JDK中DNS缓存的分析

在JAVA中使用InetAddress.getByName(String host) 方法来获取给定hostname的IP地址。为了减少DNS解析的请求次数，提高解析效率，InetAddress中提供cache来缓存解析结果。

下面就此cache进行简单的分析：

该缓存实现比较简单，巧妙的利用LinkedHashMap的特性来进行过期条目的检测和移除。

    /*** Represents a cache entry*/static final class CacheEntry {CacheEntry(Object address, long expiration) {this.address = address;this.expiration = expiration;}Object address;long expiration;}//CacheEntry实例代表一个缓存记录，一个记录中包括address（IP 地址） 和 expiration/*** A cache that manages entries based on a policy specified* at creation time.*/static final class Cache {private LinkedHashMap cache;private Type type;enum Type {Positive, Negative};//此缓存只提供两种缓存类型 Positive: DNS解析成功    Negative：DNS解析失败/*** Create cache*/public Cache(Type type) {this.type = type;cache = new LinkedHashMap();//LinkedHashMap 保存了记录的插入顺序，当遍历LindedHashMap时得到的数据是最先插入的数据，此特性很重要在put方法中有所体现}private int getPolicy() {//获取配置的缓存策略   0: 不缓存  -1: 代表永久缓存  >=1：代表缓存的时间(unit: second)if (type == Type.Positive) {return InetAddressCachePolicy.get();} else {return InetAddressCachePolicy.getNegative();}}/*** Add an entry to the cache. If there's already an* entry then for this host then the entry will be* replaced.*/public Cache put(String host, Object address) {int policy = getPolicy();if (policy == InetAddressCachePolicy.NEVER) {return this;}// purge any expired entriesif (policy != InetAddressCachePolicy.FOREVER) {// As we iterate in insertion order we can// terminate when a non-expired entry is found.LinkedList expired = new LinkedList();Iterator i = cache.keySet().iterator();//每次put的时候都对缓存记录做一个清理，由于每个条目的过期时间是一样的，所以先插入的记录就先到期long now = System.currentTimeMillis();while (i.hasNext()) {String key = (String)i.next();CacheEntry entry = (CacheEntry)cache.get(key);if (entry.expiration >= 0 && entry.expiration < now) {expired.add(key);} else {break;}}i = expired.iterator();while (i.hasNext()) {cache.remove(i.next());}}// create new entry and add it to the cache// -- as a HashMap replaces existing entries we//    don't need to explicitly check if there is//    already an entry for this host.long expiration;if (policy == InetAddressCachePolicy.FOREVER) {expiration = -1;} else {expiration = System.currentTimeMillis() + (policy * 1000);}CacheEntry entry = new CacheEntry(address, expiration);cache.put(host, entry);return this;}/*** Query the cache for the specific host. If found then* return its CacheEntry, or null if not found.*/public CacheEntry get(String host) {int policy = getPolicy();if (policy == InetAddressCachePolicy.NEVER) {return null;}CacheEntry entry = (CacheEntry)cache.get(host);// check if entry has expiredif (entry != null && policy != InetAddressCachePolicy.FOREVER) {//命中缓存条目后先判断是否过期if (entry.expiration >= 0 &&entry.expiration < System.currentTimeMillis()) {cache.remove(host);entry = null;}}return entry;}}