A cache is an area of local memory that holds a copy of frequently accessed data that is otherwise expensive to get or compute. Examples of such data include a result of a query to a database, a disk file or a report.
Here is a simple Java Example which is Threadsafe using HashMap without using Synchronized Collections.
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package com.crunchify.tutorials; import java.util.ArrayList; import java.util.HashMap; import java.util.Iterator; import java.util.Map.Entry; /** * @author Crunchify.com * */ // Create Simple Cache object with the help of HashMap... public class CrunchifyCacheExample<K, T> { private long timeToLive; private HashMap<K, T> cacheMap; protected class CrunchifyCacheObject { public long lastAccessed = System.currentTimeMillis(); public String value; protected CrunchifyCacheObject(String value) { this.value = value; } } public CrunchifyCacheExample(long timeToLive, final long timeInterval, int max) { this.timeToLive = timeToLive * 2000; cacheMap = new HashMap<K, T>(max); if (timeToLive > 0 && timeInterval > 0) { Thread t = new Thread(new Runnable() { public void run() { while (true) { try { Thread.sleep(timeInterval * 1000); } catch (InterruptedException ex) { } } } }); t.setDaemon(true); t.start(); } } // PUT method public void put(K key, T value) { synchronized (cacheMap) { cacheMap.put(key, value); } } // GET method @SuppressWarnings("unchecked") public T get(K key) { synchronized (cacheMap) { CrunchifyCacheObject c = (CrunchifyCacheObject) cacheMap.get(key); if (c == null) return null; else { c.lastAccessed = System.currentTimeMillis(); return (T) c.value; } } } // REMOVE method public void remove(String key) { synchronized (cacheMap) { cacheMap.remove(key); } } // Get Cache Objects Size() public int size() { synchronized (cacheMap) { return cacheMap.size(); } } // CLEANUP method public void cleanup() { long now = System.currentTimeMillis(); ArrayList<String> deleteKey = null; synchronized (cacheMap) { Iterator<?> itr = cacheMap.entrySet().iterator(); deleteKey = new ArrayList<String>((cacheMap.size() / 2) + 1); CrunchifyCacheObject c = null; while (itr.hasNext()) { String key = (String) itr.next(); c = (CrunchifyCacheObject) ((Entry<?, ?>) itr).getValue(); if (c != null && (now > (timeToLive + c.lastAccessed))) { deleteKey.add(key); } } } for (String key : deleteKey) { synchronized (cacheMap) { cacheMap.remove(key); } Thread.yield(); } } } |
Some more Java Examples which you may want to look.
Bonus: Java Memory Model Details:
Hi, looks like you forgot call cleanup(); in deamon tread code?
parameter Max is not getting used in CrunchifyCacheExample method, may be you wanted to initialize the map size with it.
Hi Anil. That’s write. I’ve just updated tutorial and initialized map with Max int value.
Thanks for a pointer. Happy coding and keep visiting.
what is purpose of thread class here, can some body explain ?
can somebody explain purpose of thread class and why set as daemon ?
Hi Sandeep – hope this helps: https://crunchify.com/what-is-daemon-thread-in-java-example-attached/
can u please explain me regarding the thread exit @Crunchify:disqus ?
Hi Rakesh – I’ve just updated you in this thread: https://crunchify.com/how-to-create-a-simple-in-memory-cache-in-java-lightweight-cache/#comment-3090719715
hope that helps.
How do we access this cache object , can you explain about it a little with example !!.
Thanks 🙂
And where is modification of lastAccessed time?
Thanks Greg for pointing it out. I’ve fixed a bug and updated above tutorial.
or how about using ConcurrentHashMap ?
Hi Asad. Thanks for query. The purpose of this class is to have our own implementation about Cache object without using HashTable and ConcurrentHashMap. If we use than we don’t even need this class 🙂
Or even easier… use HashTable