java读取文件方法大全

下文笔者讲述使用java代码读取文件的方法大全,如下所示

实现思路:
    使用BufferedReader, Scanner, StreamTokenizer, DataInputStream, SequenceInputStream, FileChannel
	这些类即可实现读取文件的目的

例:

使用InputStream读取

    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();
    }

从Classpath路径下读取文件

 从src/main/resources读取文件fileTest.txt

    @Test
    public void test() throws IOException {
        String expectedData = "Hello,World!";
        Class<ReadFileTest> clazz = ReadFileTest.class;
        InputStream inputStream = clazz.getResourceAsStream("/fileTest.txt");
        String data = readFromInputStream(inputStream);
 
        Assert.assertThat(data, containsString(expectedData));
    }

使用commons-io库

引入相应的依赖
<dependency>
	<groupId>commons-io</groupId>
	<artifactId>commons-io</artifactId>
	<version>1.4</version>
</dependency>
 
@Test
public void useCommonIO() throws IOException { 
	ClassLoader classLoader = getClass().getClassLoader();
	File file = new File(classLoader.getResource("fileTest.txt").getFile());
	String data = FileUtils.readFileToString(file, "UTF-8");
}

@Test
public void useCommonIO2() throws IOException {
	String expectedData = "Hello,World!";

	FileInputStream fis = new FileInputStream("src/test/resources/fileTest.txt");
	String data = IOUtils.toString(fis, "UTF-8");

	assertEquals(expectedData, data.trim());
}

BufferedReader

@Test
public void bufferedReader() throws IOException { 
    String file ="src/test/resources/fileTest.txt";
 
    BufferedReader reader = new BufferedReader(new FileReader(file));
    String currentLine = reader.readLine();
    reader.close(); 
}

Java NIO读取文件

    @Test
    public void readSmallFile()  throws IOException {  
        Path path = Paths.get("src/test/resources/fileTest.txt");
 
        String read = Files.readAllLines(path).get(0); 
    }

读取大文件

 
    @Test
    public void readLargeFile() throws IOException { 
        Path path = Paths.get("src/test/resources/fileTest.txt");
 
        BufferedReader reader = Files.newBufferedReader(path);
        String line = reader.readLine(); 
    }
	

Files.lines

在JDK8中
  Files类增加了lines方法
   这个方法将返回Stream<String>
    跟文件操作一样
    Stream需要显式调用的close()

Scanner

下面我们将使用Scanner读取文件，使用逗号(,)作为定界符(delimiter)。


@Test
public void whenReadWithScanner_thenCorrect()
        throws IOException {
    String file = "src/test/resources/fileTest.txt";
    Scanner scanner = new Scanner(new File(file));
    scanner.useDelimiter(",");
 
    System.out.Println(scanner.hasNext()); 
 
    scanner.close();
}
Scanner默认的定界符是空格 

StreamTokenizer

tokenizer会指出下一个token的类型，String或Number。

tokenizer.nval - 如果类型为Number时，读取该字段

tokenizer.sval - 如果类型为String时，读取该字段

@Test
public void readWithTokenize()
        throws IOException {
    String file = "src/test/resources/fileTestTokenizer.txt";
    FileReader reader = new FileReader(file);
    StreamTokenizer tokenizer = new StreamTokenizer(reader);
 
    //  1
    tokenizer.nextToken();
    assertEquals(StreamTokenizer.TT_WORD, tokenizer.ttype);
    assertEquals("Hello", tokenizer.sval);
 
    //  2
    tokenizer.nextToken();
    assertEquals(StreamTokenizer.TT_NUMBER, tokenizer.ttype);
    assertEquals(1, tokenizer.nval, 0.0000001);
 
    //  3
    tokenizer.nextToken();
    assertEquals(StreamTokenizer.TT_EOF, tokenizer.ttype);
    reader.close();
}

DataInputStream

使用DataInputStream

@Test
public void whenReadWithDataInputStream() throws IOException { 
    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);
    }
}

FileChannel

注意事项:
    FileChannel运行速度会超过standard IO

@Test
public void whenReadWithFileChannel()
        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();
}

读取utf-8编码的文件

@Test
public void whenReadUTFEncodedFile()
        throws IOException { 
    String file = "src/test/resources/fileTestUtf8.txt";
    BufferedReader reader = new BufferedReader
            (new InputStreamReader(new FileInputStream(file), "UTF-8"));
    String currentLine = reader.readLine();
    reader.close(); 
}

从URL读取数据

@Test
public void readFromURL() throws IOException {
    URL urlObject = new URL("http://www.java265.com");
    URLConnection urlConnection = urlObject.openConnection();
    InputStream inputStream = urlConnection.getInputStream();
    String data = readFromInputStream(inputStream);
}

从jar包中读取文件

 
@Test
public void readFromJar() throws IOException { 
    Class clazz = Test.class;
    InputStream inputStream = clazz.getResourceAsStream("/test.txt");
    String data = readFromInputStream(inputStream); 
}