io.neba.core.logviewer.Tail Maven / Gradle / Ivy

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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

  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 io.neba.core.logviewer;

import org.eclipse.jetty.websocket.api.RemoteEndpoint;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;

import static java.lang.Math.max;
import static java.lang.Thread.sleep;
import static java.nio.ByteBuffer.allocate;
import static java.nio.file.Files.newByteChannel;
import static java.nio.file.StandardOpenOption.READ;
import static org.apache.commons.io.IOUtils.closeQuietly;

/**
 * A non-blocking tail implementation allowing to read an arbitrary number of bytes from the end of a file
 * and follow changes to it.
 *
 * @author Olaf Otto
 */
public class Tail implements Runnable {
    private static final int AWAIT_FILE_ROTATION_MILLIS = 1000;
    private static final int TAIL_CHECK_INTERVAL_MILLIS = 500;

    private final Logger logger = LoggerFactory.getLogger(getClass());

    private final RemoteEndpoint remoteEndpoint;
    private final File file;
    private final long bytesToTail;

    private boolean stopped = false;

    /**
     * @param remoteEndpoint must not be null.
     * @param file must not be null.
     * @param bytesToTail the number of bytes up to which are immediately read from the file.
     */
    Tail(RemoteEndpoint remoteEndpoint, File file, long bytesToTail) {
        if (remoteEndpoint == null) {
            throw new IllegalArgumentException("constructor parameter remoteEndpoint must not be null");
        }
        if (file == null) {
            throw new IllegalArgumentException("constructor parameter file must not be null");
        }
        this.bytesToTail = bytesToTail;
        this.remoteEndpoint = remoteEndpoint;
        this.file = file;
    }

    @Override
    public void run() {
        SeekableByteChannel channel = null;

        try {
            channel = newByteChannel(this.file.toPath(), READ);

            long availableInByte = this.file.length();
            long startingFromInByte = max(availableInByte - this.bytesToTail, 0);

            channel.position(startingFromInByte);

            long position = startingFromInByte;

            // Read up to this amount of data from the file at once.
            ByteBuffer readBuffer = allocate(4096);
            while (!this.stopped) {

                // The file might be temporarily gone during rotation. Wait, then decide
                // whether the file is considered gone permanently or whether a rotation has occurred.
                if (!this.file.exists()) {
                    sleep(AWAIT_FILE_ROTATION_MILLIS);
                }
                if (!this.file.exists()) {
                    this.remoteEndpoint.sendString("file not found");
                    return;
                }

                if (position > this.file.length()) {
                    this.remoteEndpoint.sendString("file rotated");
                    position = 0;
                    closeQuietly(channel);
                    channel = newByteChannel(this.file.toPath(), READ);
                }

                int read = channel.read(readBuffer);

                if (read == -1) {
                    sleep(TAIL_CHECK_INTERVAL_MILLIS);
                    continue;
                }

                position = channel.position();

                readBuffer.flip();
                this.remoteEndpoint.sendBytes(readBuffer);
                readBuffer.clear();
            }
        } catch (IOException e) {
            this.logger.error("Unable to tail " + this.file.getAbsolutePath() + ".", e);
        } catch (InterruptedException e) {
            if (!this.stopped) {
                this.logger.error("Stopped tailing " + this.file.getAbsolutePath() + ", got interrupted.", e);
            }
        } finally {
            closeQuietly(channel);
        }
    }

    public void stop() {
        this.stopped = true;
    }
}