org.springframework.batch.item.support.SingleItemPeekableItemReader.java Source code

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Here is the source code for org.springframework.batch.item.support.SingleItemPeekableItemReader.java

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/*
 * Copyright 2006-2010 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.support;

import java.util.Map.Entry;

import org.springframework.batch.item.ExecutionContext;
import org.springframework.batch.item.ItemReader;
import org.springframework.batch.item.ItemStream;
import org.springframework.batch.item.ItemStreamException;
import org.springframework.batch.item.ItemStreamReader;
import org.springframework.batch.item.ParseException;
import org.springframework.batch.item.PeekableItemReader;
import org.springframework.batch.item.UnexpectedInputException;

/**
 * <p>
 * A {@link PeekableItemReader} that allows the user to peek one item ahead.
 * Repeated calls to {@link #peek()} will return the same item, and this will be
 * the next item returned from {@link #read()}.
 * </p>
 * 
 * <p>
 * Intentionally <b>not</b> thread-safe: it wouldn't be possible to honour the peek in
 * multiple threads because only one of the threads that peeked would get that
 * item in the next call to read.
 * </p>
 * 
 * @author Dave Syer
 * 
 */
public class SingleItemPeekableItemReader<T> implements ItemStreamReader<T>, PeekableItemReader<T> {

    private ItemReader<T> delegate;

    private T next;

    private ExecutionContext executionContext = new ExecutionContext();

    /**
     * The item reader to use as a delegate. Items are read from the delegate
     * and passed to the caller in {@link #read()}.
     * 
     * @param delegate the delegate to set
     */
    public void setDelegate(ItemReader<T> delegate) {
        this.delegate = delegate;
    }

    /**
     * Get the next item from the delegate (whether or not it has already been
     * peeked at).
     * 
     * @see ItemReader#read()
     */
    @Override
    public T read() throws Exception, UnexpectedInputException, ParseException {
        if (next != null) {
            T item = next;
            next = null;
            // executionContext = new ExecutionContext();
            return item;
        }
        return delegate.read();
    }

    /**
     * Peek at the next item, ensuring that if the delegate is an
     * {@link ItemStream} the state is stored for the next call to
     * {@link #update(ExecutionContext)}.
     * 
     * @return the next item (or null if there is none).
     * 
     * @see PeekableItemReader#peek()
     */
    @Override
    public T peek() throws Exception, UnexpectedInputException, ParseException {
        if (next == null) {
            updateDelegate(executionContext);
            next = delegate.read();
        }
        return next;
    }

    /**
     * If the delegate is an {@link ItemStream}, just pass the call on,
     * otherwise reset the peek cache.
     * 
     * @throws ItemStreamException if there is a problem
     * @see ItemStream#close()
     */
    @Override
    public void close() throws ItemStreamException {
        next = null;
        if (delegate instanceof ItemStream) {
            ((ItemStream) delegate).close();
        }
        executionContext = new ExecutionContext();
    }

    /**
     * If the delegate is an {@link ItemStream}, just pass the call on,
     * otherwise reset the peek cache.
     * 
     * @param executionContext the current context
     * @throws ItemStreamException if there is a problem
     * @see ItemStream#open(ExecutionContext)
     */
    @Override
    public void open(ExecutionContext executionContext) throws ItemStreamException {
        next = null;
        if (delegate instanceof ItemStream) {
            ((ItemStream) delegate).open(executionContext);
        }
        executionContext = new ExecutionContext();
    }

    /**
     * If there is a cached peek, then retrieve the execution context state from
     * that point. If there is no peek cached, then call directly to the
     * delegate.
     * 
     * @param executionContext the current context
     * @throws ItemStreamException if there is a problem
     * @see ItemStream#update(ExecutionContext)
     */
    @Override
    public void update(ExecutionContext executionContext) throws ItemStreamException {
        if (next != null) {
            // Get the last state from the delegate instead of using
            // current value.
            for (Entry<String, Object> entry : this.executionContext.entrySet()) {
                executionContext.put(entry.getKey(), entry.getValue());
            }
            return;
        }
        updateDelegate(executionContext);
    }

    private void updateDelegate(ExecutionContext executionContext) {
        if (delegate instanceof ItemStream) {
            ((ItemStream) delegate).update(executionContext);
        }
    }

}