Java IO 流操作大全：文件讀寫的四種核心方式與性能對比

前言：IO 流體系的核心分類

Java IO 按處理單位分為字節流（InputStream/OutputStream）與字符流（Reader/Writer）；按功能分為節點流（直接操作目標）與處理流（包裝增強）。本文通過完整代碼示例，系統總結文件讀寫的四種核心模式並對比性能。

一、文件讀取的四種方式

1. 字節流單字節讀取（適合二進制文件）

public static void readByBytes(String filename) {
    try (FileInputStream in = new FileInputStream(filename)) {
        int b;
        while ((b = in.read()) != -1) { System.out.print((char)b); }
    } catch (IOException e) { e.printStackTrace(); }
}

2. 字節流批量讀取（緩衝區，高性能）

public static void readByBytesBuffer(String filename) {
    try (FileInputStream in = new FileInputStream(filename)) {
        byte[] buf = new byte[1024];
        int len;
        while ((len = in.read(buf)) != -1) { System.out.write(buf, 0, len); }
    } catch (IOException e) { e.printStackTrace(); }
}

3. 字符流讀取（文本文件，自動編碼轉換）

public static void readByChars(String filename) {
    try (InputStreamReader reader = new InputStreamReader(new FileInputStream(filename), "UTF-8")) {
        char[] buf = new char[1024];
        int len;
        while ((len = reader.read(buf)) != -1) { System.out.print(new String(buf, 0, len)); }
    } catch (IOException e) { e.printStackTrace(); }
}

4. 行讀取（BufferedReader，處理格式化文本）

public static void readByLines(String filename) {
    try (BufferedReader br = new BufferedReader(new FileReader(filename))) {
        String line;
        while ((line = br.readLine()) != null) { System.out.println(line); }
    } catch (IOException e) { e.printStackTrace(); }
}

5. 隨機訪問

public static void readRandomAccess(String filename) {
    try (RandomAccessFile raf = new RandomAccessFile(filename, "r")) {
        raf.seek(100); // 定位
        byte[] buf = new byte[10];
        raf.read(buf);
        System.out.write(buf);
    } catch (IOException e) { e.printStackTrace(); }
}

二、文件寫入的三種方式與性能測試

public static void writePerformanceTest() throws IOException {
    String path = "E:/test/";
    int times = 10000;
    // 1. FileOutputStream 直接寫
    long start = System.currentTimeMillis();
    try (FileOutputStream fos = new FileOutputStream(path + "fos.txt")) {
        for (int i = 0; i < times; i++) fos.write('a');
    }
    System.out.println("FileOutputStream: " + (System.currentTimeMillis() - start) + "ms");

    // 2. BufferedWriter 緩衝寫入
    start = System.currentTimeMillis();
    try (BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(path + "bw.txt"), "UTF-8"))) {
        for (int i = 0; i < times; i++) { bw.write('a'); bw.newLine(); }
    }
    System.out.println("BufferedWriter: " + (System.currentTimeMillis() - start) + "ms");

    // 3. FileWriter 便捷寫入
    start = System.currentTimeMillis();
    try (FileWriter fw = new FileWriter(path + "fw.txt")) {
        for (int i = 0; i < times; i++) fw.write('a');
    }
    System.out.println("FileWriter: " + (System.currentTimeMillis() - start) + "ms");
}

三、性能結論與最佳實踐

• 讀取：優先使用 BufferedReader（文本）或 BufferedInputStream（二進制）。
• 寫入：大量數據必須用 BufferedWriter 或 BufferedOutputStream。
• 編碼：字符流操作時顯式指定 Charset（如 UTF-8），避免平台默認編碼導致亂碼。
• 資源管理：統一使用 try-with-resources（JDK 7+）自動關閉流。