1. import java.io.BufferedReader;
2. import java.io.IOException;
3. import java.io.InputStream;
4. import java.io.InputStreamReader;
5. import java.lang.reflect.Field;
6. import java.lang.reflect.Method;
7. import java.net.HttpURLConnection;
8. import java.net.InetAddress;
9. import java.net.MalformedURLException;
10. import java.net.URL;
11. import java.net.UnknownHostException;
12. import java.util.HashMap;
13. import java.util.List;
14. import java.util.Map;
15. import sun.net.spi.nameservice.NameService;
16. public class JavaDNSCacheTest {
17.         /**
18.          * 经测试,二种方式在Windows环境都可以, Linux环境待测
19.          *
20.          * @param args
21.          * @throws Exception
22.          */
23.         public static void main(String[] args) throws Exception {
24.                 /* 1. 动态替换AddressCache */
25.                 // changeDNSWithAddressCache();
26.                 /* 1. 动态代理NameService */
27.                 changeDNSWithNameService();
28.         }
29.         public static void changeDNSWithNameService() throws Exception {
30.                 /* 1. 获取反身类 */
31.                 Class<?> addressClass = InetAddress.class;
32.                 /* 2. 获取addressCache字段 */
33.                 try {
34.                         Field nameServiceField = addressClass.getDeclaredField("nameService");// 对于Jrockit或IBM JDK
35.                         nameServiceField.setAccessible(true);
36.                         sun.net.spi.nameservice.NameService nameService = (NameService) nameServiceField.get(null);
37.                         nameServiceField.set(null, new NameServiceProxy(nameService));
38.                         nameServiceField.setAccessible(false);
39.                 } catch (NoSuchFieldException e) {
40.                         Field nameServicesField = addressClass.getDeclaredField("nameServices");// 对于OpenJDK
41.                         nameServicesField.setAccessible(true);
42.                         List<sun.net.spi.nameservice.NameService> nameServices = (List<sun.net.spi.nameservice.NameService>) nameServicesField.get(null);
43.                         if (nameServices != null && nameServices.size() > 0) {
44.                                 /* 置换为代理实例 */
45.                                 nameServices.set(0, new NameServiceProxy(nameServices.get(0)));
46.                         } else {
47.                                 // 可能为空吗? 待测
48.                         }
49.                         nameServicesField.setAccessible(false);
50.                 }
51.                 getHttpConent("www.baidu.com");
52.         }
53.         public static void changeDNSWithCddressCache() throws Exception {
54.                 /* 1. 获取反身类 */
55.                 Class<?> addressClass = InetAddress.class;
56.                 /* 2. 获取addressCache字段 */
57.                 Field addressCacheField = addressClass.getDeclaredField("addressCache");
58.                 /* 3. 获取addressCache */
59.                 addressCacheField.setAccessible(true);
60.                 Object addressCache = addressCacheField.get(null);
61.                 addressCacheField.setAccessible(false);
62.                 /* 4. 获取addressCache的反射类 */
63.                 Class<?> addressCacheClass = addressCache.getClass();
64.                 /* 5. 获取addressCache的put方法 */
65.                 Method putMethod = addressCacheClass.getDeclaredMethod("put", String.class, InetAddress[].class);
66.                 /* 5. 修改addressCache将wwww.baidu.com换成指定任意IP */
67.                 putMethod.setAccessible(true);
68.                 putMethod.invoke(addressCache, "www.baidu.com", new InetAddress[] { InetAddress.getByAddress(new byte[] { 115, (byte) 239, (byte) 210, 26 }) });
69.                 putMethod.setAccessible(false);
70.                 /* 6.测试,看看是否连通 */
71.                 getHttpConent("www.baidu.com");
72.         }
73.         private static void getHttpConent(String host) throws MalformedURLException, IOException {
74.                 HttpURLConnection conn = (HttpURLConnection) new URL("http://" + host).openConnection();
75.                 try {
76.                         conn.setConnectTimeout(3000);// 减少连接时间,方便测试
77.                         conn.setDefaultUseCaches(false);
78.                         conn.setDoInput(true);
79.                         conn.setRequestMethod("GET");
80.                         conn.connect();
81.                         int code = conn.getResponseCode();
82.                         System.out.format("REST[%d]\n", code);
83.                         InputStream in = null;
84.                         in = conn.getErrorStream();// 如果非2xx,则由errorStream获取输出.
85.                         if (in == null) {
86.                                 in = conn.getInputStream();
87.                         }
88.                         if (in != null) {
89.                                 BufferedReader reader = new BufferedReader(new InputStreamReader(in));
90.                                 for (String line = null; (line = reader.readLine()) != null;) {
91.                                         System.out.println(line);
92.                                 }
93.                         }
94.                 } finally {
95.                         if (conn != null) {
96.                                 conn.disconnect();
97.                         }
98.                 }
99.         }
100.         @SuppressWarnings("restriction")
101.         public static class NameServiceProxy implements sun.net.spi.nameservice.NameService {
102.                 final sun.net.spi.nameservice.NameService nameService;
103.                 final Map<String, InetAddress[]> mapping = new HashMap<String, InetAddress[]>();
104.                 {
105.                         try {
106.                                 mapping.put("www.baidu.com", new InetAddress[] { InetAddress.getByAddress(new byte[] { 115, (byte) 239, (byte) 210, 26 }) });
107.                         } catch (UnknownHostException e) {
108.                                 e.printStackTrace();
109.                         }
110.                 }// 实例初始化表
111.                 public NameServiceProxy(sun.net.spi.nameservice.NameService nameService) {
112.                         this.nameService = nameService;
113.                 }
114.                 @Override
115.                 public String getHostByAddr(byte[] addr) throws UnknownHostException {
116.                         return this.nameService.getHostByAddr(addr);
117.                 }
118.                 @Override
119.                 public InetAddress[] lookupAllHostAddr(String host) throws UnknownHostException {
120.                         if (mapping.containsKey(host)) {
121.                                 return mapping.get(host);
122.                         } else {
123.                                 return this.nameService.lookupAllHostAddr(host);
124.                         }
125.                 }
126.         }
127. }

Java动态替换InetAddress中DNS的做法简单分析2