Java 文件 IO 性能大比拼：Stream vs NIO ByteBuffer vs MappedByteBuffer

測試場景與代碼

對比三種文件讀寫方式性能，測試數據量：2000×2000 = 400 萬個整數。完整 JUnit 測試代碼如下：

1. 傳統 Stream (BufferedInput/OutputStream)

@Test
public void useStream() throws IOException {
    long start = System.currentTimeMillis();
    int num = 2000 * 2000;
    DataOutputStream out = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(FILE)));
    for (int i = 0; i < num; i++) out.writeInt(i);
    out.close();
    DataInputStream in = new DataInputStream(new BufferedInputStream(new FileInputStream(FILE)));
    try { while (true) in.readInt(); } catch (EOFException ignored) {}
    in.close();
    System.out.println("Stream ms: " + (System.currentTimeMillis() - start)); // ~1000ms
}

2. NIO ByteBuffer + FileChannel

@Test
public void useNioByteBuffer() throws IOException {
    long start = System.currentTimeMillis();
    int num = 2000 * 2000;
    FileChannel fcOut = new FileOutputStream(FILE).getChannel();
    ByteBuffer buf = ByteBuffer.allocate(num * 5);
    for (int i = 0; i < num; i++) buf.putInt(i);
    buf.flip();
    fcOut.write(buf);
    fcOut.close();
    FileChannel fcIn = new FileInputStream(FILE).getChannel();
    ByteBuffer bufIn = ByteBuffer.allocate(num * 5);
    fcIn.read(bufIn);
    fcIn.close();
    System.out.println("NIO ByteBuffer ms: " + (System.currentTimeMillis() - start)); // ~220ms
}

3. NIO MappedByteBuffer (記憶體映射)

@Test
public void useRandomAccess() throws IOException {
    long start = System.currentTimeMillis();
    int num = 2000 * 2000;
    RandomAccessFile raf = new RandomAccessFile(FILE, "rw");
    FileChannel fc = raf.getChannel();
    MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, num * 4);
    for (int i = 0; i < num; i++) mbb.putInt(i);
    fc.close();
    // 讀取驗證...
    System.out.println("MappedByteBuffer ms: " + (System.currentTimeMillis() - start)); // ~112ms
}

結論

MappedByteBuffer (112ms) > NIO ByteBuffer (220ms) > Stream (1000ms)。記憶體映射文件直接操作用戶空間，省去核心態拷貝，性能碾壓傳統 IO。未來文件讀寫優先考慮 MappedByteBuffer 或 NIO 通道方式。