Android端WebView场景下接入阿里云公共DNS Android SDK最佳实践方案

• 方案概述

public void setWebViewClient(WebViewClient client);

WebView提供了setWebViewClient接口对网络请求进行拦截，通过重载WebViewClient中的ShouldInterceptRequest方法，我们可以拦截到所有的网络请求：

public class WebViewClient{
       // API < 21
       public WebResourceResponse shouldInterceptRequest(WebView view,String url){
             .....
       } 
       // API >= 21
      public WebResourceResponse shouldInterceptRequest(WebView view,WebResourceRequest request) {
            .....
       }
     ......
 }

• 当API<21时，shouldInterceptRequest的版本为：

public WebResourceResponse shouldInterceptRequest(WebView view,String url)

此时仅能获取到请求URL，请求方法、头部信息以及body等均无法获取，强行拦截该请求可能无法得到正确响应。所以当API<21时，不对请求进行拦截：

public WebResourceResponse shouldInterceptRequest(WebView view,String url) {
      return super.shouldInterceptRequest(view, url);
}

• 当API≥21时，shouldInterceptRequest提供的方法为：

public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
     String scheme = request.getUrl().getScheme().trim();
     String method = request.getMethod();
     Map<String, String> headerFields = request.getRequestHeaders();
     // 只能正常处理不带body的请求
     if ((scheme.equalsIgnoreCase("http") || scheme.equalsIgnoreCase("https"))&& method.equalsIgnoreCase("get")) {
            ......
     } else {
           return super.shouldInterceptRequest(view, reqeust);
     }
}

• 提供WebResourceResponse回调

webview拦截网络请求时，需要返回一个WebResourceResponse：

public WebResourceResponse(String mimeType, String encoding, InputStream data) ;

创建WebResourceResponse对象需要提供：请求的MIME类型、请求的编码、请求的输入流。

其中请求的输入流可以通过URLConnection.getInputStream()获取，而MIME类型和encoding可以通过请求的ContentType获取到，即通过URLConnection.getContentType ()获得。

text/html;charset=utf-8

String contentType = conn.getContentType();
String mime = getMime(contentType);
String charset = getCharset(contentType);

// 不含有MIME类型的请求不拦截
if (TextUtils.isEmpty(mime)) {
        return super.shouldInterceptRequest(view, request);
 } else {
       if (!TextUtils.isEmpty(charset)) {
             // 如果同时获取到MIME和charset可以直接拦截
             return new WebResourceResponse(mime, charset, connection.getInputStream());
       } else {
             //获取不到编码信息
             // 二进制资源无需编码信息，可以进行拦截
             if (isBinaryRes(mime)) {
                   Log.e(TAG, "binary resource for " + mime);
                   return new WebResourceResponse(mime, charset, connection.getInputStream());
            }else {
                   // 非二进制资源需要编码信息，不拦截
                  Log.e(TAG, "non binary resource for " + mime);
                 return super.shouldInterceptRequest(view, request);
           }
       }
  }

• 设置请求头Host

public WebResourceResponse shouldInterceptRequest(WebView view, WebResourceRequest request) {
     ...... 
     URL url = new URL(request.getUrl().toString());
     conn = (HttpURLConnection) url.openConnection();
     //通过阿里云公共DNS Android SDK提供API获得IP
     String ip = null;
     String[] ipv4Array = mDNSResolver.getIpv4ByHostFromCache(url.getHost(),true);
     if (ipv4Array != null && ipv4Array.length > 0) {
        ip = ipv4Array[0];
     }
     if (ip != null) {
           //Log.d(TAG, "get IP: " + ip + " for host: " + url.getHost() + "from pdns resolver success!");
          String newUrl = path.replaceFirst(url.getHost(), ip);
          conn = (HttpURLConnection) new URL(newUrl).openConnection(); 
          for (Map.Entry<String, String> field : headers.entrySet()) {
                //设置Http请求的Head头部信息
               conn.setRequestProperty(field.getKey(), field.getValue( ));
          }
      }
      // conn.setRequestProperty("Host", url.getHost());
    } 
}

• 接入的场景

1.重定向

通过拦截服务器的Get请求，如果服务端的response包含重定向，此时需要判断原有请求中是否含有Cookie。如果原有请求头含义Cookie，重定向后Cookie会发生改变，这种场景下放弃拦截。如果原有请求头不包含Cookie，则需重新二次请求。

int code = conn.getResponseCode();
if (code >= 300 && code < 400) {
      if (请求报头中含有cookie) {
             // 不拦截
            return super.shouldInterceptRequest(view, request);
      }

      //临时重定向和永久重定向location的大小写有区分
      String location = conn.getHeaderField("Location");
      if (location == null) {
            location = conn.getHeaderField("location");
      }

      if (!(location.startsWith("http://") || location.startsWith("https://"))) {
             //补全url
             URL originalUrl = new URL(path);
             location = originalUrl.getProtocol() + "://"+ originalUrl.getHost() + location;
      }
      Log.e(TAG, "code:" + code + "; location:" + location + ";path" + path);
     发起二次请求
  } else {
        // redirect finish.
        Log.e(TAG, "redirect finish");
        ......
 }

2. HTTPS证书校验

如果拦截到的请求是HTTPS请求，此时还需要进行证书校验：

if (conn instanceof HttpsURLConnection) {
     final HttpsURLConnection httpsURLConnection = (HttpsURLConnection)conn;
     //https场景证书校验
     httpsURLConnection.setHostnameVerifier(new HostnameVerifier() {
         @Override
         public boolean verify(String hostname, SSLSession session) {
             String host = httpsURLConnection.getRequestProperty("Host");
             if (null == host) {
                   host = httpsURLConnection.getURL().getHost();
             }
             return HttpsURLConnection.getDefaultHostnameVerifier().verify(host, session);
       }
   });
}

3. HTTPS+SNI

如果HTTPS请求涉及到SNI场景，需要自定义SSLSocket，开发者可以参考Android端HTTPS（含SNI）业务场景"IP直连"方案说明