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HttpClient/HttpURLConnection + HttpDns最佳實踐
在Android端如果OkHttp作為網絡請求框架，由於其提供了自定義DNS服務接口，可以很優雅地結合HttpDns，相關實現可參考:HttpDns+OkHttp最佳實踐。
如果您使用HttpClient或HttpURLConnection發起網絡請求，盡管無法直接自定義Dns服務，但是由於HttpClient和HttpURLConnection也通過InetAddress進行域名解析，通過修改InetAddress的DNS緩存，同樣可以比通用方案更為優雅地使用HttpDns。
InetAddress在虛擬機層麵提供了域名解析能力，通過調用InetAddress.getByName(String host)即可獲取域名對應的IP。調用InetAddress.getByName(String host)時，InetAddress會首先檢查本地是否保存有對應域名的ip緩存，如果有且未過期則直接返回；如果沒有則調用係統DNS服務（Android的DNS也是采用NetBSD-derived resolver library來實現）獲取相應域名的IP，並在寫入本地緩存後返回該IP。
核心代碼位於java.net.InetAddress.lookupHostByName(String host, int netId)
public class InetAddress implements Serializable {
...
/**
* Resolves a hostname to its IP addresses using a cache.
*
* @param host the hostname to resolve.
* @param netId the network to perform resolution upon.
* @return the IP addresses of the host.
*/
private static InetAddress[] lookupHostByName(String host, int netId)
throws UnknownHostException {
BlockGuard.getThreadPolicy().onNetwork();
// Do we have a result cached?
Object cachedResult = addressCache.get(host, netId);
if (cachedResult != null) {
if (cachedResult instanceof InetAddress[]) {
// A cached positive result.
return (InetAddress[]) cachedResult;
} else {
// A cached negative result.
throw new UnknownHostException((String) cachedResult);
}
}
try {
StructAddrinfo hints = new StructAddrinfo();
hints.ai_flags = AI_ADDRCONFIG;
hints.ai_family = AF_UNSPEC;
// If we don't specify a socket type, every address will appear twice, once
// for SOCK_STREAM and one for SOCK_DGRAM. Since we do not return the family
// anyway, just pick one.
hints.ai_socktype = SOCK_STREAM;
InetAddress[] addresses = Libcore.os.android_getaddrinfo(host, hints, netId);
// TODO: should getaddrinfo set the hostname of the InetAddresses it returns?
for (InetAddress address : addresses) {
address.hostName = host;
}
addressCache.put(host, netId, addresses);
return addresses;
} catch (GaiException gaiException) {
...
}
}
}
其中addressCache為InetAddress的本地緩存：
private static final AddressCache addressCache = new AddressCache();
結合InetAddress的解析策略，我們可以通過如下方法實現自定義DNS服務：
- 通過HttpDns SDK獲取目標域名的ip
- 利用反射的方式獲取到
InetAddress.addressCache對象 - 利用反射方式調用
addressCache.put()方法，域名和ip的對應關係寫入InetAddress緩存
具體實現可參考以下代碼：
public class CustomDns {
public static void writeSystemDnsCache(String hostName, String ip) {
try {
Class inetAddressClass = InetAddress.class;
Field field = inetAddressClass.getDeclaredField("addressCache");
field.setAccessible(true);
Object object = field.get(inetAddressClass);
Class cacheClass = object.getClass();
Method putMethod;
if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
//put方法在api21及以上為put(String host, int netId, InetAddress[] address)
putMethod = cacheClass.getDeclaredMethod("put", String.class, int.class, InetAddress[].class);
} else {
//put方法在api20及以下為put(String host, InetAddress[] address)
putMethod = cacheClass.getDeclaredMethod("put", String.class, InetAddress[].class);
}
putMethod.setAccessible(true);
String[] ipStr = ip.split("\\.");
byte[] ipBuf = new byte[4];
for(int i = 0; i < 4; i++) {
ipBuf[i] = (byte) (Integer.parseInt(ipStr[i]) & 0xff);
}
if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
putMethod.invoke(object, hostName, 0, new InetAddress[] {InetAddress.getByAddress(ipBuf)});
} else {
putMethod.invoke(object, hostName, new InetAddress[] {InetAddress.getByAddress(ipBuf)});
}
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (UnknownHostException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
和通用方案相比，使用該方法具有下列優勢：
- 實現簡單
- 通用性強，該方案在HTTPS,SNI以及設置Cookie等場景均適用。規避了證書校驗，域名檢查等環節
- 全局生效，
InetAddress.addressCache為全局單例，該方案對所有使用InetAddress作為域名解析服務的請求全部生效
-
AddressCache的默認TTL為2S，且默認最多可以保存16條緩存記錄:class AddressCache { ... /** * When the cache contains more entries than this, we start dropping the oldest ones. * This should be a power of two to avoid wasted space in our custom map. */ private static final int MAX_ENTRIES = 16; // The TTL for the Java-level cache is short, just 2s. private static final long TTL_NANOS = 2 * 1000000000L; } }Android虛擬機下反射規則與JVM存在差異，無法直接修改final變量的值。所以使用該方法請務必注意IP過期時間及緩存數量。另外針對該問題可嚐試另一種解決方案：重寫AddressCache類，並通過ClassLoader優先加載，覆蓋係統類。
AddressCache.put方法在 API 21進行了改動，增加了netId參數，為保證兼容性需要針對不同版本區別處理。具體方案參考上文代碼該方式可以解決HTTPS，SNI以及設置cookie等場景，但不適用於WebView場景。Android Webview使用
Chromium或Webkit作為內核(Android 4.4開始，Webview內核由Chromium替代Webkit)。上述兩者均繞開InetAddress而直接使用係統DNS服務，所以該方案對此場景無效。
最後更新：2017-04-14 13:32:07