java实现清理DNS Cache的方法

本文实例讲述了java实现清理DNS Cache的方法。分享给大家供大家参考。具体分析如下：

一、测试环境

OS：Windows7 x64

JDK：1.6.0_45

二、本人找到四种方式清理jvm的DNS缓存，大家可以根据自己的情况选用。

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. 通过反射清理，如本文的clearCache方法

三、代码
复制代码代码如下: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");
}
}

希望本文所述对大家的Java程序设计有所帮助。