Java 文件複製性能大比拚：四種 I/O 方式實測與分析

測試環境

• 檔案：1.mp3 (約 100MB)
• JDK：1.8+
• OS：Windows 10 / SSD

四種實作方式

public class FileCopyBenchmark {

    // 1. 單字節讀寫 (最慢)
    public static void copyByte(File src, File dest) throws IOException {
        try (FileInputStream in = new FileInputStream(src);
             FileOutputStream out = new FileOutputStream(dest)) {
            int b;
            while ((b = in.read()) != -1) out.write(b);
        }
    }

    // 2. 字節陣列讀寫 (10KB 緩衝)
    public static void copyByteArray(File src, File dest) throws IOException {
        try (FileInputStream in = new FileInputStream(src);
             FileOutputStream out = new FileOutputStream(dest)) {
            byte[] buf = new byte[10 * 1024];
            int len;
            while ((len = in.read(buf)) != -1) out.write(buf, 0, len);
        }
    }

    // 3. 緩衝流單字節 (BufferedInputStream/OutputStream)
    public static void copyBufferedByte(File src, File dest) throws IOException {
        try (BufferedInputStream in = new BufferedInputStream(new FileInputStream(src));
             BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(dest))) {
            int b;
            while ((b = in.read()) != -1) out.write(b);
        }
    }

    // 4. 緩衝流 + 字節陣列 (推薦生產用)
    public static void copyBufferedArray(File src, File dest) throws IOException {
        try (FileInputStream in = new FileInputStream(src);
             BufferedOutputStream out = new BufferedOutputStream(new FileOutputStream(dest))) {
            byte[] buf = new byte[10 * 1024];
            int len;
            while ((len = in.read(buf)) != -1) out.write(buf, 0, len);
        }
    }
}

實測結果 (100MB 檔案)

核心結論

1. 絕對不要用單字節讀寫 (read()/write(int)) 處理大文件。
2. 字節陣列是性能關鍵：應用層緩衝減少 JNI/系統調用次數。
3. BufferedOutputStream 比 BufferedInputStream 更關鍵：聚合寫入減少磁碟 I/O。
4. 實務上推薦 NIO FileChannel.transferTo/transferFrom (零拷貝) 或 Files.copy() (JDK 7+)，性能更優且代碼最簡潔：

// 現代最佳實踐 (零拷貝，內核態直接傳輸)
Files.copy(src.toPath(), dest.toPath(), StandardCopyOption.REPLACE_EXISTING);

補充：NIO transferTo 實測

public static void copyNio(File src, File dest) throws IOException {
    try (FileChannel in = FileChannel.open(src.toPath(), StandardOpenOption.READ);
         FileChannel out = FileChannel.open(dest.toPath(), StandardOpenOption.WRITE, StandardOpenOption.CREATE)) {
        in.transferTo(0, in.size(), out);
    }
}

通常比 Buffered+Array 快 10%-30%，且 CPU 佔用極低。