java實作清理DNS Cache的方法

本文實例講述了java實作清理DNS Cache的方法。分享給大家供大家參考。具體分析如下：

一、測試環境

OS：Windows7 x64

JDK：1.6.0_45

二、本人找到四種方式清理jvm的DNS緩存，大家可以依照自己的狀況選用。

1. 在首次呼叫InetAddress.getByName()前，設定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啟動參數中設定-Dsun.net.inetaddr.ttl=0
4. 透過反射清理，如本文的clearCache方法

三、代碼
複製代碼代碼如下: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. 在首次呼叫InetAddress.getByName()前，設定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啟動參數中設定-Dsun.net.inetaddr.ttl=0
* 4. 呼叫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();
//在下一行設定斷點.
//放在此處無效，因為類別載入的時候就確定了這個值（應該在使用InetAddress.getByName之前設定）已經快取了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 {
//修改快取資料開始
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");
}
}

希望本文所述對大家的Java程式設計有幫助。