package xiaofei;    import java.lang.reflect.Field;  import java.net.InetAddress;  import java.net.UnknownHostException;  import java.util.Map;    /** * @author xiaofei.wxf * @date 13-12-18 */  public class DNSCacheTest {      /**      * 1. 在首次调用InetAddress.getByName()前，设置java.security.Security.setProperty("networkaddress.cache.ttl", "0");      * 2. 修改jre/lib/security/java.security 下的 networkaddress.cache.ttl 属性      * 3. jvm启动参数中设置-Dsun.net.inetaddr.ttl=0      * 4. 调用clearCache方法清除      *      * @param args      * @throws UnknownHostException      */      public static void main(String[] args) throws UnknownHostException, NoSuchFieldException, IllegalAccessException {          java.security.Security.setProperty("networkaddress.cache.ttl", "0");          InetAddress addr1 = InetAddress.getByName("www.baidu.com");          System.out.println(addr1.getHostAddress());          //clearCache();          //在下一行设置断点.          //放在此处无效，因为类加载的时候就确定了这个值（应该在使用InetAddress.getByName之前设置）已经缓存了cache          //java.security.Security.setProperty("networkaddress.cache.ttl", "0");          InetAddress addr2 = InetAddress.getByName("www.baidu.com");          System.out.println(addr2.getHostAddress());          InetAddress addr3 = InetAddress.getByName("www.google.com");          System.out.println(addr3.getHostAddress());          InetAddress addr4 = InetAddress.getByName("www.google.com");          System.out.println(addr4.getHostAddress());          //clearCache();      }        public static void clearCache() throws NoSuchFieldException, IllegalAccessException {          //修改缓存数据开始          Class clazz = java.net.InetAddress.class;          final Field cacheField = clazz.getDeclaredField("addressCache");          cacheField.setAccessible(true);          final Object obj = cacheField.get(clazz);          Class cacheClazz = obj.getClass();          final Field cachePolicyField = cacheClazz.getDeclaredField("type");          final Field cacheMapField = cacheClazz.getDeclaredField("cache");          cachePolicyField.setAccessible(true);          cacheMapField.setAccessible(true);          final Map cacheMap = (Map)cacheMapField.get(obj);          System.out.println(cacheMap);          cacheMap.remove("www.baidu.com");      }  }