Java读取文件的N种方法

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

   

@Testpublic 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));}

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。

       

<dependency><groupId>commons-io</groupId><artifactId>commons-io</artifactId><version>1.4</version></dependency>@Testpublic 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。

   

@Testpublic 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

   

@Testpublic 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。

   

@Testpublic 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<String>。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);//  1tokenizer.nextToken();assertEquals(StreamTokenizer.TT_WORD, tokenizer.ttype);assertEquals("Hello", tokenizer.sval);//  2tokenizer.nextToken();assertEquals(StreamTokenizer.TT_NUMBER, tokenizer.ttype);assertEquals(1, tokenizer.nval, 0.0000001);//  3tokenizer.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));
}