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The Spring Batch Infrastructure is a set of low-level components, interfaces and tools for batch processing applications and optimisations.

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/*
 * Copyright 2006-2007 the original author or authors.
 *
 * 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.
 */

package org.springframework.batch.item;

/**
 * Strategy interface for providing the data. 

 * 
 * Implementations are expected to be stateful and will be called multiple times
 * for each batch, with each call to {@link #next} returning a different value
 * and finally returning null when all input data is exhausted.

 * 
 * Implementations need to be thread safe and clients of a {@link ItemReader}
 * need to be aware that this is the case.

 * 
 * A richer interface (e.g. with a look ahead or peek) is not feasible because
 * we need to support transactions in an asynchronous batch.
 * 
 * @author Rob Harrop
 * @author Dave Syer
 * @author Lucas Ward
 * @since 1.0
 */
public interface ItemReader {

	/**
	 * Reads a piece of input data and advance to the next one. Implementations
	 * must return null at the end of the input
	 * data set. In a transactional setting, caller might get the same item
	 * twice from successive calls (or otherwise), if the first call was in a
	 * transaction that rolled back.
	 * 
	 * @throws Exception if an underlying resource is unavailable.
	 */
	Object read() throws Exception, UnexpectedInputException, NoWorkFoundException, ParseException;

	/**
	 * Mark the stream so that it can be reset later and the items backed out.

	 * 
	 * Mark is called before reading a new chunk of items - in case of rollback
	 * mark will not be called again before re-processing the chunk.

	 * 
	 * In a multi-threaded setting implementations have to ensure that only the
	 * state from the current thread is saved.
	 * 
	 * @throws MarkFailedException if there is a problem with the mark. If a
	 * mark fails inside a transaction, it would be worrying, but not normally
	 * fatal.
	 */
	void mark() throws MarkFailedException;

	/**
	 * Reset the stream to the last mark. After a reset the stream state will be
	 * such that changes (items read or written) since the last call to mark
	 * will not be visible after a call to close.

	 * 
	 * In a multi-threaded setting implementations have to ensure that only the
	 * state from the current thread is reset.
	 * 
	 * @throws ResetFailedException if there is a problem with the reset. If a
	 * reset fails inside a transaction, it would normally be fatal, and would
	 * leave the stream in an inconsistent state. So while this is an unchecked
	 * exception, it may be important for a client to catch it explicitly.
	 */
	void reset() throws ResetFailedException;

}