在上一节，我们已经成功实现了一个简单的HTTP服务器，但是，好像和Servlet没啥关系，因为整个操作都是基于HttpExchange接口做的。

而Servlet处理HTTP的接口是基于HttpServletRequest和HttpServletResponse，前者负责读取HTTP请求，后者负责写入HTTP响应。

怎么把基于HttpExchange的操作转换为基于HttpServletRequest和HttpServletResponse？答案是使用Adapter模式。

首先我们定义HttpExchangeAdapter，它持有一个HttpExchange实例，并实现HttpExchangeRequest和HttpExchangeResponse接口：

interface HttpExchangeRequest {
  String getRequestMethod();
  URI getRequestURI();
}

interface HttpExchangeResponse {
  Headers getResponseHeaders();
  void sendResponseHeaders(int rCode, long responseLength) throws IOException;
  OutputStream getResponseBody();
}

public class HttpExchangeAdapter implements HttpExchangeRequest, HttpExchangeResponse {
  final HttpExchange exchange;

  public HttpExchangeAdapter(HttpExchange exchange) {
    this.exchange = exchange;
  }

  // 实现方法
  ...
}

紧接着我们编写HttpServletRequestImpl，它内部持有HttpServletRequest，并实现了HttpServletRequest接口：

public class HttpServletRequestImpl implements HttpServletRequest {
  final HttpExchangeRequest exchangeRequest;

  public HttpServletRequestImpl(HttpExchangeRequest exchangeRequest) {
    this.exchangeRequest = exchangeRequest;
  }

  // 实现方法
  ...
}

同理，编写HttpServletResponseImpl如下：

public class HttpServletResponseImpl implements HttpServletResponse {
  final HttpExchangeResponse exchangeResponse;

  public HttpServletResponseImpl(HttpExchangeResponse exchangeResponse) {
    this.exchangeResponse = exchangeResponse;
  }

  // 实现方法
  ...
}

用一个图表示从HttpExchange转换为HttpServletRequest和HttpServletResponse如下：

  ┌──────────────────────┐ ┌───────────────────────┐
  │ HttpServletRequest │ │ HttpServletResponse │
  └──────────────────────┘ └───────────────────────┘
        ▲            ▲
        │            │
  ┌──────────────────────┐ ┌───────────────────────┐
  │HttpServletRequestImpl│ │HttpServletResponseImpl│
┌──│- exchangeRequest   │ │- exchangeResponse ────┼──┐
│ └──────────────────────┘ └───────────────────────┘ │
│                           │
│ ┌──────────────────────┐ ┌───────────────────────┐ │
└─▶│ HttpExchangeRequest │ │ HttpExchangeResponse │◀─┘
  └──────────────────────┘ └───────────────────────┘
           ▲     ▲
           │     │
           │     │
         ┌───────────────────┐
         │HttpExchangeAdapter│  ┌────────────┐
         │- httpExchange ────┼──▶│HttpExchange│
         └───────────────────┘  └────────────┘

接下来我们改造处理HTTP请求的HttpHandler接口：

public class HttpConnector implements HttpHandler {
  @Override
  public void handle(HttpExchange exchange) throws IOException {
    var adapter = new HttpExchangeAdapter(exchange);
    var request = new HttpServletRequestImpl(adapter);
    var response = new HttpServletResponseImpl(adapter);
    process(request, response);
  }

  void process(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO
  }
}

在handle(HttpExchange)方法内部，我们创建的对象如下：

1. HttpExchangeAdapter实例：它内部引用了jdk.httpserver提供的HttpExchange实例；
2. HttpServletRequestImpl实例：它内部引用了HttpExchangeAdapter实例，但是转换为HttpExchangeRequest接口；
3. HttpServletResponseImpl实例：它内部引用了HttpExchangeAdapter实例，但是转换为HttpExchangeResponse接口。

所以实际上创建的实例只有3个。最后调用process(HttpServletRequest, HttpServletResponse)方法，这个方法内部就可以按照Servlet标准来处理HTTP请求了，因为方法参数是标准的Servlet接口：

void process(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  String name = request.getParameter("name");
  String html = "<h1>Hello, " + (name == null ? "world" : name) + ".</h1>";
  response.setContentType("text/html");
  PrintWriter pw = response.getWriter();
  pw.write(html);
  pw.close();
}

目前，我们仅实现了代码调用时用到的getParameter()、setContentType()和getWriter()这几个方法。如果补全HttpServletRequest和HttpServletResponse接口所有的方法定义，我们就得到了完整的HttpServletRequest和HttpServletResponse接口实现。

运行代码，在浏览器输入http://localhost:8080/?name=World，结果如下：

参考源码

可以从GitHub或Gitee下载源码。

GitHub

小结

为了实现Servlet服务器，我们必须把jdk.httpserver提供的输入输出HttpExchange转换为Servlet标准定义的HttpServletRequest和HttpServletResponse接口，转换方式是Adapter模式；

转换后的HttpExchangeAdapter类再用HttpExchangeRequest和HttpExchangeResponse把读取和写入功能分开，使得结构更加清晰。