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在java.net包描述中, 简要说明了一些关键的接口. 其中负责networking identifiers的是Addresses. 这个类的具体实现类是InetAddress, 底层封装了Inet4Address与Inet6Address的异同, 可以看成一个Facade工具类.
- A Low Level API, which deals with the following abstractions:
- Addresses, which are networking identifiers, like IP addresses.
- Sockets, which are basic bidirectional data communication mechanisms.
- Interfaces, which describe network interfaces.
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在OpenJDK的InetAddress源码中, 根据dns或hostname解析IP的代码部分:
- private static InetAddress[] getAllByName0 (String host, InetAddress reqAddr, boolean check)
- throws UnknownHostException {
-
- /* If it gets here it is presumed to be a hostname */
- /* Cache.get can return: null, unknownAddress, or InetAddress[] */
-
- /* make sure the connection to the host is allowed, before we
- * give out a hostname
- */
- if (check) {
- SecurityManager security = System.getSecurityManager();
- if (security != null) {
- security.checkConnect(host, -1);
- }
- }
-
- InetAddress[] addresses = getCachedAddresses(host);
-
- /* If no entry in cache, then do the host lookup */
- if (addresses == null) {
- addresses = getAddressesFromNameService(host, reqAddr);
- }
-
- if (addresses == unknown_array)
- throw new UnknownHostException(host);
-
- return addresses.clone();
- }
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关键的二个方法是:
getCachedAddresses(host);
getAddressesFromNameService(host, reqAddr);
前者从addressCache, 或negativeCache根据dns/hostname解析缓存的IP.
后者从遍历nameServices,调用每个NameService的lookupAllHostAddr(host)查找IP, 然后将host:IP缓存到前面的cache中.
根据上述, 实现动态解析DNS, 有二种方式:
1. 反射addressCache, 或negativeCache, 将host:IP通过Cache的put()方法放入.
2. 反射nameServices,将代理的NameService实例放入.
二种做法的难处:
1. addressCache, 或negativeCache都是 java.net.InetAddress.Cache, 其内部的CacheEntry受二组JVM选项影响:
networkaddress.cache.ttl
networkaddress.cache.negative.ttl
在ttl后, CacheEntry的get()只会返回null.
2. nameServices只是OpenJDK的实现. 换言之,只是SUN一家的. 其他JDK不用此属性名.
写段代码看看Jrockit与IBM JVM中InetAddress内部的属性:
- Class<InetAddress> type = InetAddress.class;
-
- Field[] fields = type.getDeclaredFields();
-
- for (Field f : fields) {
- System.out.println(f.getName() + ":" + f.getType());
- }
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OpenJDK:
- IPv4
- IPv6
- preferIPv6Address
- holder
- nameServices
- canonicalHostName
- serialVersionUID
- addressCache
- negativeCache
- addressCacheInit
- unknown_array
- impl
- lookupTable
- cachedLocalHost
- cacheTime
- maxCacheTime
- cacheLock
- FIELDS_OFFSET
- UNSAFE
- serialPersistentFields
- $assertionsDisabled
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JRockit:
- IPv4
- IPv6
- preferIPv6Address
- hostName
- address
- family
- nameService
- canonicalHostName
- serialVersionUID
- addressCache
- negativeCache
- addressCacheInit
- unknown_array
- impl
- lookupTable
- $assertionsDisabled
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IBM JDK
- IPv4:int
- IPv6:int
- preferIPv6Address
- hostName:class java.lang.String
- address:int
- family:int
- nameService:interface sun.net.spi.nameservice.NameService
- canonicalHostName:class java.lang.String
- serialVersionUID:long
- addressCache:class java.net.InetAddress$Cache
- negativeCache:class java.net.InetAddress$Cache
- localHostName:class java.lang.String
- localHostNameLock:class java.lang.Object
- cacheLocalHost:boolean
- addressCacheInit:boolean
- unknown_array:class [Ljava.net.InetAddress;
- impl:interface java.net.InetAddressImpl
- lookupTable:class java.util.HashMap
- $assertionsDisabled:boolean
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看到这里, 知道蛋疼了吧. 三种JDK中,
OpenJDK中是nameservices是个List<NameService>,
Jrockit与IBM JVM中却是nameservice, 只是单独的NameService实例.
所以要用第2种做法, 你至少要满足这三种主流JDK的需求.
简单实现二种做法:
做法1, 动态替换AddressCache.
做法2, 动态代理NameService.
源码如下
- 长度限制, 源码查看回贴.
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暂时测试到这吧, 有兴趣的同学可以一起完善. 争取同时满足OpenJDK, Jrockit, IBM JDK三种主流环境的DNS动态解析类.
Java动态替换InetAddress中DNS的做法简单分析1
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原文地址：http://www.cnblogs.com/zolo/p/5849242.html
