Looking for a drop-in replacement for a java.util.Map

Question

Problem

Following up on this question, it seems that a file- or disk-based Map implementation may be the right solution to the problems I mentioned there.

Answer 1

UPDATE (some 4 years after first post...): beware that in newer versions of ehcache, persistence of cache items is available only in the pay product. Thanks @boday for pointing this out.

ehcache is great. It will give you the flexibility you need to implement the map in memory, disk or memory with spillover to disk. If you use this very simple wrapper for java.util.Map then using it is blindingly simple:

import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.Set;

import net.sf.ehcache.Cache;
import net.sf.ehcache.Element;

import org.apache.log4j.Logger;

import com.google.common.collect.Sets;

public class EhCacheMapAdapter implements Map {
    @SuppressWarnings("unused")
    private final static Logger logger = Logger
            .getLogger(EhCacheMapAdapter.class);

    public Cache ehCache;

    public EhCacheMapAdapter(Cache ehCache) {
        super();
        this.ehCache = ehCache;
    } // end constructor

    @Override
    public void clear() {
        ehCache.removeAll();
    } // end method

    @Override
    public boolean containsKey(Object key) {
        return ehCache.isKeyInCache(key);
    } // end method

    @Override
    public boolean containsValue(Object value) {
        return ehCache.isValueInCache(value);
    } // end method

    @Override
    public Set> entrySet() {
        throw new UnsupportedOperationException();
    } // end method

    @SuppressWarnings("unchecked")
    @Override
    public V get(Object key) {
        if( key == null ) return null;
        Element element = ehCache.get(key);
        if( element == null ) return null;
        return (V)element.getObjectValue();
    } // end method

    @Override
    public boolean isEmpty() {
        return ehCache.getSize() == 0;
    } // end method

    @SuppressWarnings("unchecked")
    @Override
    public Set keySet() {
        List l = ehCache.getKeys();
        return Sets.newHashSet(l);
    } // end method

    @SuppressWarnings("unchecked")
    @Override
    public V put(K key, V value) {
        Object o = this.get(key);
        if( o != null ) return (V)o;
        Element e = new Element(key,value);
        ehCache.put(e);
        return null;
    } // end method


    @Override
    public V remove(Object key) {
        V retObj = null;
        if( this.containsKey(key) ) {
            retObj = this.get(key);
        } // end if
        ehCache.remove(key);
        return retObj;
    } // end method

    @Override
    public int size() {
        return ehCache.getSize();
    } // end method

    @Override
    public Collection values() {
        throw new UnsupportedOperationException();
    } // end method

    @Override
    public void putAll(Map m) {
        for( K key : m.keySet() ) {
            this.put(key, m.get(key));
        } // end for
    } // end method
} // end class