Overview (Apache Commons Object Pooling Package)

Packages
Package	Description
org.apache.commons.pool	Object pooling API.
org.apache.commons.pool.impl	Object pooling API implementations.

Generic Object pooling API with several implementations.

The org.apache.commons.pool package defines a simple interface for a pool of object instances, and a handful of base classes that may be useful when creating pool implementations. The API supports pooling of unique objects which can be requested via a key as well as pools where all objects are equivalent.

The org.apache.commons.pool.impl package contains several pool implementations. StackObjectPool is useful for supporting reuse of a limited number of instances while allowing new instances to be created as needed to support high demand. GenericObjectPool has many configuration options and can support a limited set of objects such as would be useful in a database connection pool. SoftReferenceObjectPool has no limit on the number of objects in the pool, but the garbage collector can remove idle objects from the pool as needed. There are also keyed versions of the first two.

Here is a simple example of pooling HashMap's. First create an ObjectPoolFactory

    public class HashMapFactory
        extends BasePoolableObjectFactory
    {
        /**
         * Creates an instance that can be returned by the pool.
         * @return an instance that can be returned by the pool.
         */
        public Object makeObject()
            throws Exception
        {
            return new HashMap();
        }

        /**
         * Uninitialize an instance to be returned to the pool.
         * @param obj the instance to be passivated
         */
        public void passivateObject(Object obj)
            throws Exception
        {
            Map map = (Map)obj;
            map.clear();
        }
    }

A class that makes frequent use of a Map could then use a pool as shown below:

    public class Foo
    {
        private ObjectPool pool;
        public Foo()
        {
            PoolableObjectFactory factory = new HashMapFactory();
            pool = new StackObjectPool(factory, 1000);
        }

        public doSomething()
        {
            ...
            Map map = null;
            try
            {
                map = (Map)pool.borrowObject();
                // use map
                ...
            }
            finally
            {
                if (map != null)
                {
                    pool.returnObject(map);
                }
            }
            ...
        }
    }

The above example shows how one would use an ObjectPool. The other supplied implementations or another special purpose pool would be used similarly.