java - HashMap is broken/ performance issues -



java - HashMap is broken/ performance issues -

currently have hashmap implemented

private static map<string, item> cached = new hashmap<string, item>();

and item object properties date expirationtime , , byte[] data

this map used when multiple threads concurrently start hitting this. check

1.

class="lang-java prettyprint-override">public static final byte[] getcacheddata(httpservletrequest request) throws servletexception { string url = getfullurl(request); map<string, item> cache = getcache(request); // chec item item = null; synchronized (cache) { item = cache.get(url); if (null == item) homecoming null; // create sure not on hr old. if (item.expirationtime.gettime() < system.currenttimemillis()) { cache.remove(url); item = null; } } if (null == item) { log.info("expiring item: " + url); homecoming null; } homecoming item.data; }

2. if info returned null, create , info , cache in hashmap

class="lang-java prettyprint-override">public static void cachedatax(httpservletrequest request, byte[] data, integer minutes) throws servletexception { item item = new item(data); string url = getfullurl(request); map<string, item> cache = getcache(request); log.info("caching item: " + url + " - bytes: " + data.length); synchronized (cache) { calendar cal = calendar.getinstance(); cal.add(calendar.minute, minutes); item.expirationtime = cal.gettime(); cache.put(url, item); } }

seems if multiple threads access key (url in case) , info gets added cache more 1 time @ same key location [ getcachedata homecoming null multiple threads since hashmap has not finished writing info first thread ]

any suggestions how solve issue ?

in cachedatax add together check existence of item before add together (inside of synchronized block).

synchronized (cache) { if (cache.get(url) == null) { calendar cal = calendar.getinstance(); cal.add(calendar.minute, minutes); item.expirationtime = cal.gettime(); cache.put(url, item); } }

this ensure multiple threads have done lookup , returned null cannot add together same info cache. 1 add together , others threads silently ignore since cache has been updated.

java performance hashmap concurrenthashmap

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