/*
    * Copyright 2006-2013 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.database;
  
  import java.util.List;
  
  import org.apache.commons.logging.Log;
  import org.apache.commons.logging.LogFactory;
  import org.springframework.batch.item.support.AbstractItemCountingItemStreamItemReader;
  import org.springframework.beans.factory.InitializingBean;
  import org.springframework.util.Assert;
  import org.springframework.util.ClassUtils;
  
  /**
   * Abstract {@link org.springframework.batch.item.ItemReader} for to extend when
   * reading database records in a paging fashion.
   *
   * <p>
   * Implementations should execute queries using paged requests of a size
   * specified in {@link #setPageSize(int)}. Additional pages are requested when
   * needed as {@link #read()} method is called, returning an object corresponding
   * to current position.
   * </p>
   *
   * @author Thomas Risberg
   * @author Dave Syer
   * @since 2.0
   */
  public abstract class AbstractPagingItemReader<T> extends AbstractItemCountingItemStreamItemReader<T> implements
  InitializingBean {
  
  	protected Log logger = LogFactory.getLog(getClass());
  
  	private volatile boolean initialized = false;
  
  	private int pageSize = 10;
  
  	private volatile int current = 0;
  
  	private volatile int page = 0;
  
  	protected volatile List<T> results;
  
  	private Object lock = new Object();
  
  	public AbstractPagingItemReader() {
  		setName(ClassUtils.getShortName(AbstractPagingItemReader.class));
  	}
  
  	/**
  	 * The current page number.
  	 * @return the current page
  	 */
  	public int getPage() {
  		return page;
  	}
  
  	/**
  	 * The page size configured for this reader.
  	 * @return the page size
  	 */
  	public int getPageSize() {
  		return pageSize;
  	}
  
  	/**
  	 * The number of rows to retrieve at a time.
  	 *
  	 * @param pageSize the number of rows to fetch per page
  	 */
  	public void setPageSize(int pageSize) {
  		this.pageSize = pageSize;
  	}
  
  	/**
  	 * Check mandatory properties.
  	 * @see org.springframework.beans.factory.InitializingBean#afterPropertiesSet()
  	 */
  	@Override
  	public void afterPropertiesSet() throws Exception {
  		Assert.isTrue(pageSize > 0, "pageSize must be greater than zero");
  	}
  
  	@Override
  	protected T doRead() throws Exception {
  
 		synchronized (lock) {
 
 			if (results == null || current >= pageSize) {
 
 				if (logger.isDebugEnabled()) {
 					logger.debug("Reading page " + getPage());
 				}
 
 				doReadPage();
 				page++;
 				if (current >= pageSize) {
 					current = 0;
 				}
 
 			}
 
 			int next = current++;
 			if (next < results.size()) {
 				return results.get(next);
 			}
 			else {
 				return null;
 			}
 
 		}
 
 	}
 
 	abstract protected void doReadPage();
 
 	@Override
 	protected void doOpen() throws Exception {
 
 		Assert.state(!initialized, "Cannot open an already opened ItemReader, call close first");
 		initialized = true;
 
 	}
 
 	@Override
 	protected void doClose() throws Exception {
 
 		synchronized (lock) {
 			initialized = false;
 			current = 0;
 			page = 0;
 			results = null;
 		}
 
 	}
 
 	@Override
 	protected void jumpToItem(int itemIndex) throws Exception {
 
 		synchronized (lock) {
 			page = itemIndex / pageSize;
 			current = itemIndex % pageSize;
 		}
 
 		doJumpToPage(itemIndex);
 
 		if (logger.isDebugEnabled()) {
 			logger.debug("Jumping to page " + getPage() + " and index " + current);
 		}
 
 	}
 
 	abstract protected void doJumpToPage(int itemIndex);
 
 }