Sunday, January 4, 2015

Google Guava Cache Implementation

Caches are very useful in a range of use cases. for frequent data you can have it in cache instead of doing some computing intensive operation that will help you to achieve a good performance.

you can build a cache using a simple hashmap implementation where you put your data in the key/value pair form. putIfAbsent() method of hashmap is very handy to update the hashmap if lookup for the key fails.

in the multithreaded environment you can suffer a dirty read in case you are using a simple hashmap implementation. for the multithreaded environment you need of focus on the ConcurrentHashMap which will provide you concurrency.

ConcurrentMap based caching is good but you need to adopt a use defined eviction policy to remove element which are not used frequently.
Google guava api provide you more flexible caching based on the ConcurrentMap with more cleaner implementation with a limitation that it not store data in files or on some server e.g. Memcached.Apart from the eviction policy it also support the putIfAbsent scenerio well.

in the coming example lets see how simple is to create and use Guava based cache.
package com.rajkrrsingh.test.guava;

import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;

public class GuavaCache {
 
  private static final long MAX_SIZE = 100;
 
  private final LoadingCache<String, String> cache;
 
  public GuavaCache() {
    cache = CacheBuilder.newBuilder().maximumSize( MAX_SIZE ).build( new CacheLoader<String, String>() {
        @Override
        public String load( String key ) throws Exception {
          return key.toUpperCase();
        }
      }
    );
  }
 
  public String getEntry( String key ) {
    return cache.getUnchecked( key );
  }
 
  
  public static void main(String[] args) {
 GuavaCache gchache = new GuavaCache();
 System.out.println(gchache.getEntry("hello"));
 System.out.println(gchache.cache.size());
 for (int i = 0; i < 150; i++) {
  System.out.println(gchache.getEntry("hello"+i));
  System.out.println(gchache.cache.size());
 }
 // checking the eviction policy
 System.out.println(gchache.cache.getIfPresent("hello"));
}
}
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