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/*
* Copyright (c) 2010-2015 Pivotal Software, Inc. All rights reserved.
*
* Licensed under the Apache License, Version 2.0 (the "License"); you
* may not use this file except in compliance with the License. You
* may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
* implied. See the License for the specific language governing
* permissions and limitations under the License. See accompanying
* LICENSE file.
*/
package com.gemstone.gemfire.cache;
/**
* Allows data from outside of the VM to be placed into a region.
* When {@link com.gemstone.gemfire.cache.Region#get(Object)} is called for a region
* entry that has a null value, the
* {@link CacheLoader#load load} method of the
* region's cache loader is invoked. The load method
* creates the value for the desired key by performing an operation such
* as a database query. The load may also perform a
* {@linkplain LoaderHelper#netSearch net search}
* that will look for the value in a cache instance hosted by
* another member of the distributed system.
*
* @author Dave Monnie
*
*
* @see AttributesFactory#setCacheLoader
* @see RegionAttributes#getCacheLoader
* @see AttributesMutator#setCacheLoader
* @since 2.0
*/
public interface CacheLoader extends CacheCallback {
/**
* Loads a value. Application writers should implement this
* method to customize the loading of a value. This method is called
* by the caching service when the requested value is not in the cache.
* Any exception (including an unchecked exception) thrown by this
* method is propagated back to and thrown by the invocation of
* {@link Region#get(Object, Object)} that triggered this load.
*
*
* @param helper a LoaderHelper object that is passed in from cache service
* and provides access to the key, region, argument, and netSearch.
* @return the value supplied for this key, or null if no value can be
* supplied. A local loader will always be invoked if one exists.
* Otherwise one remote loader is invoked.
* Returning null causes
* {@link Region#get(Object, Object)} to return null.
* @throws CacheLoaderException, if an error occurs. This exception or any
* other exception thrown by this method will be propagated back to the
* application from the get method.
*
* @see Region#get(Object, Object) Region.get
*/
public V load(LoaderHelper helper)
throws CacheLoaderException;
}