com.cloudera.sqoop.io.LobReaderCache.java Source code

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Here is the source code for com.cloudera.sqoop.io.LobReaderCache.java

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/**
 * Licensed to Cloudera, Inc. under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  Cloudera, Inc. licenses this file
 * to you 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.cloudera.sqoop.io;

import java.io.IOException;
import java.util.Map;
import java.util.TreeMap;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.conf.Configuration;

/**
 * A cache of open LobFile.Reader objects.
 * This maps from filenames to the open Reader, if any.  This uses the
 * Singleton pattern. While nothing prevents multiple LobReaderCache
 * instances, it is most useful to have a single global cache. This cache is
 * internally synchronized; only one thread can insert or retrieve a reader
 * from the cache at a time. 
 */
public final class LobReaderCache {

    public static final Log LOG = LogFactory.getLog(LobReaderCache.class.getName());

    private Map<Path, LobFile.Reader> readerMap;

    private LobReaderCache() {
        this.readerMap = new TreeMap<Path, LobFile.Reader>();
    }

    private static final LobReaderCache CACHE;
    static {
        CACHE = new LobReaderCache();
    }

    /**
     * @return the singleton LobReaderCache instance.
     */
    public static LobReaderCache getCache() {
        return CACHE;
    }

    /**
     * Created a fully-qualified path object.
     * @param path the path to fully-qualify with its fs URI.
     * @param conf the current Hadoop FS configuration.
     * @return a new path representing the same location as the input 'path',
     * but with a fully-qualified URI.
     */
    public static Path qualify(Path path, Configuration conf) throws IOException {
        if (null == path) {
            return null;
        }

        FileSystem fs = path.getFileSystem(conf);
        if (null == fs) {
            fs = FileSystem.get(conf);
        }
        return path.makeQualified(fs);
    }

    /**
     * Open a LobFile for read access, returning a cached reader if one is
     * available, or a new reader otherwise.
     * @param path the path to the LobFile to open
     * @param conf the configuration to use to access the FS.
     * @throws IOException if there's an error opening the file.
     */
    public LobFile.Reader get(Path path, Configuration conf) throws IOException {

        LobFile.Reader reader = null;
        Path canonicalPath = qualify(path, conf);
        // Look up an entry in the cache.
        synchronized (this) {
            reader = readerMap.remove(canonicalPath);
        }

        if (null != reader && !reader.isClosed()) {
            // Cache hit. return it.
            LOG.debug("Using cached reader for " + canonicalPath);
            return reader;
        }

        // Cache miss; open the file.
        LOG.debug("No cached reader available for " + canonicalPath);
        return LobFile.open(path, conf);
    }

    /**
     * Return a reader back to the cache. If there's already a reader for
     * this path, then the current reader is closed.
     * @param reader the opened reader. Any record-specific subreaders should be
     * closed.
     * @throws IOException if there's an error accessing the path's filesystem.
     */
    public void recycle(LobFile.Reader reader) throws IOException {
        Path canonicalPath = reader.getPath();

        // Check if the cache has a reader for this path already. If not, add this.
        boolean cached = false;
        synchronized (this) {
            if (readerMap.get(canonicalPath) == null) {
                LOG.debug("Caching reader for path: " + canonicalPath);
                readerMap.put(canonicalPath, reader);
                cached = true;
            }
        }

        if (!cached) {
            LOG.debug("Reader already present for path: " + canonicalPath + "; closing.");
            reader.close();
        }
    }

    @Override
    protected synchronized void finalize() throws Throwable {
        for (LobFile.Reader r : readerMap.values()) {
            r.close();
        }

        super.finalize();
    }
}