OkHttp中的攔截器機制源碼解析

一、攔截器鏈流程圖

二、getResponseWithInterceptorChain 方法

之前說到主要是通過這個方法來返回response那麼來看看這個方法裏究竟是做了什麼呢

 Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.
    List<Interceptor> interceptors = new ArrayList<>();
    interceptors.addAll(client.interceptors());
    interceptors.add(retryAndFollowUpInterceptor);
    interceptors.add(new BridgeInterceptor(client.cookieJar()));
    interceptors.add(new CacheInterceptor(client.internalCache()));
    interceptors.add(new ConnectInterceptor(client));
    if (!forWebSocket) {
      interceptors.addAll(client.networkInterceptors());
    }
    interceptors.add(new CallServerInterceptor(forWebSocket));

    Interceptor.Chain chain = new RealInterceptorChain(interceptors, null, null, null, 0,
        originalRequest, this, eventListener, client.connectTimeoutMillis(),
        client.readTimeoutMillis(), client.writeTimeoutMillis());

    return chain.proceed(originalRequest);
  }

首先添加自定義配置的攔截器

然後以此添加系統內置的攔截器

然後創建RealInterceptorChain並把這個集合添加進去，這個就是個攔截器鏈

通過chain.proceed(originalRequest);這個方法執行

proceed這個方法也很重要 可以看一下

 public Response proceed(Request request, StreamAllocation streamAllocation, HttpCodec httpCodec,
      RealConnection 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 && !this.connection.supportsUrl(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");
    }

    //注意處
    // Call the next interceptor in the chain.
    RealInterceptorChain next = new RealInterceptorChain(interceptors, streamAllocation, httpCodec,
        connection, index + 1, request, call, eventListener, connectTimeout, readTimeout,
        writeTimeout);
    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");
    }

    if (response.body() == null) {
      throw new IllegalStateException(
          "interceptor " + interceptor + " returned a response with no body");
    }

    return response;
  }

這裏主要看標註的注意處 這裏主要通過index+1創建了下一個攔截器，也就行程了攔截器鏈

然後通過每個攔截器中都有intercept這個方法，這個方法中返回response然後進行返回。

這裏有個流程是很有意思的就是在proceed()方法裏會會返回一個Response 而 這個Response是通過intercep()這個方法並傳入一個攔截器返回的，而在各個攔截器中又通過傳入的攔截器調回去，這也是攔截器流程的核心機制。從而實現了攔截器鏈，只有所有的攔截器執行完畢後，一個網絡請求的響應response 纔會被返回。

三、內置攔截器

OkHttp中的內置連接器都基實現了通用的接口

1、RetryAndFollowUpInterceptor 接口重定向攔截器

這個攔截器主要是根據結果判斷然後進行重試的攔截器

 @Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    Call call = realChain.call();
    EventListener eventListener = realChain.eventListener();

    streamAllocation = new StreamAllocation(client.connectionPool(), createAddress(request.url()),
        call, eventListener, callStackTrace);

    int followUpCount = 0;
    Response priorResponse = null;
    while (true) {
      if (canceled) {//注意處1
        streamAllocation.release();
        throw new IOException("Canceled");
      }

      Response response;
      boolean releaseConnection = true;
      try {//注意處2
        response = realChain.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()), call, eventListener, 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;
    }
  }

具體分析下流程
首先會創建一個StreamAllocation對象

其實主要看while循環裏面的方法，這裏是一個死循環來進行嘗試，首先會判斷是否取消了請求，如果取消進行釋放，並拋出異常，如注意處1.

然後主注意處2要進行請求，然後通過catch判斷各種需要重新連接的情況，如果重新連接就通過continue直接在進行一遍循環。

當代碼可以執行到 followUpRequest 方法就表示這個請求是成功的，但是服務器返回的狀態碼可能不是 200 ok 的情況，這時還需要對該請求進行檢測，其主要就是通過返回碼進行判斷的，然後如果沒問題就會返回respnse並結束循環。

如果沒有正常返回結果的話後面會關閉請求還會做一些異常檢查，如重新連接的次數是否超過了最大次數之類的。

2、BridgeInterceptor 請求和響應轉化攔截器

這個攔截器主要負責設置內容長度，編碼方式以及一些壓縮等配置，主要是添加頭部信息的功能。

 @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 && userRequest.header("Range") == 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);
      String contentType = networkResponse.header("Content-Type");
      responseBuilder.body(new RealResponseBody(contentType, -1L, Okio.buffer(responseBody)));
    }

    return responseBuilder.build();
  }

這裏通過注意出可以看到其中一個主要的功能就是，如果剛開始我們沒有配置一些請求頭信息會添加一些默認的請求頭信息。

這裏還有一個重要功能就是 判斷是否需要使用Gzip壓縮功能。以及將網絡請求回來的響應Response轉化爲永華可用的response

3、CacheInterceptor 緩存攔截器

“`
@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 (cache != null) {
  if (HttpHeaders.hasBody(response) && CacheStrategy.isCacheable(response, networkRequest)) {
    // Offer this request to the cache.
    CacheRequest cacheRequest = cache.put(response);
    return cacheWritingResponse(cacheRequest, response);
  }

  if (HttpMethod.invalidatesCache(networkRequest.method())) {
    try {
      cache.remove(networkRequest);
    } catch (IOException ignored) {
      // The cache cannot be written.
    }
  }
}

return response;

}

主要流程，先判斷是否有緩存。然後如果緩存不可用會關閉，然後在會判斷網絡禁止的話和緩存都不可用的話會創建一個響應返回504

// 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();
}

如果前面的條件都不符合，會讀取網絡結果也就是會跳轉到下一個攔截器， if (networkResponse.code() == HTTP_NOT_MODIFIED)  這裏會判斷如果是304的話使用緩存，還會對緩存進行比對和更新。然後並返回response。在這之前會先判斷cacheResponse是否爲空，如果爲空則走後面的代碼，會創建一個response。

### 4、ConnectInterceptor 網絡連接攔截器

@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, chain, doExtensiveHealthChecks);
RealConnection connection = streamAllocation.connection();

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

}

這裏的StreamAllocation其實是在重定向攔截器中創建，但是他傳入到ConnectInterceptor中進行使用。然後會通過streamAllocation來創建httpCodec，httpCodec用來編碼request和解碼response。然後會通過streamAllocation.connection();來創建一個RealConnection，RealConnection主要來進行實際的網絡傳輸。然後還是通過proceed方法傳入到下一個攔截器。
### 5、CallServerInterceptor 
這個攔截器是攔截器鏈中最後一個攔截器，是真正的發起請求和處理返回響應的攔截器。
 ```
 @Override public Response intercept(Chain chain) throws IOException {
    RealInterceptorChain realChain = (RealInterceptorChain) chain;
    HttpCodec httpCodec = realChain.httpStream();
    StreamAllocation streamAllocation = realChain.streamAllocation();
    RealConnection connection = (RealConnection) realChain.connection();
    Request request = realChain.request();

    long sentRequestMillis = System.currentTimeMillis();

    realChain.eventListener().requestHeadersStart(realChain.call());
    httpCodec.writeRequestHeaders(request);
    realChain.eventListener().requestHeadersEnd(realChain.call(), request);

    Response.Builder responseBuilder = null;
    if (HttpMethod.permitsRequestBody(request.method()) && request.body() != null) {
      // If there's a "Expect: 100-continue" header on the request, wait for a "HTTP/1.1 100
      // Continue" response before transmitting the request body. If we don't get that, return
      // what we did get (such as a 4xx response) without ever transmitting the request body.
      if ("100-continue".equalsIgnoreCase(request.header("Expect"))) {
        httpCodec.flushRequest();
        realChain.eventListener().responseHeadersStart(realChain.call());
        responseBuilder = httpCodec.readResponseHeaders(true);
      }

      if (responseBuilder == null) {
        // Write the request body if the "Expect: 100-continue" expectation was met.
        realChain.eventListener().requestBodyStart(realChain.call());
        long contentLength = request.body().contentLength();
        CountingSink requestBodyOut =
            new CountingSink(httpCodec.createRequestBody(request, contentLength));
        BufferedSink bufferedRequestBody = Okio.buffer(requestBodyOut);

        request.body().writeTo(bufferedRequestBody);
        bufferedRequestBody.close();
        realChain.eventListener()
            .requestBodyEnd(realChain.call(), requestBodyOut.successfulCount);
      } else if (!connection.isMultiplexed()) {
        // If the "Expect: 100-continue" expectation wasn't met, prevent the HTTP/1 connection
        // from being reused. Otherwise we're still obligated to transmit the request body to
        // leave the connection in a consistent state.
        streamAllocation.noNewStreams();
      }
    }

    httpCodec.finishRequest();

    if (responseBuilder == null) {
      realChain.eventListener().responseHeadersStart(realChain.call());
      responseBuilder = httpCodec.readResponseHeaders(false);
    }

    Response response = responseBuilder
        .request(request)
        .handshake(streamAllocation.connection().handshake())
        .sentRequestAtMillis(sentRequestMillis)
        .receivedResponseAtMillis(System.currentTimeMillis())
        .build();

    realChain.eventListener()
        .responseHeadersEnd(realChain.call(), response);

    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();
    }

    if ((code == 204 || code == 205) && response.body().contentLength() > 0) {
      throw new ProtocolException(
          "HTTP " + code + " had non-zero Content-Length: " + response.body().contentLength());
    }

    return response;
  }

流程
這裏首先會獲取到之前各個攔截器中的一些參數，首先會 httpCodec.writeRequestHeaders(request);通過這個httpCodec寫入請求頭信息，然後會寫入請求的body信息，完成寫入後會調用 httpCodec.finishRequest();
方法表示寫入信息完成。

然後會讀取響應信息，會先讀取響應頭信息，響應如果讀取或者創建完成後會 通過streamAllocation.noNewStreams();這個方法進行關閉流，還會判斷如果響應碼是204或者205的話拋出一個異常。

其實這個攔截器鏈無非就是2個工作，發起請求，然後處理響應。至於中間會有一些封裝請求信息判斷響應信息等操作。

四、總結

這裏攔截器的流程基本分析完了，其實主要還是看清大體流程和設計思路，具體細節的部分沒有過多深入，因爲這些流程設計模式纔是真正要學習的重點。從源碼看來 好多地方還是很值得學習的。