文件读取
文件读写
文件读取
BufferedReader
我们可以用 BufferedReader 封装出常见的读取工具类：
@Test
public void whenReadWithBufferedReader_thenCorrect()
throws IOException {
String expected_value = "Hello, world!";
String file ="src/test/resources/fileTest.txt";
BufferedReader reader = new BufferedReader(new FileReader(file));
String currentLine = reader.readLine();
reader.close();
assertEquals(expected_value, currentLine);
}
private String readFromInputStream(InputStream inputStream)
throws IOException {
StringBuilder resultStringBuilder = new StringBuilder();
try (BufferedReader br
= new BufferedReader(new InputStreamReader(inputStream))) {
String line;
while ((line = br.readLine()) != null) {
resultStringBuilder.append(line).append("\n");
}
}
return resultStringBuilder.toString();
}
我们也可以指定不同的编码：
@Test
public void whenReadUTFEncodedFile_thenCorrect()
throws IOException {
String expected_value = "青空";
String file = "src/test/resources/fileTestUtf8.txt";
BufferedReader reader = new BufferedReader
(new InputStreamReader(new FileInputStream(file), "UTF-8"));
String currentLine = reader.readLine();
reader.close();
assertEquals(expected_value, currentLine);
}
从 Classpath 中读取文件
@Test
public void givenFileNameAsAbsolutePath_whenUsingClasspath_thenFileData() {
String expectedData = "Hello, world!";
Class clazz = FileOperationsTest.class;
InputStream inputStream = clazz.getResourceAsStream("/fileTest.txt");
String data = readFromInputStream(inputStream);
Assert.assertThat(data, containsString(expectedData));
}
在上面的代码片段中，我们使用 getResourceAsStream 方法使用当前类加载文件，并传递了要加载的文件的绝对路径。
ClassLoader classLoader = getClass().getClassLoader();
InputStream inputStream = classLoader.getResourceAsStream("fileTest.txt");
String data = readFromInputStream(inputStream);
我们使用 getClass().getClassLoader(). 获得当前类的 classLoader。主要区别在于，在 ClassLoader 实例上使用 getResourceAsStream 时，将从类路径的根开始的路径视为绝对路径。当针对 Class 实例使用时，该路径可以是相对于包的路径，也可以是绝对路径，该路径由前导斜线提示。
当然，请注意，实际上，应始终关闭打开的流，例如本例中的 InputStream：
InputStream inputStream = null;
try {
File file = new File(classLoader.getResource("fileTest.txt").getFile());
inputStream = new FileInputStream(file);
//...
}
finally {
if (inputStream != null) {
try {
inputStream.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
我们也能够读取某个 Jar 包中的文件：
@Test
public void givenFileName_whenUsingJarFile_thenFileData() {
String expectedData = "BSD License";
Class clazz = Matchers.class;
InputStream inputStream = clazz.getResourceAsStream("/LICENSE.txt");
String data = readFromInputStream(inputStream);
Assert.assertThat(data, containsString(expectedData));
}
commons-io
@Test
public void givenFileName_whenUsingFileUtils_thenFileData() {
String expectedData = "Hello, world!";
ClassLoader classLoader = getClass().getClassLoader();
File file = new File(classLoader.getResource("fileTest.txt").getFile());
String data = FileUtils.readFileToString(file, "UTF-8");
assertEquals(expectedData, data.trim());
}
@Test
public void givenFileName_whenUsingIOUtils_thenFileData() {
String expectedData = "Hello, world!";
FileInputStream fis = new FileInputStream("src/test/resources/fileTest.txt");
String data = IOUtils.toString(fis, "UTF-8");
assertEquals(expectedData, data.trim());
}
在这里，我们将 File 对象传递给 FileUtils 类的 readFileToString() 方法。该实用程序类无需写入任何样板代码即可创建内容，而无需编写任何代码来创建 InputStream 实例和读取数据。
Scanner
@Test
public void whenReadWithScanner_thenCorrect()
throws IOException {
String file = "src/test/resources/fileTest.txt";
Scanner scanner = new Scanner(new File(file));
scanner.useDelimiter(" ");
assertTrue(scanner.hasNext());
assertEquals("Hello,", scanner.next());
assertEquals("world!", scanner.next());
scanner.close();
}
当从控制台读取内容时，或者当内容包含带有已知定界符的原始值（例如，由空格分隔的整数列表）时，Scanner 类很有用。
StreamTokenizer
@Test
public void whenReadWithStreamTokenizer_thenCorrectTokens()
throws IOException {
String file = "src/test/resources/fileTestTokenizer.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();
}
DataInputStream
我们可以使用 DataInputStream 从文件中读取二进制或原始数据类型。
@Test
public void whenReadWithDataInputStream_thenCorrect() throws IOException {
String expectedValue = "Hello, world!";
String file ="src/test/resources/fileTest.txt";
String result = null;
DataInputStream reader = new DataInputStream(new FileInputStream(file));
int nBytesToRead = reader.available();
if(nBytesToRead > 0) {
byte[] bytes = new byte[nBytesToRead];
reader.read(bytes);
result = new String(bytes);
}
assertEquals(expectedValue, result);
}
Files
在 JDK7 之后 NIO 也是得到了极大的更新，我们可以使用 Files 的 readAllBytes 方法来读取文件：
Path path = Paths.get(getClass().getClassLoader()
.getResource("fileTest.txt").toURI());
byte[] fileBytes = Files.readAllBytes(path);
String data = new String(fileBytes);
// Use Stream
Stream<String> lines = Files.lines(path);
String data = lines.collect(Collectors.joining("\n"));
lines.close();
// 按行读取
@Test
public void whenReadSmallFileJava7_thenCorrect()
throws IOException {
String expected_value = "Hello, world!";
Path path = Paths.get("src/test/resources/fileTest.txt");
String read = Files.readAllLines(path).get(0);
assertEquals(expected_value, read);
}
// 读取大文件
@Test
public void whenReadLargeFileJava7_thenCorrect()
throws IOException {
String expected_value = "Hello, world!";
Path path = Paths.get("src/test/resources/fileTest.txt");
BufferedReader reader = Files.newBufferedReader(path);
String line = reader.readLine();
assertEquals(expected_value, line);
}
Java 11
@Test
public void readFileIntoString() throws IOException {
String content = Files.readString(Paths.get("[FILE_NAME]"));
assertNotNull(content);
}
随机读取
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;
}
FileChannel
如果我们正在读取大文件，则 FileChannel 可能比标准 IO 更快。以下代码使用 FileChannel 和 RandomAccessFile 从文件读取数据字节：
@Test
public void whenReadWithFileChannel_thenCorrect()
throws IOException {
String expected_value = "Hello, world!";
String file = "src/test/resources/fileTest.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();
}