強烈建議閱讀：
責任鏈模式

okhttp 基本使用及概要（todo:）

官網關於攔截器說明
(說明了普通攔截器和網路攔截器的使用及區別)

攔截器介紹

從下面的圖看出，其實okhttp的核心就是圍繞攔截器展開的。

攔截器的工作原理

開啟Okhttp的原始碼，發出具體請求的地方為RealCall的execute方法

@Override public Response execute() throws IOException {
    synchronized (this) {
      if (executed) throw new IllegalStateException("Already Executed");
      executed = true;
    }
    captureCallStackTrace();
    try {
      client.dispatcher().executed(this);
      // 這行程式碼是核心，用於獲取返回體
      Response result = getResponseWithInterceptorChain();
      if (result == null) throw new IOException("Canceled");
      return result;
    } finally {
      client.dispatcher().finished(this);
    }
  }
 

getResponseWithInterceptorChain方法就是一種責任鏈的處理模式，具體程式碼如下：

Response getResponseWithInterceptorChain() throws IOException {
    // Build a full stack of interceptors.(構造攔截器列表)
    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));


    // 構造責任鏈
    Interceptor.Chain chain = new RealInterceptorChain(
        interceptors, null, null, null, 0, originalRequest);
    
    // 進行鏈式處理
    return chain.proceed(originalRequest);
  }
 

0. 攔截器介面

public interface Interceptor {

  //當前攔截器對一次請求的處理，也是攔截器的核心介面
  Response intercept(Chain chain) throws IOException;
    
  // 攔截器鏈 介面
  interface Chain {
    Request request();

    // 處理一次請求
    Response proceed(Request request) throws IOException;

    /**
     * Returns the connection the request will be executed on. This is only available in the chains
     * of network interceptors; for application interceptors this is always null.
     */
    @Nullable Connection connection();
  }
}

 

1. 攔截器——RetryAndFollowUpInterceptor

 private static final int MAX_FOLLOW_UPS = 20;
@Override public Response intercept(Chain chain) throws IOException {
    Request request = chain.request();

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

    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;
      }  finally {
        // We're throwing an unchecked exception. Release any resources.
        if (releaseConnection) {
          streamAllocation.streamFailed(null);
          streamAllocation.release();
        }
      }
      
      if (++followUpCount > MAX_FOLLOW_UPS) {
        streamAllocation.release();
        throw new ProtocolException("Too many follow-up requests: " + followUpCount);
      }
    }
  }

1.建立streamAllocation物件（裡面包含我們所有對http請求對元件）
2.呼叫(RealInterceptorChain)proceed進行網路請求
3.根據異常結果或者響應結果判斷是否需要重新請求
4.呼叫下一個攔截器，對response進行處理，返回給上一個攔截器

2 攔截器——BridgeInterceptor

1.將使用者構建Request請求轉化為能夠進行網路訪問的請求
2.將這個符合網路請求的Request進行網路請求
3.將網路請求回來的響應Respond轉化為使用者可用的response（包括gzip解壓）

/**
 * Bridges from application code to network code. First it builds a network request from a user
 * request. Then it proceeds to call the network. Finally it builds a user response from the network
 * response.
 */
public final class BridgeInterceptor implements Interceptor {
  private final CookieJar cookieJar;

  public BridgeInterceptor(CookieJar cookieJar) {
    this.cookieJar = cookieJar;
  }

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

    // 請求體的構造及header相關引數的構造
    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());
    }


    // 呼叫責任鏈的下一個攔截器
    Response networkResponse = chain.proceed(requestBuilder.build());

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

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

    // 對返回體的處理： 透明的gzip傳輸
    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();
  }

  /** Returns a 'Cookie' HTTP request header with all cookies, like {@code a=b; c=d}. */
  private String cookieHeader(List<Cookie> cookies) {
    StringBuilder cookieHeader = new StringBuilder();
    for (int i = 0, size = cookies.size(); i < size; i++) {
      if (i > 0) {
        cookieHeader.append("; ");
      }
      Cookie cookie = cookies.get(i);
      cookieHeader.append(cookie.name()).append('=').append(cookie.value());
    }
    return cookieHeader.toString();
  }
}

3 攔截器——CacheInterceptor

請求結構快取的使用：

//okhttp的快取使用非常簡單，只需要OkHttpClient建立的時候設定cache就可以了
new OkHttpClient().newBuilder().cache(new Cache(new File("cache"),
         1024*1024*10)).connectTimeout(5,TimeUnit.SECONDS).build();

核心程式碼如下：

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

    long now = System.currentTimeMillis();

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

    if (cache != null) {
3     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.
4   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.
5   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.
6  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();

7  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;
  }

1.cache不為空蓯中獲取cacheCandidate
2.通過FacheStrategy.Factory獲取到快取策略，獲取網路請求與響應快取
3.呼叫cache的同步方法trackResponse，保證請求一致性
4. 沒網和沒快取的請求下直接返回一個504錯誤響應

具體的是使用快取還是進行網路請求可參考CacheStrategy中相關程式碼，主要是一些條件判斷。

4 攔截器——ConnectInterceptor

獲取一個RealConnection物件，然後建立Socket連結，並且呼叫下一個也是最後一個攔截器來完成Okhttp的整個操作。

/** Opens a connection to the target server and proceeds to the next interceptor. */
public final class ConnectInterceptor implements Interceptor {
  public final OkHttpClient client;

  public ConnectInterceptor(OkHttpClient client) {
    this.client = client;
  }

  @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");
    
    // newStream中會從連線池中獲取RealConnection/建立一個RealConnection並進行連線。
    HttpCodec httpCodec = streamAllocation.newStream(client, doExtensiveHealthChecks);
    RealConnection connection = streamAllocation.connection();

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

5 攔截器——CallServerInterceptor

1.發起真正的網路請求
2.介紹伺服器返回的響應

public final class CallServerInterceptor implements Interceptor {
  private final boolean forWebSocket;

  public CallServerInterceptor(boolean forWebSocket) {
    this.forWebSocket = forWebSocket;
  }

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

    long sentRequestMillis = System.currentTimeMillis();
2   httpCodec.writeRequestHeaders(request);

3   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();
    }

4   httpCodec.finishRequest();

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

    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 {
5     response = response.newBuilder()
          .body(httpCodec.openResponseBody(response))
          .build();
    }

6   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;
  }
}

1.HttpCodec(http中所有流物件都封裝到了HttpCodec實現類中),可以簡單的理解為它能編碼request和解碼response
2.可以看到證實了上面說的HttpCodec寫入了請求的頭部資訊
3.想socket寫入請求體
4.表明完成了http請求request的寫入工作
5.讀取響應的body資訊
6.關閉連線

參考：

https://www.jianshu.com/p/ac886644d6a8