/*
 * Copyright 2006-2018 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
 *
 *      https://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;

import org.springframework.lang.Nullable;

/**
 * <p>
 * A specialisation of {@link ItemReader} that allows the user to look ahead into the
 * stream of items. This is useful, for instance, when reading flat file data that
 * contains record separator lines which are actually part of the next record.
 * </p>
 *
 * <p>
 * The detailed contract for {@link #peek()} has to be defined by the implementation
 * because there is no general way to define it in a concurrent environment. The
 * definition of "the next read()" operation is tenuous if multiple clients are reading
 * concurrently, and the ability to peek implies that some state is likely to be stored,
 * so implementations of {@link PeekableItemReader} may well be restricted to single
 * threaded use.
 * </p>
 *
 * @author Dave Syer
 * @author Mahmoud Ben Hassine
 *
 */
public interface PeekableItemReader<T> extends ItemReader<T> {

	/**
	 * Get the next item that would be returned by {@link #read()}, without affecting the
	 * result of {@link #read()}.
	 * @return the next item or {@code null} if the data source is exhausted
	 * @throws Exception if there is a problem
	 */
	@Nullable
	T peek() throws Exception, UnexpectedInputException, ParseException;

}
