Java读取文件方式

骑猪看日落 2023-09-26 22:01 64阅读 0赞

## IO流读取 ##

### 文本内容 ###

![4a3389bf80384b8d9b4f5019b0d1c3f6.png][]

### 按行读取文件内容 ###

#### 指定编码格式（推荐） ####

public static void main(String[] args) throws UnsupportedEncodingException {
            read("D:\\test.txt");
    
        }
    
        public static void read(String path) {
            BufferedReader reader = null;
            try {
                reader = new BufferedReader(
                        new InputStreamReader(new FileInputStream(path), "UTF-8"));
                String line;
                while ((line = reader.readLine()) != null) {
                    System.out.println(line);
                }
    
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (reader != null) {
                    try {
                        reader.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
    
        }

![3bfce5d2bcf04508937df3798936ca41.png][]

**注意，如果控制台输出为乱码，需要度idea进行配置,依次选择****file-Setting-editor-File Encoding**

![e137055849204b57aec4dbe04e424a09.png][]

#### 不指定编码格式 ####

public static void main(String[] args) throws IOException {
            read("D:\\test.txt");
    
        }
    
        public static void read(String path) throws IOException {
            BufferedReader reader = new BufferedReader(new FileReader(path));
            String line;
            while ((line = reader.readLine()) != null)
            {
                System.out.println(line);
            }
            reader.close();
        }

可能出现乱码。

### 按字符读取文件内容 ###

public static void read(String path) throws IOException {
            Reader reader = new InputStreamReader(new FileInputStream(path));
            int tempchar;
            while ((tempchar = reader.read()) != -1) {
                System.out.print((char) tempchar);
            }
            reader.close();
        }

### 按字节读取文件内容 ###

常用于读取图片，声音，影像等

#### 单字节读取 ####

public static void read(String path) throws IOException {
            FileInputStream reader = new FileInputStream(path);
            int tempchar;
            while ((tempchar = reader.read()) != -1) {
                System.out.print((char) tempchar);
            }
            reader.close();
        }

#### 多字节读取 ####

public static void read(String path) throws IOException {
            FileInputStream reader = new FileInputStream(path);
            byte[] tempByte = new byte[1024];
            int len = 0 ;
            while((len = reader.read(tempByte))!= -1){
                for (int i = 0; i <len; i++) {
                    System.out.print((char)tempByte[i]);
                }
            }
            reader.close();
        }

## Scanner ##

第一种方式是Scanner，从JDK1.5开始提供的API，特点是可以按行读取、按分割符去读取文件数据，既可以读取String类型，也可以读取Int类型、Long类型等基础数据类型的数据。

public static void read(String path) throws IOException {
            try (Scanner sc = new Scanner(new FileReader(path))) {
                while (sc.hasNextLine()) {  //按行读取字符串
                    String line = sc.nextLine();
                    System.out.println(line);
                }
            }
        }

## JDK1.7提供的NIO读取文件 ##

### 小文件 ###

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
    
            List<String> content = new ArrayList<>(0);
    
            try {
                content = Files.readAllLines(Paths.get(path), Charset.forName(CHARSET_NAME));
            } catch (Exception e) {
                e.printStackTrace();
            }
    
           content.forEach(System.out::println);
    
        }

### 大文件 ###

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
    
            List<String> content = new ArrayList<>(0);
    
    
            try (BufferedReader br = Files.newBufferedReader(Paths.get(path), Charset.forName(CHARSET_NAME))) {
                String line;
                while ((line = br.readLine()) != null) {
                    content.add(line);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
    
           content.forEach(System.out::println);
    
        }

## JDK1.4提供的NIO读取文件（适用于超大文件） ##

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
    
            final int ASCII_LF = 10; // 换行符
            final int ASCII_CR = 13; // 回车符
    
            List<String> content = new ArrayList<>();
    
            try (FileChannel fileChannel = new RandomAccessFile(path, "r").getChannel()) {
                ByteBuffer byteBuffer = ByteBuffer.allocate(1024 * 100);
                byte[] lineByte;
    
                byte[] temp = new byte[0];
    
                while (fileChannel.read(byteBuffer) != -1) {
                    // 获取缓冲区位置，即读取长度
                    int readSize = byteBuffer.position();
    
                    // 将读取位置置0，并将读取位置标为废弃
                    byteBuffer.rewind();
    
                    // 读取内容
                    byte[] readByte = new byte[readSize];
                    byteBuffer.get(readByte);
    
                    // 清除缓存区
                    byteBuffer.clear();
    
                    // 读取内容是否包含一整行
                    boolean hasLF = false;
    
                    int startNum = 0;
    
                    for (int i = 0; i < readSize; i++) {
                        if (readByte[i] == ASCII_LF) {
                            hasLF = true;
                            int tempNum = temp.length;
                            int lineNum = i - startNum;
    
                            // 数组大小已经去掉换行符
                            lineByte = new byte[tempNum + lineNum];
                            System.arraycopy(temp, 0, lineByte, 0, tempNum);
                            temp = new byte[0];
                            System.arraycopy(readByte, startNum, lineByte, tempNum, lineNum);
    
                            String line = new String(lineByte, 0, lineByte.length, CHARSET_NAME);
    
                            content.add(line);
    
                            // 过滤回车符和换行符
                            if (i + 1 < readSize && readByte[i + 1] == ASCII_CR) {
                                startNum = i + 2;
                            } else {
                                startNum = i + 1;
                            }
                        }
                    }
                    if (hasLF) {
                        temp = new byte[readByte.length - startNum];
                        System.arraycopy(readByte, startNum, temp, 0, temp.length);
                    } else {
                        // 单次读取的内容不足一行的情况
                        byte[] toTemp = new byte[temp.length + readByte.length];
                        System.arraycopy(temp, 0, toTemp, 0, temp.length);
                        System.arraycopy(readByte, 0, toTemp, temp.length, readByte.length);
                        temp = toTemp;
                    }
                }
    
                // 最后一行
                if (temp.length > 0) {
                    String lastLine = new String(temp, 0, temp.length, CHARSET_NAME);
    
                    content.add(lastLine);
                }
            } catch (Exception e) {
                e.printStackTrace();
            }
           content.forEach(System.out::println);
    
        }

## JDK1.7 Files.readAllBytes ##

public static void read(String path) throws IOException {
    
            byte[] bytes = Files.readAllBytes(Paths.get(path));
    
            String content = new String(bytes, StandardCharsets.UTF_8);
            System.out.println(content);
        }

## JDK1.8 Stream流 ##

可能会出现内存溢出问题 java.lang.OutOfMemoryError

### Files.lines ###

public static void read(String path) throws IOException {
            // 读取文件内容到Stream流中，按行读取
            Stream<String> lines = Files.lines(Paths.get(path));
    
            // 随机行顺序进行数据处理
            lines.forEach(System.out::println);
        }

forEach获取Stream流中的行数据不能保证顺序，但速度快。如果你想按顺序去处理文件中的行数据，可以使用forEachOrdered，但处理效率会下降。

lines.forEachOrdered(System.out::println);

或者利用CPU多和的能力，进行数据的并行处理parallel()，适合比较大的文件

lines.parallel().forEachOrdered(System.out::println);

### Files.readAllLines ###

public static void read(String path) throws IOException {
        // 读取文件内容到Stream流中，按行读取
        List<String> lines =  Files.readAllLines(Paths.get(path),
                StandardCharsets.UTF_8);
    
        // 随机行顺序进行数据处理
        lines.forEach(System.out::println);
    }

## JDK 11 Files.readString() ##

文件不能超过2G，同时要注意你的服务器及JVM内存。这种方法适合快速读取小文本文件。

System.out.println(Files.readString(Paths.get(path)));

**依赖hutool**

### IoUtil工具类 ###

<dependency>
        <groupId>cn.hutool</groupId>
        <artifactId>hutool-core</artifactId>
        <version>5.8.10</version>
    </dependency>
    
    或者：
    <dependency>
        <groupId>cn.hutool</groupId>
        <artifactId>hutool-all</artifactId>
        <version>5.8.10</version>
    </dependency>

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
    
    
            List<String> content = new ArrayList<>();
    
            try {
                IoUtil.readLines(new FileInputStream(path), CharsetUtil.charset(CHARSET_NAME), content);
            } catch (Exception e) {
                e.printStackTrace();
            }
           content.forEach(System.out::println);
    
        }

### FileUtil工具类 ###

public static void fileOfHutool() {
        
            final String CHARSET_NAME = "UTF-8";
    
            List<String> content = FileUtil.readLines(path, CHARSET_NAME);
    
            content.forEach(System.out::println);
          
        }

## 依赖cmmons-io ##

<dependency>
        <groupId>commons-io</groupId>
        <artifactId>commons-io</artifactId>
        <version>2.11.0</version>
    </dependency>

### FileUtils工具类 ###

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
            List<String> content = new ArrayList<>(0);
    
            try {
                content = FileUtils.readLines(new File(fileName), CHARSET_NAME);
            } catch (Exception e) {
                e.printStackTrace();
            }
           content.forEach(System.out::println);
    
        }

### IOUtils工具类 ###

public static void read(String path) throws IOException {
            final String CHARSET_NAME = "UTF-8";
            List<String> content = new ArrayList<>(0);
            try {
                content = IOUtils.readLines(new FileInputStream(fileName), CHARSET_NAME);
            } catch (Exception e) {
                e.printStackTrace();
            }
           content.forEach(System.out::println);
    
        }

[4a3389bf80384b8d9b4f5019b0d1c3f6.png]: https://img-blog.csdnimg.cn/4a3389bf80384b8d9b4f5019b0d1c3f6.png
[3bfce5d2bcf04508937df3798936ca41.png]: https://img-blog.csdnimg.cn/3bfce5d2bcf04508937df3798936ca41.png
[e137055849204b57aec4dbe04e424a09.png]: https://img-blog.csdnimg.cn/e137055849204b57aec4dbe04e424a09.png