读文件
http://www.baeldung.com/java-read-file
Java – Read from File
1. Overview
In this tutorial we’ll explore different ways to read from a File in Java; we’ll make use ofBufferedReader, Scanner, StreamTokenizer, DataInputStream, SequenceInputStream andFileChannel.
Then, we will discuss how to read a UTF-8 encoded file and how to create String from contents of a file.
Finally, we’ll explore the new techniques to read from file in Java 7.
This article is part of the “Java – Back to Basic” series here on Baeldung.
2. Read with BufferedReader
Let’s start with a simple way to read from file using BufferedReader; the file itself contains:
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Hello world |
The following code reads from the file using BufferedReader:
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@Testpublic void whenReadWithBufferedReader_thenCorrect() throws IOException { String expected_value = "Hello world"; String file ="src/test/resources/test_read.txt"; BufferedReader reader = new BufferedReader(new FileReader(file)); String currentLine = reader.readLine(); reader.close(); assertEquals(expected_value, currentLine);} |
Note that readLine() will return null when the end of the file is reached.
3. Read with Scanner
Next, let’s use a Scanner to read from the File – the file contains:
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Hello world 1 |
We’ll use a simple whitespace as the delimiter:
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@Testpublic void whenReadWithScanner_thenCorrect() throws IOException { String file = "src/test/resources/test_read.txt"; Scanner scanner = new Scanner(new File(file)); scanner.useDelimiter(" "); assertTrue(scanner.hasNext()); assertEquals("Hello", scanner.next()); assertEquals("world", scanner.next()); assertEquals(1, scanner.nextInt()); scanner.close();} |
Note that the default delimiter is the whitespace, but multiple delimiters can be used with aScanner.
4. Read with StreamTokenizer
Next, let’s read a text file into tokens using a StreamTokenizer.
The way the tokenizer works is – first, we need to figure out what the next token is – String or number; we do that by looking at the tokenizer.ttype field.
Then, we’ll read the actual token based on this type:
- tokenizer.nval – if the type was a number
- tokenizer.sval – if the type was a String
The file simply contains:
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Hello 1 |
The following code reads from the file both the String and the number:
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@Testpublic void whenReadWithStreamTokenizer_thenCorrectTokens() throws IOException { String file = "src/test/resources/test_read.txt"; FileReader reader = new FileReader(file); StreamTokenizer tokenizer = new StreamTokenizer(reader); // token 1 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_WORD, tokenizer.ttype); assertEquals("Hello", tokenizer.sval); // token 2 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_NUMBER, tokenizer.ttype); assertEquals(1, tokenizer.nval, 0.0000001); // token 3 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_EOF, tokenizer.ttype); reader.close();} |
Note how the end of file token is used at the end.
5. Read with DataInputStream
We can use DataInputStream to read binary or primitive data type from file.
Let’s start with the actual file itself:
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Hello |
The following test reads the file using a DataInputStream:
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@Testpublic void whenReadWithDataInputStream_thenCorrect() throws IOException { String expectedValue = "Hello"; String file ="src/test/resources/test_read.txt"; DataInputStream reader = new DataInputStream(new FileInputStream(file)); String result = reader.readUTF(); reader.close(); assertEquals(expectedValue, result);} |
6. Read with SequenceInputStream
Now, let’s look at how to concatenate two input streams into one usingSequenceInputStream; the 2 input files will simply contain:
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2000 |
and:
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5000 |
Let’s now use a SequenceInputStream to read the two files and merge them into one:
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@Testpublic void whenReadingTwoFilesWithSequenceInputStream_thenCorrect() throws IOException { int expectedValue1 = 2000; int expectedValue2 = 5000; String file1 = "src/test/resources/test_read1.txt"; String file2 = "src/test/resources/test_read2.txt"; FileInputStream stream1 = new FileInputStream(file1); FileInputStream stream2 = new FileInputStream(file2); SequenceInputStream sequence = new SequenceInputStream(stream1, stream2); DataInputStream reader = new DataInputStream(sequence); assertEquals(expectedValue1, reader.readInt()); assertEquals(expectedValue2, reader.readInt()); reader.close(); stream2.close();} |
7. Read with FileChannel
If we are reading a large file, FileChannel can be faster than standard IO.
The contents of the file:
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Hello world |
The following code reads data bytes from the file using FileChannel and RandomAccessFile:
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@Testpublic void whenReadWithFileChannel_thenCorrect() throws IOException { String expected_value = "Hello world"; String file = "src/test/resources/test_read.txt"; RandomAccessFile reader = new RandomAccessFile(file, "r"); FileChannel channel = reader.getChannel(); int bufferSize = 1024; if (bufferSize > channel.size()) { bufferSize = (int) channel.size(); } ByteBuffer buff = ByteBuffer.allocate(bufferSize); channel.read(buff); buff.flip(); assertEquals(expected_value, new String(buff.array())); channel.close(); reader.close();} |
8. Read UTF-8 encoded file
Now, let’s see how to read a UTF-8 encoded file using BufferedReader:
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@Testpublic void whenReadUTFEncodedFile_thenCorrect() throws IOException { String expected_value = "青空"; String file = "src/test/resources/test_read.txt"; BufferedReader reader = new BufferedReader (new InputStreamReader(new FileInputStream(file), "UTF-8")); String currentLine = reader.readLine(); reader.close(); assertEquals(expected_value, currentLine);} |
9. Read a file into a String
We can make good use of StringBuilder to read the entire contents of a file into a String. Let’s start with the file:
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Hello worldTest line |
The following code append data read from the file into a StringBuilder line by line:
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@Testpublic void whenReadFileContentsIntoString_thenCorrect() throws IOException { String expected_value = "Hello world n Test line n"; String file = "src/test/resources/test_read.txt"; BufferedReader reader = new BufferedReader(new FileReader(file)); StringBuilder builder = new StringBuilder(); String currentLine = reader.readLine(); while (currentLine != null) { builder.append(currentLine); builder.append("n"); currentLine = reader.readLine(); } reader.close(); assertEquals(expected_value, builder.toString());} |
10. Read from File using Java 7
Java 7 introduces a new way of working with files and the filesystem – let’s make use of that to read files.
10.1. Read a Small File with Java 7
The file contents:
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Hello world |
The following code shows how to read small file using the new Files class:
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@Testpublic void whenReadSmallFileJava7_thenCorrect() throws IOException { String expected_value = "Hello world"; Path path = Paths.get("src/test/resources/test_read.txt"); String read = Files.readAllLines(path).get(0); assertEquals(expected_value, read);} |
Note that you can use the readAllBytes() method as well if you need binary data.
10.2. Read a Large File with Java 7
If we want to read a large file with Files class, we can use the BufferedReader.
The file contents:
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Hello world |
The following code reads the file using the new Files class and BufferedReader:
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@Testpublic void whenReadLargeFileJava7_thenCorrect() throws IOException { String expected_value = "Hello world"; Path path = Paths.get("src/test/resources/test_read.txt"); BufferedReader reader = Files.newBufferedReader(path); String line = reader.readLine(); assertEquals(expected_value, line);} |
11. Conclusion
As you can see, there are many possibilities of reading data from a file using plain Java. You can go for BufferedReader to read line by line, Scanner to read using different delimiters, StreamTokenizer to read file into tokens, DataInputStream to read binary data and primitive data types, SequenceInput Stream to link multiple files into one stream,FileChannel to read faster from large files, etc.
写文件
Java – Write to File
1. Overview
In this tutorial we’ll explore different ways to write to a file using Java. We’ll make use ofBufferedWriter, PrintWriter, FileOutputStream, DataOutputStream, RandomAccessFile,FileChannel and the Java 7 Files utility class.
We’ll also take a look at locking the file while writing and discuss some final take-aways on writing to file.
This article is part of the “Java – Back to Basic” series here on Baeldung.
2. Write with BufferedWriter
Let’s start simple – and use BufferedWriter to write a String to a new file:
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public void whenWriteStringUsingBufferedWritter_thenCorrect() throws IOException { String str = "Hello"; BufferedWriter writer = new BufferedWriter(new FileWriter(fileName)); writer.write(str); writer.close();} |
The output in the file will be:
Hello |
We can then append a String to the existing file:
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@Testpublic void whenAppendStringUsingBufferedWritter_thenOldContentShouldExistToo() throws IOException { String str = "World"; BufferedWriter writer = new BufferedWriter(new FileWriter(fileName, true)); writer.append(' '); writer.append(str); writer.close();} |
The file will then be:
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Hello World |
3. Write with PrintWriter
Next – let’s see how we can use a PrintWriter to write formatted text to a file:
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@Testpublic void givenWritingStringToFile_whenUsingPrintWriter_thenCorrect() throws IOException { FileWriter fileWriter = new FileWriter(fileName); PrintWriter printWriter = new PrintWriter(fileWriter); printWriter.print("Some String"); printWriter.printf("Product name is %s and its price is %d $", "iPhone", 1000); printWriter.close();} |
The resulting file will contain:
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Some StringProduct name is iPhone and its price is 1000$ |
Note how we’re not only writing a raw String to file, but also some formatted text with theprintf method.
We can create the writer using FileWriter, BufferedWriter or even System.out.
4. Write with FileOutputStream
Let’s now see how we can use FileOutputStream to write binary data to a file. The following code converts a String int bytes and writes the bytes to file using aFileOutputStream:
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@Testpublic void givenWritingStringToFile_whenUsingFileOutputStream_thenCorrect() throws IOException { String str = "Hello"; FileOutputStream outputStream = new FileOutputStream(fileName); byte[] strToBytes = str.getBytes(); outputStream.write(strToBytes); outputStream.close();} |
The output in the file will of course be:
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Hello |
5. Write with DataOutputStream
Next – let’s take a look at how we can use a DataOutputStream to write a String to file:
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@Testpublic void givenWritingToFile_whenUsingDataOutputStream_thenCorrect() throws IOException { String value = "Hello"; FileOutputStream fos = new FileOutputStream(fileName); DataOutputStream outStream = new DataOutputStream(new BufferedOutputStream(fos)); outStream.writeUTF(value); outStream.close(); // verify the results String result; FileInputStream fis = new FileInputStream(fileName); DataInputStream reader = new DataInputStream(fis); result = reader.readUTF(); reader.close(); assertEquals(value, result);} |
6. Write with RandomAccessFile
Let’s now illustrate how to write and edit inside an existing file – rather than just writing to a completely new file or appending to an existing one. Simply put – we need random access.
RandomAccessFile enable us to write at a specific position in the file given the offset – from the beginning of the file – in bytes. The following code writes an integer value with offset given from the beginning of the file:
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private void writeToPosition(String filename, int data, long position) throws IOException { RandomAccessFile writer = new RandomAccessFile(filename, "rw"); writer.seek(position); writer.writeInt(data); writer.close();} |
If we want to read the int stored at specific location, we can use the following method:
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private int readFromPosition(String filename, long position) throws IOException { int result = 0; RandomAccessFile reader = new RandomAccessFile(filename, "r"); reader.seek(position); result = reader.readInt(); reader.close(); return result;} |
To test our functions, let’s write an integer – edit it – and, finally, read it back:
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@Testpublic void whenWritingToSpecificPositionInFile_thenCorrect() throws IOException { int data1 = 2014; int data2 = 1500; writeToPosition(fileName, data1, 4); assertEquals(data1, readFromPosition(fileName, 4)); writeToPosition(fileName2, data2, 4); assertEquals(data2, readFromPosition(fileName, 4));} |
7. Write with FileChannel
If you are dealing with large files, FileChannel can be faster than standard IO. The following code write String to a file using FileChannel:
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@Testpublic void givenWritingToFile_whenUsingFileChannel_thenCorrect() throws IOException { RandomAccessFile stream = new RandomAccessFile(fileName, "rw"); FileChannel channel = stream.getChannel(); String value = "Hello"; byte[] strBytes = value.getBytes(); ByteBuffer buffer = ByteBuffer.allocate(strBytes.length); buffer.put(strBytes); buffer.flip(); channel.write(buffer); stream.close(); channel.close(); // verify RandomAccessFile reader = new RandomAccessFile(fileName, "r"); assertEquals(value, reader.readLine()); reader.close();} |
8. Write to file using Java 7
Java 7 introduces a new way of working with the filesystem, along with a new utility class –Files. Using the Files class, we can create, move, copy, delete files and directories as well; it also can be used to read and write to a file:
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@Testpublic void givenUsingJava7_whenWritingToFile_thenCorrect() throws IOException { String str = "Hello"; Path path = Paths.get(fileName); byte[] strToBytes = str.getBytes(); Files.write(path, strToBytes); String read = Files.readAllLines(path).get(0); assertEquals(str, read);} |
9. Write to temporary file
Now, let’s try to write to temporary file. The following code creates a temporary file and writes a String to it:
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@Testpublic void whenWriteToTmpFile_thenCorrect() throws IOException { String toWrite = "Hello"; File tmpFile = File.createTempFile("test", ".tmp"); FileWriter writer = new FileWriter(tmpFile); writer.write(toWrite); writer.close(); BufferedReader reader = new BufferedReader(new FileReader(tmpFile)); assertEquals(toWrite, reader.readLine()); reader.close();} |
So, as you can see – it’s just the creation of the temporary file that is interesting and different – after that point, writing to the file is the same.
10. Lock File Before Writing
Finally, when writing to a file, you sometimes need to make extra sure that no one else is writing to that file at the same time. Basically – you need to be able to lock that file while writing.
Let’s make use of the FileChannel to try locking the file before writing to it:
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@Testpublic void whenTryToLockFile_thenItShouldBeLocked() throws IOException { RandomAccessFile stream = new RandomAccessFile(fileName, "rw"); FileChannel channel = stream.getChannel(); FileLock lock = null; try { lock = channel.tryLock(); } catch (final OverlappingFileLockException e) { stream.close(); channel.close(); } stream.writeChars("test lock"); lock.release(); stream.close(); channel.close();} |
Note that if the file is already locked when we try to acquire the lock, anOverlappingFileLockException will be thrown.
11. Notes
After exploring so many methods of writing to a file, let’s discuss some important notes:
- If we try to read from a file that doesn’t exist, a FileNotFoundException will be thrown
- If we try to write to a file that doesn’t exist, the file will be created first and no exception will be thrown
- It is very important to close the stream after using it, as it is not closed implicitly, to release any resources associated with it
- In output stream, the close() method calls flush() before releasing the resources which forces any buffered bytes to be written to the stream
Looking at the common usage practices, we can see – for example – that PrintWriter is used to write formatted text; FileOutputStream to write binary data; DataOutputStream to write primitive data types; RandomAccessFile to write to a specific position; FileChannel to write faster in larger files. Some of the APIs of these classes do allow more, but this is a good place to start.
12. Conclusion
This article illustrates the many options of writing data to a File using Java.
The implementation of all these examples and code snippets can be found in my github project – this is an Eclipse based project, so it should be easy to import and run as it is.