org.apache.hadoop.hdfs.util.BestEffortLongFile.java Source code

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Here is the source code for org.apache.hadoop.hdfs.util.BestEffortLongFile.java

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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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 org.apache.hadoop.hdfs.util;

import com.google.common.io.Files;
import com.google.common.primitives.Longs;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.io.IOUtils;

import java.io.Closeable;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

/**
 * Class that represents a file on disk which stores a single <code>long</code>
 * value, but does not make any effort to make it truly durable. This is in
 * contrast to {@link PersistentLongFile} which fsync()s the value on every
 * change.
 * <p/>
 * This should be used for values which are updated frequently (such that
 * performance is important) and not required to be up-to-date for correctness.
 * <p/>
 * This class also differs in that it stores the value as binary data instead
 * of a textual string.
 */
@InterfaceAudience.Private
public class BestEffortLongFile implements Closeable {

    private final File file;
    private final long defaultVal;

    private long value;

    private FileChannel ch = null;

    private ByteBuffer buf = ByteBuffer.allocate(Long.SIZE / 8);

    public BestEffortLongFile(File file, long defaultVal) {
        this.file = file;
        this.defaultVal = defaultVal;
    }

    public long get() throws IOException {
        lazyOpen();
        return value;
    }

    public void set(long newVal) throws IOException {
        lazyOpen();
        buf.clear();
        buf.putLong(newVal);
        buf.flip();
        IOUtils.writeFully(ch, buf, 0);
        value = newVal;
    }

    private void lazyOpen() throws IOException {
        if (ch != null) {
            return;
        }

        // Load current value.
        byte[] data = null;
        try {
            data = Files.toByteArray(file);
        } catch (FileNotFoundException fnfe) {
            // Expected - this will use default value.
        }

        if (data != null && data.length != 0) {
            if (data.length != Longs.BYTES) {
                throw new IOException("File " + file + " had invalid length: " + data.length);
            }
            value = Longs.fromByteArray(data);
        } else {
            value = defaultVal;
        }

        // Now open file for future writes.
        RandomAccessFile raf = new RandomAccessFile(file, "rw");
        try {
            ch = raf.getChannel();
        } finally {
            if (ch == null) {
                IOUtils.closeStream(raf);
            }
        }
    }

    @Override
    public void close() throws IOException {
        if (ch != null) {
            ch.close();
        }
    }
}