org.springframework.batch.item.data.AbstractPaginatedDataItemReader.java Source code

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Here is the source code for org.springframework.batch.item.data.AbstractPaginatedDataItemReader.java

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/*
 * Copyright 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
 *
 *      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.data;

import org.springframework.batch.item.ItemReader;
import org.springframework.batch.item.ItemStreamReader;
import org.springframework.batch.item.support.AbstractItemCountingItemStreamItemReader;
import org.springframework.util.Assert;

import java.util.Iterator;

/**
 * A base class that handles basic reading logic based on the paginated
 * semantics of Spring Data's paginated facilities.  It also handles the
 * semantics required for restartability based on those facilities.
 * 
 * @author Michael Minella
 * @author Glenn Renfro
 * @since 2.2
 * @param <T> Type of item to be read
 */
public abstract class AbstractPaginatedDataItemReader<T> extends AbstractItemCountingItemStreamItemReader<T> {

    protected volatile int page = 0;

    protected int pageSize = 10;

    protected Iterator<T> results;

    private Object lock = new Object();

    /**
     * The number of items to be read with each page.
     * 
     * @param pageSize the number of items.  pageSize must be greater than zero.
     */
    public void setPageSize(int pageSize) {
        Assert.isTrue(pageSize > 0, "pageSize must be greater than zero");
        this.pageSize = pageSize;
    }

    @Override
    protected T doRead() throws Exception {

        synchronized (lock) {
            if (results == null || !results.hasNext()) {

                results = doPageRead();

                page++;

                if (results == null || !results.hasNext()) {
                    return null;
                }
            }

            if (results.hasNext()) {
                return results.next();
            } else {
                return null;
            }
        }
    }

    /**
     * Method this {@link ItemStreamReader} delegates to
     * for the actual work of reading a page.  Each time
     * this method is called, the resulting {@link Iterator}
     * should contain the items read within the next page.
     * <br><br>
     * If the {@link Iterator} is empty or null when it is
     * returned, this {@link ItemReader} will assume that the
     * input has been exhausted.
     * 
     * @return an {@link Iterator} containing the items within a page.
     */
    protected abstract Iterator<T> doPageRead();

    @Override
    protected void doOpen() throws Exception {
    }

    @Override
    protected void doClose() throws Exception {
    }

    @Override
    protected void jumpToItem(int itemLastIndex) throws Exception {
        synchronized (lock) {
            page = itemLastIndex / pageSize;
            int current = itemLastIndex % pageSize;

            Iterator<T> initialPage = doPageRead();

            for (; current >= 0; current--) {
                initialPage.next();
            }
        }
    }
}