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transient URLStreamHandler handler;

public URLConnection openConnection() throws java.io.IOException {
   return handler.openConnection(this);
}

HttpUrlConnection 的对象是由 URL 对象调用 handler.openConnection 来常见完成的，这里的 handler 的类型为 URLStreamHandler，这个 handler 又是由 factory.createURLStreamHandler(protocol) 根据协议来创建的，最终它的实现为 com.android.okhttp.HttpHandler 或者 com.android.okhttp.HttpsHandler。

https://cs.android.com/android/platform/superproject/+/master:external/okhttp/repackaged/android/src/main/java/com/android/okhttp/HttpHandler.java;l=41?q=com.android.okhttp.HttpHandler

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protected URLConnection openConnection(URL url) throws IOException {
      return newOkUrlFactory(null /* proxy */).open(url);
  }

  protected OkUrlFactory newOkUrlFactory(Proxy proxy) {
      OkUrlFactory okUrlFactory = createHttpOkUrlFactory(proxy);
      okUrlFactory.client().setConnectionPool(configAwareConnectionPool.get());
      return okUrlFactory;
  }

  public static OkUrlFactory createHttpOkUrlFactory(Proxy proxy) {
      OkHttpClient client = new OkHttpClient();
      // 0 代表不限制
      client.setConnectTimeout(0, TimeUnit.MILLISECONDS);
      client.setReadTimeout(0, TimeUnit.MILLISECONDS);
      client.setWriteTimeout(0, TimeUnit.MILLISECONDS);
      // 重定向设置
      client.setFollowRedirects(HttpURLConnection.getFollowRedirects());
      // 默认 https 下不自动跟踪重定向
      client.setFollowSslRedirects(false);
      client.setConnectionSpecs(CLEARTEXT_ONLY);
      if (proxy != null) {
          client.setProxy(proxy);
      }
      OkUrlFactory okUrlFactory = new OkUrlFactory(client);
      OkUrlFactories.setUrlFilter(okUrlFactory, CLEARTEXT_FILTER);
      ResponseCache responseCache = ResponseCache.getDefault();
      if (responseCache != null) {
          AndroidInternal.setResponseCache(okUrlFactory, responseCache);
      }
      return okUrlFactory;
  }

这样就完成了 OkHttpClient 对象的创建了。它返回的 URLConnection 对象的实现为 HttpURLConnectionImpl 或者为 HttpsURLConnectionImpl，可见 这里，之后 HttpUrlConnection 发生的操作都是由它完成的。