(Android) OkHttp3.10 源碼學習筆記 5 攔截器鏈分析

接上面的分析，在獲取請求response的時候，都調用瞭如下代碼

 Response response = getResponseWithInterceptorChain();

我們跟進去看一下    

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);
  }    

上面的代碼可以看到，首先添加了client.intercepors()，它們就是application 攔截器，然後添加了上篇文章介紹的五種內部攔截器，最後又添加了network攔截器

再看下面的代碼

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

創建了RealInterceptorChain這個對象，將前面創建好的攔截器list傳入，這樣就完成了第一步。然後調用了chain的proceed()方法

@Override public Response proceed(Request request) throws IOException {
    return proceed(request, streamAllocation, httpCodec, connection);
  }

最終調用了四個參數的proceed方法，接着看

 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");
    }

    // 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;
  }

這個方法比較長，我們挑重點看

 // 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);

上面這段代碼也創建了一個攔截器鏈，這和剛纔創建的區別在於他傳入的參數是index+1,就傳入了下一個攔截器，這樣就把我們的整個攔截器構造成了一個鏈條，設計很巧妙喔。第三塊獲取response的代碼，傳入的是剛纔構造的next攔截器鏈。

總結一下

1.創建一系列攔截器，放入一個攔截器list當中

2.創建一個攔截器鏈對象，並執行攔截器鏈的proceed方法，這裏注意，執行的是下一個攔截器的intercept方法。

這裏我們看一下intercept方法，這裏執行的是chain的proceed方法，這樣就印證了我們上面的說法，整個攔截器鏈完整的交替執行。

  response = realChain.proceed(request, streamAllocation, null, null);
        releaseConnection = false;

這裏，可以把OkHttp的網絡請求看成一個個攔截器的執行他的intercept方法的過程，這樣應該比較好理解了。

再次總結一下

1.在發起請求前對Request進行處理

2.調用下一個攔截器，獲取response

3.對response處理，返回給上一個攔截器