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The MongoDB Connector for Hadoop is a plugin for Hadoop that provides the ability to use MongoDB as an input source and/or an output destination.

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/*
 * Copyright 2010-2013 10gen Inc.
 *
 * 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 com.mongodb.hadoop.util;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.bson.BSONCallback;
import org.bson.BSONDecoder;
import org.bson.BSONObject;
import org.bson.BasicBSONCallback;
import org.bson.BasicBSONDecoder;

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.atomic.AtomicBoolean;

public class BSONLoader implements Iterable, Iterator {

    private static final Log LOG = LogFactory.getLog(BSONLoader.class);

    private final BSONDecoder decoder = new BasicBSONDecoder();
    private final BSONCallback callback = new BasicBSONCallback();

    private volatile byte[] nextHdr;
    private volatile int nextLen;
    private AtomicBoolean hasMore = new AtomicBoolean(true);
    private final DataInputStream input;

    public BSONLoader(final InputStream input) {
        this.input = new DataInputStream(input);
    }

    public Iterator iterator() {
        return this;
    }

    public boolean hasNext() {
        checkHeader();
        return hasMore.get();
    }

    private synchronized void checkHeader() {
        // Read the BSON length from the start of the record
        byte[] l = new byte[4];
        try {
            input.readFully(l);
            nextLen = org.bson.io.Bits.readInt(l);
            nextHdr = l;
            hasMore.set(true);
        } catch (Exception e) {
            if (LOG.isDebugEnabled()) {
                LOG.debug("Failed to get next header: " + e, e);
            }
            hasMore.set(false);
            try {
                input.close();
            } catch (IOException e1) {
                LOG.warn(e1.getMessage(), e1);
            }
        }
    }

    public BSONObject next() {
        try {
            byte[] data = new byte[nextLen + 4];
            System.arraycopy(nextHdr, 0, data, 0, 4);
            input.readFully(data, 4, nextLen - 4);
            decoder.decode(data, callback);
            return (BSONObject) callback.get();
        } catch (IOException e) {
            /* If we can't read another length it's not an error, just return quietly. */
            LOG.info("No Length Header available." + e);
            hasMore.set(false);
            try {
                input.close();
            } catch (IOException e1) {
                LOG.warn(e1.getMessage(), e1);
            }
            throw new NoSuchElementException("Iteration completed.");
        }
    }

    public void remove() {
        throw new UnsupportedOperationException();
    }
}