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"); 
    } 
}