android4.4之后的HttpUrlConnection的实现是基于okhttp

首先看HttpUrlConnection使用

URL url = new URL("http://www.baidu.com");
HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
urlConnection.setRequestMethod("get");
urlConnection.connect();
int responseCode = urlConnection.getResponseCode();

下面看URL类的源码实现

transient URLStreamHandler handler;

public URL(String spec) throws MalformedURLException {
        this(null, spec);
    }

public URL(URL context, String spec, URLStreamHandler handler)
        throws MalformedURLException
    {
            //...
            // Get the protocol handler if not specified or the protocol
            // of the context could not be used
            if (handler == null &&
                (handler = getURLStreamHandler(protocol)) == null) {
                throw new MalformedURLException("unknown protocol: "+protocol);
            }

            this.handler = handler;

            //...
    }

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

static URLStreamHandler getURLStreamHandler(String protocol) {

        URLStreamHandler handler = (URLStreamHandler)handlers.get(protocol);
        //...

            // Fallback to built-in stream handler.
            // Makes okhttp the default http/https handler
            if (handler == null) {
                try {
                    if (protocol.equals("file")) {
                        handler = (URLStreamHandler)Class.
                            forName("sun.net.www.protocol.file.Handler").newInstance();
                    } else if (protocol.equals("ftp")) {
                        handler = (URLStreamHandler)Class.
                            forName("sun.net.www.protocol.ftp.Handler").newInstance();
                    } else if (protocol.equals("jar")) {
                        handler = (URLStreamHandler)Class.
                            forName("sun.net.www.protocol.jar.Handler").newInstance();
                    } else if (protocol.equals("http")) {
                        handler = (URLStreamHandler)Class.
                            forName("com.android.okhttp.HttpHandler").newInstance();
                    } else if (protocol.equals("https")) {
                        handler = (URLStreamHandler)Class.
                            forName("com.android.okhttp.HttpsHandler").newInstance();
                    }
                } catch (Exception e) {
                    throw new AssertionError(e);
                }
            }

            //...
        }

        return handler;

    }

URL.openConnection()中的handler实例通过getURLStreamHandler方法中反射（com.android.okhttp.HttpHandler）的形式获得。

从Android 4.4开始，HttpURLConnection的实现确实是通过调用okhttp完成的，而具体的方法则是通过HttpHandler这个桥梁，以及在OkHttpClient, HttpEngine中增加相应的方法来实现，当然，其实还涉及一些类的增加或删除。

附：

但是通过类搜索方法并没有找到okhttp.HttpHandler。下面给出解析：

OkHttp in Android is here: https://android.googlesource.com/platform/external/okhttp/+/master. 
The reason the package name is com.android.okhttp is because there are jarjar rules which repackage it under that name.

因为jarjar-rules的存在，其实路径为/external/okhttp/jarjar-rules.txt,内容如下:
rule com.squareup.** com.android.@1
rule okio.** com.android.okio.@1