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. 在首次調用InetAddress.getByName()前，設置java.security.Security.setProperty("networkaddress.cache.ttl", "0"); 
     * 2. 修改jre/lib/security/java.security 下的 networkaddress.cache.ttl 屬性 
     * 3. jvm啟動參數中設置-Dsun.net.inetaddr.ttl=0 
     * 4. 調用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();  
        //在下一行設置斷點.  
        //放在此處無效，因為類加載的時候就確定了這個值（應該在使用InetAddress.getByName之前設置）已經緩存了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 {  
        //修改緩存數據開始  
        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");  
    }  
}