Java IO 流體系全景解析：字節流、字符流、緩衝流與掃描器實戰

流的分類體系

核心代碼實戰

1. 字節流複製文件 (圖片/視頻等二進制)

public static void copyByByteStream(File src, File dest) throws IOException {
    try (FileInputStream in = new FileInputStream(src);
         FileOutputStream out = new FileOutputStream(dest)) {
        byte[] buffer = new byte[8192];
        int len;
        while ((len = in.read(buffer)) != -1) {
            out.write(buffer, 0, len);
        }
    }
}

2. 字符流處理文本文件

public static void writeText(File file, String content, boolean append) throws IOException {
    try (FileWriter writer = new FileWriter(file, append)) {
        writer.write(content);
    }
}

public static String readText(File file) throws IOException {
    StringBuilder sb = new StringBuilder();
    try (FileReader reader = new FileReader(file)) {
        char[] buffer = new char[1024];
        int len;
        while ((len = reader.read(buffer)) != -1) {
            sb.append(buffer, 0, len);
        }
    }
    return sb.toString();
}

3. 緩衝流大幅提升性能 (必用)

// 寫入
try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) {
    bw.write("高效寫入");
    bw.newLine(); // 跨平台換行
}

// 讀取逐行
try (BufferedReader br = new BufferedReader(new FileReader(file))) {
    String line;
    while ((line = br.readLine()) != null) {
        process(line);
    }
}

4. Scanner 靈活解析 (推薦文本讀取)

public static void parseWithScanner(File file) throws FileNotFoundException {
    try (Scanner scanner = new Scanner(file, StandardCharsets.UTF_8.name())) {
        // 按行讀取
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine();
        }
        // 按正則分隔符解析
        scanner.useDelimiter("\\s*,\\s*");
        while (scanner.hasNext()) {
            String token = scanner.next();
        }
    }
}

轉換流：字節與字符橋梁

// 字節流 → 字符流 (指定編碼)
Reader reader = new InputStreamReader(inputStream, "UTF-8");
Writer writer = new OutputStreamWriter(outputStream, "GBK");

// 實際應用：網絡流轉字符處理
try (BufferedReader br = new BufferedReader(
        new InputStreamReader(socket.getInputStream(), StandardCharsets.UTF_8))) {
    String response = br.readLine();
}

特殊流應用場景

NIO.2 現代文件操作 (JDK 7+)

// 文件讀寫
List<String> lines = Files.readAllLines(Paths.get("file.txt"), StandardCharsets.UTF_8);
Files.write(Paths.get("out.txt"), lines, StandardCharsets.UTF_8, StandardOpenOption.CREATE);

// 大文件流式處理
try (Stream<String> stream = Files.lines(Paths.get("big.log"))) {
    stream.filter(l -> l.contains("ERROR")).forEach(System.out::println);
}

// 文件屬性操作
Path path = Paths.get("data.txt");
Files.getLastModifiedTime(path);
Files.setPosixFilePermissions(path, PosixFilePermissions.fromString("rw-r--r--"));

最佳實踐總結

1. 二進制文件：字節流 + 緩衝 (BufferedInputStream/OutputStream)
2. 文本文件：字符流 + 緩衝 (BufferedReader/Writer) 或 Scanner
3. 必須指定編碼：構造 InputStreamReader/OutputStreamWriter 時顯式傳 charset
4. try-with-resources：自動關閉，防止洩漏
5. 大文件流式處理：避免一次性讀入內存，使用 Files.lines() 或 BufferedReader 循環
6. 性能關鍵：緩衝區大小 8KB-64KB，減少系統調用

總結

Java IO 體系雖龐大，但掌握「字節/字符、輸入/輸出、節點/處理」三個維度分類，熟練使用緩衝流、轉換流、Scanner、NIO.2，即可應對 95% 文件與網絡數據處理場景。現代開發優先選擇 NIO.2 API，代碼更簡潔、功能更強大、異常處理更規範。