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 entries        if (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 expired        if (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;    }    }