to read its entire contents in one pass, you can use the readAllBytes(Path) or readAllLines(Path, Charset) method. These methods take care of most of the work for you, such as opening and closing the stream, but are not intended for handling large files. The following code shows how to use the readAllBytes method:
Writing All Bytes or Lines to a File
You can use 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 code snippet shows how to use a write method.
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 can be used to read text from a file in an efficient manner.
The following code snippet shows how to use the newBufferedReader method to read from a file. The file is encoded in "US-ASCII."
Writing a File by Using Buffered Stream I/O
You can use 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:
Methods for Unbuffered Streams and Interoperable with java.io APIs
Reading a File by Using Stream I/O
To open a file for reading, you can use the newInputStream(Path, OpenOption. ) method. This method returns an unbuffered input stream for reading bytes from the file.
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 example opens a log file. If the file does not exist, it is created. If the file exists, it is opened for appending.
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 different 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<? extends OpenOption>, FileAttribute<?>. )
Both newByteChannel methods enable you to specify a list of OpenOption options. The same open options used by the newOutputStream methods are supported, in addition to one more option: READ is required because 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:
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.
Methods for Creating Regular and Temporary Files
Creating Files
You can create an empty file with an initial set of attributes by using the createFile(Path, FileAttribute<?>) method. For example, if, at the time of creation, you want a file to have a particular set of file permissions, use the createFile method to do so. If you 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 existence of the file and creates that file with the specified attributes, which makes the process more secure against malicious code.
The following code snippet creates a file with default attributes:
POSIX File Permissions has an example that uses createFile(Path, FileAttribute<?>) to create a file with pre-set permissions.
You can also create a new file by using the newOutputStream methods, as 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 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 allow you to specify a suffix for the filename and the first method allows you to also specify a prefix. The following code snippet gives an example of the second method:
Как прочитать файл в java
Хотя с помощью ранее рассмотренных классов можно записывать текст в файлы, однако они предназначены прежде всего дл работы с бинарными потоками данных, и их возможностей для полноценной работы с текстовыми файлами недостаточно. И для этой цели служат совсем другие классы, которые являются наследниками абстрактных классов Reader и Writer .
Запись файлов. Класс FileWriter
Класс FileWriter является производным от класса Writer. Он используется для записи текстовых файлов.
Чтобы создать объект FileWriter, можно использовать один из следующих конструкторов:
Так, в конструктор передается либо путь к файлу в виде строки, либо объект File, который ссылается на конкретный текстовый файл. Параметр append указывает, должны ли данные дозаписываться в конец файла (если параметр равен true), либо файл должен перезаписываться.
Запишем в файл какой-нибудь текст:
В конструкторе использовался параметр append со значением false — то есть файл будет перезаписываться. Затем с помощью методов, определенных в базовом классе Writer производится запись данных.
Чтение файлов. Класс FileReader
Класс FileReader наследуется от абстрактного класса Reader и предоставляет функциональность для чтения текстовых файлов.
Для создания объекта FileReader мы можем использовать один из его конструкторов:
А используя методы, определенные в базом классе Reader, произвести чтение файла:
Также мы можем считывать в промежуточный буфер из массива символов:
В данном случае считываем последовательно символы из файла в массив из 256 символов, пока не дойдем до конца файла в этом случае метод read возвратит число -1.
Java Files
File handling is an important part of any application.
Java has several methods for creating, reading, updating, and deleting files.
Java File Handling
The File class from the java.io package, allows us to work with files.
To use the File class, create an object of the class, and specify the filename or directory name:
Example
Java Create and Write To Files
Create a File
To create a file in Java, you can use the createNewFile() method. This method returns a boolean value: true if the file was successfully created, and false if the file already exists. Note that the method is enclosed in a try. catch block. This is necessary because it throws an IOException if an error occurs (if the file cannot be created for some reason):
Example
The output will be:
To create a file in a specific directory (requires permission), specify the path of the file and use double backslashes to escape the “ \ " character (for Windows). On Mac and Linux you can just write the path, like: /Users/name/filename.txt
Example
Write To a File
In the following example, we use the FileWriter class together with its write() method to write some text to the file we created in the example above. Note that when you are done writing to the file, you should close it with the close() method:
Example
The output will be:
Read a File
In the following example, we use the Scanner class to read the contents of the text file we created.
Example
The output will be:
Example
The output will be:
Note: There are many available classes in the Java API that can be used to read and write files in Java: FileReader, BufferedReader, Files, Scanner, FileInputStream, FileWriter, BufferedWriter, FileOutputStream , etc. Which one to use depends on the Java version you're working with and whether you need to read bytes or characters, and the size of the file/lines etc.
Кофе-брейк #229. Как работать с файлами и Input/Output в Java. Служебные методы класса Objects
Источник: Medium Это руководство разъясняет, как создавать, читать, записывать и удалять файлы в Java. Вы также узнаете, как работают классы File , InputStream и OutputStream .