免费发广告帖子的网站,北京赛车网站建设,全屋定制十大名牌衣柜,wordpress菜单选项如何链接在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#xff0c;提高解析效率#xff0c;InetAddress中提供cache来缓存解析结果。 下面就此cache进行简单的分析#xff1a; 该缓存实现比较简单#xff0c;巧妙的利…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addressIP 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 NegativeDNS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 theres 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// dont 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;}} 转载于:https://www.cnblogs.com/cruze/p/3707011.html
