okhttp网络框架源码浅析(二)

okhttp网络框架源码浅析(一)

interceptors责任链

不清楚责任链设计模式，可以先了解一下责任链设计模式
RealInterceptorChain

public final class RealInterceptorChain implements Interceptor.Chain {
  private final List<Interceptor> interceptors;//拦截器列表
  private final StreamAllocation streamAllocation;
  private final HttpCodec httpCodec;
  private final Connection connection;
  private final int index;//职责链索引
  private final Request request;
  private int calls;

  public RealInterceptorChain(List<Interceptor> interceptors, StreamAllocation streamAllocation,
      HttpCodec httpCodec, Connection connection, int index, Request request) {
    this.interceptors = interceptors;
    this.connection = connection;
    this.streamAllocation = streamAllocation;
    this.httpCodec = httpCodec;
    this.index = index;
    this.request = request;
  }
  public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      Connection connection) throws IOException {
    if (index >= interceptors.size()) throw new AssertionError();

    calls++;

    // If we already have a stream, confirm that the incoming request will use it.
    if (this.httpCodec != null && !sameConnection(request.url())) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must retain the same host and port");
    }

    // If we already have a stream, confirm that this is the only call to chain.proceed().
    if (this.httpCodec != null && calls > 1) {
      throw new IllegalStateException("network interceptor " + interceptors.get(index - 1)
          + " must call proceed() exactly once");
    }

    // Call the next interceptor in the chain.
    //创建责任链，next责任链内部index值为1
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);
    //这个index值为0
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

    // Confirm that the next interceptor made its required call to chain.proceed().
    if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
      throw new IllegalStateException("network interceptor " + interceptor
          + " must call proceed() exactly once");
    }

    // Confirm that the intercepted response isn't null.
    if (response == null) {
      throw new NullPointerException("interceptor " + interceptor + " returned null");
    }

    return response;
  }

Interceptor接口

public interface Interceptor {
  Response intercept(Chain chain) throws IOException;

  interface Chain {
    Request request();

    Response proceed(Request request) throws IOException;

    Connection connection();
  }
}

RealInterceptorChain 实现了Interceptor.Chain接口，其中核心方法为proceed()。
职责链含有两个重要属性，职责链索引index和职责链拦截器interceptors。

首先getResponseWithInterceptorChain()初始化拦截器链，创建责任链

    Interceptor.Chain chain = new RealInterceptorChain(
        interceptors, null, null, null, 0, originalRequest);
    return chain.proceed(originalRequest);

1.将拦截器链传入，并执行索引为0的责任链。proceed方法首先判断责任链索引是否小于拦截器列表数量。

2.将call值加1，call值表示此责任链被调用的次数。

3.判断是否已有数据流并且请求是否与当前责任链请求一致；判断是否有数据流并且此责任链只调用了一次；

4.创建新的责任链，传入相同的拦截列表，索引index+1，获取索引为0的拦截器，并执行interceptor.intercept(next)方法。

5.假若用户没有定义拦截器，那么第一个执行的是RetryAndFollowUpInterceptor拦截器，那么看看RetryAndFollowUpInterceptor.intercept()方法是如何执行的。

@Override public Response intercept(Chain chain) throws IOException {
    //获取原生请求
    Request request = chain.request();
    //创建流分配对象
    streamAllocation = new StreamAllocation(
        client.connectionPool(), createAddress(request.url()), callStackTrace);

    int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
      if (canceled) {
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response = null;
      boolean releaseConnection = true;
      try {
        response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null);
        releaseConnection = false;
      } catch (RouteException e) {
        // The attempt to connect via a route failed. The request will not have been sent.
        if (!recover(e.getLastConnectException(), false, request)) {
          throw e.getLastConnectException();
        }
        releaseConnection = false;
        continue;
      } catch (IOException e) {
        // An attempt to communicate with a server failed. The request may have been sent.
        boolean requestSendStarted = !(e instanceof ConnectionShutdownException);
        if (!recover(e, requestSendStarted, request)) throw e;
        releaseConnection = false;
        continue;
      } finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }

      // Attach the prior response if it exists. Such responses never have a body.
      if (priorResponse != null) {
        response = response.newBuilder()
            .priorResponse(priorResponse.newBuilder()
                    .body(null)
                    .build())
            .build();
      }

      Request followUp = followUpRequest(response);

      if (followUp == null) {
        if (!forWebSocket) {
          streamAllocation.release();
        }
        return response;
      }

      closeQuietly(response.body());

      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }

      if (followUp.body() instanceof UnrepeatableRequestBody) {
        streamAllocation.release();
        throw new HttpRetryException("Cannot retry streamed HTTP body", response.code());
      }

      if (!sameConnection(response, followUp.url())) {
        streamAllocation.release();
        streamAllocation = new StreamAllocation(
            client.connectionPool(), createAddress(followUp.url()), callStackTrace);
      } else if (streamAllocation.codec() != null) {
        throw new IllegalStateException("Closing the body of " + response
            + " didn't close its backing stream. Bad interceptor?");
      }

      request = followUp;
      priorResponse = response;
    }
  }

这里主要是重试连接操作，根据返回的responseCode做出相应的处理。这里重点看

//这里的责任链是之前传过来的，它的index==1，
response = ((RealInterceptorChain) chain).proceed(request, streamAllocation, null, null)

6.所以回到RealInterceptorChain.porceed()方法中发现,又会执行拦截列表第一个拦截器BridgeInterceptor.intercept方法:

    //创建责任链，这个next责任链内部index值为2
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);
    //这个index值为1
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

7.BridgeInterceptor拦截器主要给原生请求添加请求头信息，包括Content-Type，Content-Length，Transfer-Encoding等等请求头信息。

@Override public Response intercept(Chain chain) throws IOException {
    Request userRequest = chain.request();
    Request.Builder requestBuilder = userRequest.newBuilder();

    RequestBody body = userRequest.body();
    if (body != null) {
      MediaType contentType = body.contentType();
      if (contentType != null) {
        requestBuilder.header("Content-Type", contentType.toString());
      }

      long contentLength = body.contentLength();
      if (contentLength != -1) {
        requestBuilder.header("Content-Length", Long.toString(contentLength));
        requestBuilder.removeHeader("Transfer-Encoding");
      } else {
        requestBuilder.header("Transfer-Encoding", "chunked");
        requestBuilder.removeHeader("Content-Length");
      }
    }

    if (userRequest.header("Host") == null) {
      requestBuilder.header("Host", hostHeader(userRequest.url(), false));
    }

    if (userRequest.header("Connection") == null) {
      requestBuilder.header("Connection", "Keep-Alive");
    }

    // If we add an "Accept-Encoding: gzip" header field we're responsible for also decompressing
    // the transfer stream.
    boolean transparentGzip = false;
    if (userRequest.header("Accept-Encoding") == null) {
      transparentGzip = true;
      requestBuilder.header("Accept-Encoding", "gzip");
    }

    List<Cookie> cookies = cookieJar.loadForRequest(userRequest.url());
    if (!cookies.isEmpty()) {
      requestBuilder.header("Cookie", cookieHeader(cookies));
    }

    if (userRequest.header("User-Agent") == null) {
      requestBuilder.header("User-Agent", Version.userAgent());
    }

    Response networkResponse = chain.proceed(requestBuilder.build());

    HttpHeaders.receiveHeaders(cookieJar, userRequest.url(), networkResponse.headers());

    Response.Builder responseBuilder = networkResponse.newBuilder()
        .request(userRequest);

    if (transparentGzip
        && "gzip".equalsIgnoreCase(networkResponse.header("Content-Encoding"))
        && HttpHeaders.hasBody(networkResponse)) {
      GzipSource responseBody = new GzipSource(networkResponse.body().source());
      Headers strippedHeaders = networkResponse.headers().newBuilder()
          .removeAll("Content-Encoding")
          .removeAll("Content-Length")
          .build();
      responseBuilder.headers(strippedHeaders);
      responseBuilder.body(new RealResponseBody(strippedHeaders, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
  }

添加完请求头之后，BridgeInterceptor又执行了:

Response networkResponse = chain.proceed(requestBuilder.build());

那么说明它又要创建一个index为3的责任链，并且执行index为二的拦截器CacheInterceptor.intercept()方法：

@Override public Response intercept(Chain chain) throws IOException {
    Response cacheCandidate = cache != null
        ? cache.get(chain.request())
        : null;

    long now = System.currentTimeMillis();

    CacheStrategy strategy = new CacheStrategy.Factory(now, chain.request(), cacheCandidate).get();
    Request networkRequest = strategy.networkRequest;
    Response cacheResponse = strategy.cacheResponse;

    if (cache != null) {
      cache.trackResponse(strategy);
    }

    if (cacheCandidate != null && cacheResponse == null) {
      closeQuietly(cacheCandidate.body()); // The cache candidate wasn't applicable. Close it.
    }

    // If we're forbidden from using the network and the cache is insufficient, fail.
    if (networkRequest == null && cacheResponse == null) {
      return new Response.Builder()
          .request(chain.request())
          .protocol(Protocol.HTTP_1_1)
          .code(504)
          .message("Unsatisfiable Request (only-if-cached)")
          .body(Util.EMPTY_RESPONSE)
          .sentRequestAtMillis(-1L)
          .receivedResponseAtMillis(System.currentTimeMillis())
          .build();
    }

    // If we don't need the network, we're done.
    if (networkRequest == null) {
      return cacheResponse.newBuilder()
          .cacheResponse(stripBody(cacheResponse))
          .build();
    }

    Response networkResponse = null;
    try {
      networkResponse = chain.proceed(networkRequest);
    } finally {
      // If we're crashing on I/O or otherwise, don't leak the cache body.
      if (networkResponse == null && cacheCandidate != null) {
        closeQuietly(cacheCandidate.body());
      }
    }

    // If we have a cache response too, then we're doing a conditional get.
    if (cacheResponse != null) {
      if (networkResponse.code() == HTTP_NOT_MODIFIED) {
        Response response = cacheResponse.newBuilder()
            .headers(combine(cacheResponse.headers(), networkResponse.headers()))
            .sentRequestAtMillis(networkResponse.sentRequestAtMillis())
            .receivedResponseAtMillis(networkResponse.receivedResponseAtMillis())
            .cacheResponse(stripBody(cacheResponse))
            .networkResponse(stripBody(networkResponse))
            .build();
        networkResponse.body().close();

        // Update the cache after combining headers but before stripping the
        // Content-Encoding header (as performed by initContentStream()).
        cache.trackConditionalCacheHit();
        cache.update(cacheResponse, response);
        return response;
      } else {
        closeQuietly(cacheResponse.body());
      }
    }

    Response response = networkResponse.newBuilder()
        .cacheResponse(stripBody(cacheResponse))
        .networkResponse(stripBody(networkResponse))
        .build();

    if (HttpHeaders.hasBody(response)) {
      CacheRequest cacheRequest = maybeCache(response, networkResponse.request(), cache);
      response = cacheWritingResponse(cacheRequest, response);
    }

    return response;
  }

8.以上CacheInterceptor.intercept()方法中，首先获取缓存以及当前时间，判断服务器资源是否修改过或者缓存是否过期，若没有则返回缓存，无需请求网络。

若返回的缓存为空，则执行：

networkResponse = chain.proceed(networkRequest);

此时又创建了一个index为4的责任链，执行拦截链中的index为3的拦截器ConnectInterceptor.intercept()，与服务器建立连接。

@Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Request request = realChain.request();
    StreamAllocation streamAllocation = realChain.streamAllocation();

    // We need the network to satisfy this request. Possibly for validating a conditional GET.
    boolean doExtensiveHealthChecks = !request.method().equals("GET");
    HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

    return realChain.proceed(request, streamAllocation, httpCodec, connection);
  }

9.ConnectInterceptor拦截器主要与服务器建立连接返回连接，此时又执行下一个拦截器；若用户没有定义网络拦截器，则执行最后一个拦截器CallServerInterceptor.intercept()，获取服务器数据。

  @Override public Response intercept(Chain chain) throws IOException {
    HttpCodec httpCodec = ((RealInterceptorChain) chain).httpStream();
    StreamAllocation streamAllocation = ((RealInterceptorChain) chain).streamAllocation();
    Request request = chain.request();
    //获取当前时间
    long sentRequestMillis = System.currentTimeMillis();
    //写入请求头
    httpCodec.writeRequestHeaders(request);
    //获取数据流
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      Sink requestBodyOut = httpCodec.createRequestBody(request, request.body().contentLength());
      BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);
      request.body().writeTo(bufferedRequestBody);
      bufferedRequestBody.close();
    }
    //关闭数据流
    httpCodec.finishRequest();
    //写入response
    Response response = httpCodec.readResponseHeaders()
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();
    //获取响应码
    int code = response.code();
    if (forWebSocket && code == 101) {
      // Connection is upgrading, but we need to ensure interceptors see a non-null response body.
      response = response.newBuilder()
          .body(Util.EMPTY_RESPONSE)
          .build();
    } else {
      response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

    if ("close".equalsIgnoreCase(response.request().header("Connection"))
        || "close".equalsIgnoreCase(response.header("Connection"))) {
      streamAllocation.noNewStreams();
    }
    //服务器无内容或内容重置
    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }
     //返回响应数据
    return response;
  }
}

所有拦截器全部都执行一次后会返回数据，此时CallServerInterceptor会返回数据给RealInterceptorChain.proceed()，然后执行:

 // Call the next interceptor in the chain.
    RealInterceptorChain next = new RealInterceptorChain(
        interceptors, streamAllocation, httpCodec, connection, index + 1, request);
    Interceptor interceptor = interceptors.get(index);
    Response response = interceptor.intercept(next);

    // Confirm that the next interceptor made its required call to chain.proceed().
    if (httpCodec != null && index + 1 < interceptors.size() && next.calls != 1) {
      throw new IllegalStateException("network interceptor " + interceptor
          + " must call proceed() exactly once");
    }

    // Confirm that the intercepted response isn't null.
    if (response == null) {
      throw new NullPointerException("interceptor " + interceptor + " returned null");
    }

    return response;

10.①判断所有拦截器是否执行了一次；响应是否为空。最后返回给 ConnectInterceptor.intercept()；

②然后ConnectInterceptor.intercept()返回数据给CacheInterceptor.intercept()，写入缓存并更新缓存；

③然后将数据返回给BridgeInterceptor.intercept()，此时BridgeInterceptor要写入cookie到本地，然后解码gzip流数据，最后返回给RetryAndFollowUpInterceptor；

④最后返回数据给客户端，至此网络请求执行完毕。

interceptor职责链执行流程

okhttp职责链执行流程图