Closes the pool. Once the pool is closed, {@link #borrowObject()} will
fail with IllegalStateException, but {@link #returnObject(Object)} and
{@link #invalidateObject(Object)} will continue to work, with returned
objects destroyed on return.
<p>
Destroys idle instances in the pool by invoking {@link #clear()}.
</p>
Closes the pool. Once the pool is closed, {@link #borrowObject()} will fail with IllegalStateException, but {@link #returnObject(Object)} and {@link #invalidateObject(Object)} will continue to work, with returned objects destroyed on return. <p> Destroys idle instances in the pool by invoking {@link #clear()}. </p>