Java 文件讀取三種方式性能對比：標準 IO、緩衝 IO 與內存映射 (NIO)

Java 文件讀取三大模式代碼實現與性能分析

本文對比傳統阻塞 IO、帶緩衝 IO 及 NIO 內存映射三種讀取大文件的方式，附帶完整可運行代碼。

方式一：標準 IO + 自定義緩衝區 (FileChannel + ByteBuffer)

public static void readFile(String path) throws FileNotFoundException {
    long start = System.currentTimeMillis();
    int bufSize = 1024;
    FileChannel fcin = new RandomAccessFile(new File(path), "r").getChannel();
    ByteBuffer rBuffer = ByteBuffer.allocate(bufSize);
    byte[] bs = new byte[bufSize];
    while (fcin.read(rBuffer) != -1) {
        int rSize = rBuffer.position();
        rBuffer.rewind();
        rBuffer.get(bs);
        rBuffer.clear();
        String temp = new String(bs, 0, rSize);
        // 按行處理 temp...
    }
    System.out.println("傳統IO讀取，總耗時：" + (System.currentTimeMillis() - start) + "ms");
}

方式二：緩衝字符流 (BufferedReader)

public static void readFile1(String path) {
    long start = System.currentTimeMillis();
    BufferedReader br = new BufferedReader(new FileReader(new File(path)));
    String line;
    while ((line = br.readLine()) != null) { System.out.println(line); }
    br.close();
    System.out.println("緩衝字符流讀取，總耗時：" + (System.currentTimeMillis() - start) + "ms");
}

方式三：NIO 內存映射 (MappedByteBuffer)

public static void readFile3(String path) {
    long start = System.currentTimeMillis();
    File file = new File(path);
    long fileLength = file.length();
    final int BUFFER_SIZE = 0x300000; // 3MB
    MappedByteBuffer inputBuffer = new RandomAccessFile(file, "r").getChannel()
        .map(FileChannel.MapMode.READ_ONLY, 0, fileLength);
    byte[] dst = new byte[BUFFER_SIZE];
    for (int offset = 0; offset < fileLength; offset += BUFFER_SIZE) {
        int len = Math.min(BUFFER_SIZE, (int)(fileLength - offset));
        for (int i = 0; i < len; i++) dst[i] = inputBuffer.get(offset + i);
        Scanner scan = new Scanner(new ByteArrayInputStream(dst)).useDelimiter("XXX");
        while (scan.hasNext()) System.out.print(scan.next() + "XXX");
        scan.close();
    }
    System.out.println("NIO 內存映射讀取，總耗時：" + (System.currentTimeMillis() - start) + "ms");
}

性能與適用場景總結

內容來源：《磁盤 IO 工作機制》微信訂閱號：it_pupil。