在Android端如果OkHttp作为网络请求框架，由于其提供了自定义DNS服务接口，可以很优雅地结合HttpDns，相关实现可参考:HttpDns+OkHttp最佳实践。
如果您使用HttpClient或HttpURLConnection发起网络请求，尽管无法直接自定义Dns服务，但是由于HttpClient和HttpURLConnection也通过InetAddress进行域名解析，通过修改InetAddress的DNS缓存，同样可以比通用方案更为优雅地使用HttpDns。
InetAddress在虚拟机层面提供了域名解析能力，通过调用InetAddress.getByName(String host)即可获取域名对应的IP。调用InetAddress.getByName(String host)时，InetAddress会首先检查本地是否保存有对应域名的ip缓存，如果有且未过期则直接返回；如果没有则调用系统DNS服务（Android的DNS也是采用NetBSD-derived resolver library来实现）获取相应域名的IP，并在写入本地缓存后返回该IP。
核心代码位于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为InetAddress的本地缓存：
private static final AddressCache addressCache = new AddressCache();
结合InetAddress的解析策略，我们可以通过如下方法实现自定义DNS服务：
- 通过HttpDns SDK获取目标域名的ip
- 利用反射的方式获取到
InetAddress.addressCache对象 - 利用反射方式调用
addressCache.put()方法，域名和ip的对应关系写入InetAddress缓存
具体实现可参考以下代码：
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及以上为put(String host, int netId, InetAddress[] address)
putMethod = cacheClass.getDeclaredMethod("put", String.class, int.class, InetAddress[].class);
} else {
//put方法在api20及以下为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，使用该方法具有下列优势：
- 实现简单
- 通用性强，该方案在HTTPS,SNI以及设置Cookie等场景均适用。规避了证书校验，域名检查等环节
- 全局生效，
InetAddress.addressCache为全局单例，该方案对所有使用InetAddress作为域名解析服务的请求全部生效
另外使用该方案请务必注意以下几点：
-
AddressCache的默认TTL为2S，且默认最多可以保存16条缓存记录: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虚拟机下反射规则与JVM存在差异，无法直接修改final变量的值。所以使用该方法请务必注意IP过期时间及缓存数量。另外针对该问题可尝试另一种解决方案：重写AddressCache类，并通过ClassLoader优先加载，覆盖系统类。
AddressCache.put方法在 API 21进行了改动，增加了netId参数，为保证兼容性需要针对不同版本区别处理。具体方案参考上文代码该方式可以解决HTTPS，SNI以及设置cookie等场景，但不适用于WebView场景。Android Webview使用
Chromium或Webkit作为内核(Android 4.4开始，Webview内核由Chromium替代Webkit)。上述两者均绕开InetAddress而直接使用系统DNS服务，所以该方案对此场景无效。