okhttp详解4

接下来分析Transport中HttpTransport部分，主要分析数据发送和数据接收流程。

一、重要的类简述

1.HttpTransport包装版的数据请求通道，包含httpConnection成员变量，所有的数据操作都是通过httpConnection操作。

2.HttpConnection数据操作，包含souce和sink两个成员变量，由socket包装而成。HttpConnection操作数据通过source和sink处理。

二、流程分析

1.HttpTransport.java

public void writeRequestHeaders(Request request) throws IOException {
  httpEngine.writingRequestHeaders();
  String requestLine = RequestLine.get(
      request, httpEngine.getConnection().getRoute().getProxy().type());
  httpConnection.writeRequest(request.headers(), requestLine);
}

http请求由三部分组成，分别是：请求行、消息报头、请求正文。首先创建请求行，然后通过httpConnection发送请求头和请求行。

2.HttpConnection.java

/** Returns bytes of a request header for sending on an HTTP transport. */
public void writeRequest(Headers headers, String requestLine) throws IOException {
  if (state != STATE_IDLE) throw new IllegalStateException("state: " + state);
  sink.writeUtf8(requestLine).writeUtf8("\r\n");
  for (int i = 0, size = headers.size(); i < size; i ++) {
    sink.writeUtf8(headers.name(i))
        .writeUtf8(": ")
        .writeUtf8(headers.value(i))
        .writeUtf8("\r\n");
  }
  sink.writeUtf8("\r\n");
  state = STATE_OPEN_REQUEST_BODY;
}

这个部分很简单，就是按照http协议格式发送数据。

3. HttpTransport.java

   @Override public Sink createRequestBody(Request request, long contentLength) throws IOException {
       if ("chunked".equalsIgnoreCase(request.header("Transfer-Encoding"))) {
         // Stream a request body of unknown length.
         return httpConnection.newChunkedSink();
       }
   
       if (contentLength != -1) {
         // Stream a request body of a known length.
         return httpConnection.newFixedLengthSink(contentLength);
       }
   
       throw new IllegalStateException(
           "Cannot stream a request body without chunked encoding or a known content length!");
     }

   获取Request body Sink

4.HttpTransport.java

@Override public void finishRequest() throws IOException {
  httpConnection.flush();
}

5.HttpTransport.java

public void flush() throws IOException {
  sink.flush();
}

以上两部的目的是将之前写入sink的请求行、消息报头、请求正文发送出去。

6.HttpTransport.java

@Override public Response.Builder readResponseHeaders() throws IOException {
  return httpConnection.readResponse();
}

7.HttpTransport.java

public Response.Builder readResponse() throws IOException {
  if (state != STATE_OPEN_REQUEST_BODY && state != STATE_READ_RESPONSE_HEADERS) {
    throw new IllegalStateException("state: " + state);
  }

  try {
    while (true) {
      StatusLine statusLine = StatusLine.parse(source.readUtf8LineStrict());

      Response.Builder responseBuilder = new Response.Builder()
          .protocol(statusLine.protocol)
          .code(statusLine.code)
          .message(statusLine.message);

      Headers.Builder headersBuilder = new Headers.Builder();
      readHeaders(headersBuilder);
      headersBuilder.add(OkHeaders.SELECTED_PROTOCOL, statusLine.protocol.toString());
      responseBuilder.headers(headersBuilder.build());

      if (statusLine.code != HTTP_CONTINUE) {
        state = STATE_OPEN_RESPONSE_BODY;
        return responseBuilder;
      }
    }
  } catch (EOFException e) {
   ...
  }
}

读取响应头。

8.HttpTransport.java

@Override public ResponseBody openResponseBody(Response response) throws IOException {
  Source source = getTransferStream(response);
  return new RealResponseBody(response.headers(), Okio.buffer(source));
}

获取响应正文的数据。 getTransferStream这一步很关键，获取的是HttpConnection中的一个内部类Source。

9.HttpEngine.java

public Response getResponse() {
  if (userResponse == null) throw new IllegalStateException();
  return userResponse;
}

之前几步已经分析，最后请求的数据会赋值给userResponse，最后获取的时候直接返回userResponse。

10.HttpEngine.java

public void releaseConnection() throws IOException {
  if (transport != null && connection != null) {
    transport.releaseConnectionOnIdle();
  }
  connection = null;
}

11.HttpTransport.java

@Override public void releaseConnectionOnIdle() throws IOException {
  if (canReuseConnection()) {
    httpConnection.poolOnIdle();
  } else {
    httpConnection.closeOnIdle();
  }
}

释放资源，如果连接可重用则httpConnection.poolOnIdle()放置到连接池中，否则httpConnection.closeOnIdle()关闭连接。