Java读取文件的N种方法

admin 2025-10-11 23:41:01

1.概述 在这篇文章里， 我们将探索不同的方式从文件中读取数据。

首先， 学习通过标准的的Java类，从classpath、URL或者Jar中加载文件。

然后，学习通用BufferedReader, Scanner, StreamTokenizer, DataInputStream, SequenceInputStream, FileChannel读取文件内容。也会讨论如何读取UTF-8编码的文件。

最后，学习Java7和Java8中新的加载和读取文件的技术。

2.准备 2.1 输入文件

这篇文章的很多示例，从名为fileTest.txt的文件读取文本内容，文件包含

Hello,World! 有少量示例， 我们会读取不同的文件， 示例中会有说明。

2.2 辅助方法

很多示例都会用到共用的方法readFromInputStream， 该方法将InputStream转化String

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();

}

3.从Classpath读取文件 3.1 使用标准Java

从src/main/resources读取文件fileTest.txt

@Test

public void test() throws IOException {

String expectedData = "Hello,World!";

Class clazz = ReadFileTest.class;

InputStream inputStream = clazz.getResourceAsStream("/fileTest.txt");

String data = readFromInputStream(inputStream);

Assert.assertThat(data, containsString(expectedData));

}

在上面的代码中，我们通过当前类的getResourceAsStream方法加载文件，入参是绝对路径。

ClassLoader中相同的方法也可以使用。

ClassLoader classLoader = getClass().getClassLoader(); InputStream inputStream = classLoader.getResourceAsStream("fileTest.txt"); String data = readFromInputStream(inputStream); 这两种方法的主要区别是， 当前类的ClassLoader的getResourceAsStream方法，入参路径是从classpath开始。

而类实例的入参是相对于包路径，但路径开始使用'/'符号， 也是绝对路径。

特别要注意的是， 文件打开读取完数据后， 始终需要关闭

inputStream.close(); 3.2 使用commons-io库

另一个比较常用的方法是使用commons-io包里的FileUtils.readFileToString方法。

commons-io

commons-io

1.4

@Test

public void useCommonIO() throws IOException {

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());

}

该方法入参是File对象。这个工具类的优点是不用编码InputStream实例的相关代码。 这个库还提供了IOUtils类。

@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());

}

4.BufferedReader

@Test

public void bufferedReader() 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);

}

当读取结束的时候， reader.readLine()会返回null

5.Java NIO NIO是在JDK7中添加。

5.1读取小文件

首先看一下关于Files.readAllLines的示例

@Test

public void readSmallFile() 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);

}

入参Path对象，Path可以认为是java.io.File的升级版本，提供一些额外的功能。

如果读取的是二进制文件，可以使用Files.readAllBytes()方法

5.2读取大文件

如果想要读取大文件， 我们可以使用Files类和BufferedReader类。

@Test

public void readLargeFile() 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);

}

5.3Files.lines

在JDK8中，Files类增加了lines方法，这个方法将返回Stream。跟文件操作一样，Stream需要显式调用的close()。Files API提供了很多简单读取文件的方法。

6.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(",");

assertTrue(scanner.hasNext());

assertEquals("Hello", scanner.next());

assertEquals("World!", scanner.next());

scanner.close();

}

Scanner默认的定界符是空格。它适用于从控制台读取输入或者内容有固定定界符的内容时。

7.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();

}

8.DataInputStream 如果要读取二进制文件或者原生数据，可以使用DataInputStream

@Test

public void whenReadWithDataInputStream() 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);

}

package com.pl;

import java.io.DataInputStream;

import java.io.FileInputStream;

import java.io.IOException;

public class ccs_2 {

public static void main(String[] args) throws IOException {

String expectedValue = "";

String file = "C:\\Users\\Administrator\\Desktop\\1.txt";

//result = null;

DataInputStream reader = new DataInputStream(new FileInputStream(file));

int nBytesToRead = reader.available();

if (nBytesToRead > 0) {

byte[] bytes = new byte[nBytesToRead];

reader.read(bytes);

expectedValue = new String(bytes);

}

System.out.printf("\n====" + expectedValue);

}

}

9.FileChannel 如果读取的是一个大文件，FileChannel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();

}

package com.pl;

import java.io.IOException;

import java.io.RandomAccessFile;

import java.nio.ByteBuffer;

import java.nio.channels.FileChannel;

public class ccs_2 {

public static void main(String[] args) throws IOException {

//throws IOException {

String expected_value = "";

String file = "C:\\Users\\Administrator\\Desktop\\1.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();

expected_value = new String(buff.array());

//assertEquals(expected_value, new String(buff.array()));

channel.close();

reader.close();

System.out.printf("\n====" + expected_value);

}

}

10.读取utf-8编码的文件

@Test

public void whenReadUTFEncodedFile()

throws IOException {

String expected_value = "你好，世界！";

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);

}

package com.pl;

import java.io.BufferedReader;

import java.io.FileInputStream;

import java.io.IOException;

import java.io.InputStreamReader;

public class ccs_2 {

public static void main(String[] args) throws IOException {

// File file = new File("C:\\Program Files");

//

// //System.out.println(file.list());

// String[] A = file.list();

// for (int i = 0; i < A.length; i++) {

// System.out.printf("\n=====" + A[i]);

// }

String expected_value = "你好，世界！";

String file = "C:\\Users\\Administrator\\Desktop\\1.txt";

BufferedReader reader = new BufferedReader

(new InputStreamReader(new FileInputStream(file), "UTF-8"));

String currentLine = reader.readLine();

reader.close();

System.out.printf("\n====" + currentLine);

}

}

11.从URL读取数据

@Test

public void readFromURL() throws IOException {

URL urlObject = new URL("https://www.baidu.com");

URLConnection urlConnection = urlObject.openConnection();

InputStream inputStream = urlConnection.getInputStream();

String data = readFromInputStream(inputStream);

}

12.从jar包中读取文件 我们的目标是读取junit-4.12.jar包中的LICENSE-junit.txt文件。clazz只需要这个Jar中的类就行。

@Test

public void readFromJar() throws IOException {

String expectedData = "Eclipse Public License";

Class clazz = Test.class;

InputStream inputStream = clazz.getResourceAsStream("/LICENSE-junit.txt");

String data = readFromInputStream(inputStream);

Assert.assertThat(data, containsString(expectedData));

}

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