java io 面试要点

// 字节流读文本，需手动解码
InputStream in = new FileInputStream("a.txt");
byte[] buf = new byte[1024];
int len = in.read(buf);
String s = new String(buf, 0, len, StandardCharsets.UTF_8);

Reader reader = new InputStreamReader(
    new FileInputStream("a.txt"), StandardCharsets.UTF_8);

// 字节流 → 字符流
Reader r = new InputStreamReader(
    new FileInputStream("a.txt"), "UTF-8");

// 字符流 → 字节流
Writer w = new OutputStreamWriter(
    new FileOutputStream("a.txt"), "UTF-8");

// 不推荐，编码依赖系统
FileReader fr = new FileReader("a.txt");

// 推荐，显式指定编码
Reader r = new InputStreamReader(
    new FileInputStream("a.txt"), StandardCharsets.UTF_8);

// 无缓冲：每次 read 都可能触发系统调用
InputStream in = new FileInputStream("a.txt");

// 有缓冲：一次读 8KB，减少 IO 次数
InputStream in = new BufferedInputStream(
    new FileInputStream("a.txt"), 8192);

Writer w = new BufferedWriter(new FileWriter("a.txt"));
w.write("hello");
w.flush();  // 立即写入，不关闭
w.write("world");
w.close();  // 关闭并释放

public static String readLine(InputStream in) throws IOException {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    int b;
    while ((b = in.read()) != -1) {
        if (b == '\n') break;
        if (b != '\r') bos.write(b);
    }
    return bos.size() == 0 && b == -1 ? null 
        : bos.toString("UTF-8");
}

ServerSocket server = new ServerSocket(8080);
while (true) {
    Socket socket = server.accept(); // 阻塞
    new Thread(() -> {
        InputStream in = socket.getInputStream();
        byte[] buf = new byte[1024];
        in.read(buf); // 阻塞
        // 处理...
    }).start();
}

Selector selector = Selector.open();
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.configureBlocking(false);
ssc.register(selector, SelectionKey.OP_ACCEPT);

while (true) {
    selector.select(); // 阻塞直到有事件
    Set<SelectionKey> keys = selector.selectedKeys();
    // 处理事件...
}

// AIO 示例
AsynchronousSocketChannel channel = 
    AsynchronousSocketChannel.open();
channel.read(buffer, null, new CompletionHandler<Integer, Object>() {
    @Override
    public void completed(Integer result, Object attachment) {
        // 读完成后回调
    }
    @Override
    public void failed(Throwable exc, Object attachment) {}
});

// Stream：单向
InputStream in = ...;  // 只能读
OutputStream out = ...; // 只能写

// Channel：双向
SocketChannel ch = ...;
ch.read(buffer);
ch.write(buffer);

ByteBuffer buf = ByteBuffer.allocate(1024);
buf.put("hello".getBytes()); // position=5

// 不 flip 直接读，position=5，读不到数据
buf.flip(); // limit=5, position=0
while (buf.hasRemaining()) {
    System.out.print((char) buf.get());
}

ByteBuffer heap = ByteBuffer.allocate(1024);      // 堆
ByteBuffer direct = ByteBuffer.allocateDirect(1024); // 堆外

ssc.register(selector, SelectionKey.OP_ACCEPT);
while (true) {
    selector.select();
    Iterator<SelectionKey> it = 
        selector.selectedKeys().iterator();
    while (it.hasNext()) {
        SelectionKey key = it.next();
        if (key.isAcceptable()) { /* 处理连接 */ }
        if (key.isReadable())  { /* 处理读 */ }
        it.remove(); // 必须移除
    }
}

Set<SelectionKey> keys = selector.selectedKeys();
Iterator<SelectionKey> it = keys.iterator();
while (it.hasNext()) {
    SelectionKey key = it.next();
    // 处理...
    it.remove(); // 关键
}

磁盘 → 内核缓冲区 → 用户缓冲区 → Socket 缓冲区 → 网卡
        (DMA)      (CPU拷贝)      (CPU拷贝)     (DMA)

// 传统方式
byte[] buf = new byte[1024];
while ((len = fileInputStream.read(buf)) != -1) {
    socketOutputStream.write(buf, 0, len);
}

MappedByteBuffer buf = 
    fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, size);
socketChannel.write(buf);

// Java 中通过 FileChannel.transferTo 使用
fileChannel.transferTo(position, count, socketChannel);

磁盘 → 内核页缓存 → Socket 缓冲区 → 网卡
       (DMA)        (CPU拷贝)      (DMA)

磁盘 → 内核页缓存 → 网卡
       (DMA)      (SG-DMA)

// 服务端发送文件
FileChannel fileChannel = new FileInputStream("a.txt").getChannel();
SocketChannel socketChannel = SocketChannel.open(
    new InetSocketAddress("localhost", 8080));

// 零拷贝发送
fileChannel.transferTo(0, fileChannel.size(), socketChannel);

// Netty 零拷贝发送文件
FileRegion region = new DefaultFileRegion(
    new FileInputStream("a.txt").getChannel(), 0, size);
ctx.writeAndFlush(region);