Java –如何读取文件

墨蓝 2023-02-15 05:05 19阅读 0赞

在Java中，几乎没有读取文件的方法。

1. Java 8 – `Files.lines` ，它将返回一个`Stream`

//@Since 1.8
    
    	Stream<String> lines = Files.lines(Paths.get("app.log"));
    
    	List<String> content = lines.collect(Collectors.toList());

2. Java 7 – `Files.readAllBytes`或`Files.readAllLines`

//@Since 1.7
    	
    	// Returns a byte[]
    	byte[] bytes = Files.readAllBytes(Paths.get("app.log"));
    	String content = new String(bytes);
    	
    	// Returns a List String
    	List<String> content = Files.readAllLines(Paths.get("app.log"));

3.和这个经典的`BufferedReader`

try (FileReader reader = new FileReader("app.log");
    		BufferedReader br = new BufferedReader(reader)) {
    
    		// read line by line
    		String line;
    		while ((line = br.readLine()) != null) {
    			System.out.println(line);
    		}
    
         } catch (IOException e) {
                //e
         }

## 1. Files.lines ##

在Java 8中，我们可以使用`Files.lines`将文件读入`Stream` ，它将自动关闭资源（打开的文件）。

app.log

Line 1
    Line 2
    Line 3
    Line 4
    Line 5

@ FileExample1.java

package com.mkyong;
    
    import java.io.IOException;
    import java.nio.file.Files;
    import java.nio.file.Paths;
    import java.util.List;
    import java.util.stream.Collectors;
    import java.util.stream.Stream;
    
    public class FileExample1 {
    
        public static void main(String[] args) {
    
            try {
    
                Stream<String> lines = Files.lines(Paths.get("app.log"));
                List<String> content = lines.collect(Collectors.toList());
                content.forEach(x -> System.out.println(x));
    
            } catch (IOException e) {
                System.err.format("IOException: %s%n", e);
            }
        }
    
    }

Terminal

Line 1
    Line 2
    Line 3
    Line 4
    Line 5

## 2. Files.readAllBytes ##

在Java 7中，我们可以使用`Files.readAllBytes`或`Files.readAllLines`读取文件，它将自动关闭资源（打开的文件）。

FileExample2.java

package com.mkyong;
    
    import java.io.IOException;
    import java.nio.file.Files;
    import java.nio.file.Paths;
    
    public class FileExample2 {
    
        public static void main(String[] args) {
    
            try {
    
                byte[] bytes = Files.readAllBytes(Paths.get("app.log"));
                String content = new String(bytes);
    
                System.out.println(content);
    
            } catch (IOException e) {
                System.err.format("IOException: %s%n", e);
            }
        }
    
    }

输出量

Terminal

Line 1
    Line 2
    Line 3
    Line 4
    Line 5

## 3. BufferedReader ##

3.1一个经典的`BufferedReader`带有`try-with-resources`可以自动关闭资源。

FileExample3.java

package com.mkyong;
    
    import java.io.BufferedReader;
    import java.io.FileReader;
    import java.io.IOException;
    
    public class FileExample3 {
    
        public static void main(String[] args) {
    
            try (FileReader reader = new FileReader("app.log");
                 BufferedReader br = new BufferedReader(reader)) {
    
                // read line by line
                String line;
                while ((line = br.readLine()) != null) {
                    System.out.println(line);
                }
    
            } catch (IOException e) {
                System.err.format("IOException: %s%n", e);
            }
        }
    
    }

3.2在过去，我们必须手动关闭所有内容。

FileExample3.java

package com.mkyong.calculator;
    
    import java.io.BufferedReader;
    import java.io.FileReader;
    import java.io.IOException;
    
    public class FileExample3 {
    
        public static void main(String[] args) {
    
            BufferedReader br = null;
            FileReader fr = null;
    
            try {
    
                fr = new FileReader("app.log");
                br = new BufferedReader(fr);
    
                // read line by line
                String line;
                while ((line = br.readLine()) != null) {
                    System.out.println(line);
                }
    
            } catch (IOException e) {
                System.err.format("IOException: %s%n", e);
            } finally {
                try {
                    if (br != null)
                        br.close();
    
                    if (fr != null)
                        fr.close();
                } catch (IOException ex) {
                    System.err.format("IOException: %s%n", ex);
                }
            }
    
        }
    
    }

## 4.扫描仪 ##

让我们以经典的`Scanner`示例结束本文。

FileExample4.java

package com.mkyong;
    
    import java.io.FileReader;
    import java.io.IOException;
    import java.util.Scanner;
    
    public class FileExample4 {
    
        public static void main(String[] args) {
    
            try (FileReader fr = new FileReader("app.log");
                 Scanner scanner = new Scanner(fr)) {
    
                StringBuilder sb = new StringBuilder();
                while (scanner.hasNext()) {
                    sb.append(scanner.nextLine()).append("\n");
                }
    
                System.out.println(sb.toString());
    
            } catch (IOException e) {
                System.err.format("IOException: %s%n", e);
            }
    
        }
    
    }

## 参考文献 ##

*  [文件JavaDocs][JavaDocs]
 *  [BufferedReader JavaDocs][]
 *  [如何用Java读取文件– BufferedReader][Java_ BufferedReader]
 *  [扫描器JavaDocs][JavaDocs 1]
 *  [Java教程–读取，写入和创建文件][Java]

标签： [io][] [java][] [nio][] [读取文件][Link 1]

> 翻译自: [https://mkyong.com/java/java-how-to-read-a-file/][https_mkyong.com_java_java-how-to-read-a-file]

[JavaDocs]: https://docs.oracle.com/javase/8/docs/api/java/nio/file/Files.html
[BufferedReader JavaDocs]: https://docs.oracle.com/javase/8/docs/api/java/io/BufferedReader.html
[Java_ BufferedReader]: https://www.mkyong.com/java/how-to-read-file-from-java-bufferedreader-example/
[JavaDocs 1]: https://docs.oracle.com/javase/8/docs/api/java/util/Scanner.html
[Java]: https://docs.oracle.com/javase/tutorial/essential/io/file.html
[io]: https://mkyong.com/tag/io/
[java]: https://mkyong.com/tag/java/
[nio]: https://mkyong.com/tag/nio/
[Link 1]: https://mkyong.com/tag/read-file/
[https_mkyong.com_java_java-how-to-read-a-file]: https://mkyong.com/java/java-how-to-read-a-file/