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Commons IO example source code file (Tailer.java)

This example Commons IO source code file (Tailer.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons IO tags/keywords

file, filenotfoundexception, interruptedexception, interruptedexception, io, ioexception, randomaccessfile, randomaccessfile, string, tailer, tailer, tailerlistener, tailerlistener, the, thread

The Commons IO Tailer.java source code

/*
 * 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.commons.io.input;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;

import org.apache.commons.io.FileUtils;
import org.apache.commons.io.IOUtils;

/**
 * Simple implementation of the unix "tail -f" functionality.
 * <p>
 * Example Usage:
 * <pre>
 *      // Simplest invocation using static helper method:
 *      TailerListener listener = ...
 *      Tailer tailer = Tailer.create(file, listener, delay);
 *      
 *      // Alternative method using executor:
 *      TailerListener listener = ...
 *      Tailer tailer = new Tailer(file, listener, delay);
 *      Executor executor ...
 *      executor.execute(tailer);
 *      
 *      // Alternative method if you want to handle the threading yourself:
 *      TailerListener listener = ...
 *      Tailer tailer = new Tailer(file, listener, delay);
 *      Thread thread = new Thread(tailer);
 *      thread.setDaemon(true); // optional
 *      thread.start();
 *      
 *      // Remember to stop the tailer when you have done with it:
 *      tailer.stop();
 * </pre>
 *
 * @version $Id: Tailer.java 1021884 2010-10-12 18:49:16Z ggregory $
 * @since Commons IO 2.0
 */
public class Tailer implements Runnable {

    /**
     * The file which will be tailed.
     */
    private final File file;

    /**
     * The amount of time to wait for the file to be updated.
     */
    private final long delay;

    /**
     * Whether to tail from the end or start of file
     */
    private final boolean end;

    /**
     * The listener to notify of events when tailing.
     */
    private final TailerListener listener;

    /**
     * The tailer will run as long as this value is true.
     */
    private volatile boolean run = true;

    /**
     * Creates a Tailer for the given file, starting from the beginning, with the default delay of 1.0s.
     * @param file The file to follow.
     * @param listener the TailerListener to use.
     */
    public Tailer(File file, TailerListener listener) {
        this(file, listener, 1000);
    }

    /**
     * Creates a Tailer for the given file, starting from the beginning.
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delay the delay between checks of the file for new content in milliseconds.
     */
    public Tailer(File file, TailerListener listener, long delay) {
        this(file, listener, 1000, false);
    }

    /**
     * Creates a Tailer for the given file, with a delay other than the default 1.0s.
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delay the delay between checks of the file for new content in milliseconds.
     * @param end Set to true to tail from the end of the file, false to tail from the beginning of the file.
     */
    public Tailer(File file, TailerListener listener, long delay, boolean end) {

        this.file = file;
        this.delay = delay;
        this.end = end;

        // Save and prepare the listener
        this.listener = listener;
        listener.init(this);
    }

    /**
     * Creates and starts a Tailer for the given file.
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delay the delay between checks of the file for new content in milliseconds.
     * @param end Set to true to tail from the end of the file, false to tail from the beginning of the file.
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener, long delay, boolean end) {
        Tailer tailer = new Tailer(file, listener, delay, end);
        Thread thread = new Thread(tailer);
        thread.setDaemon(true);
        thread.start();
        return tailer;
    }

    /**
     * Creates and starts a Tailer for the given file, starting at the beginning of the file
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delay the delay between checks of the file for new content in milliseconds.
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener, long delay) {
        return create(file, listener, delay, false);
    }

    /**
     * Creates and starts a Tailer for the given file, starting at the beginning of the file
     * with the default delay of 1.0s
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener) {
        return create(file, listener, 1000, false);
    }

    /**
     * Return the file.
     *
     * @return the file
     */
    public File getFile() {
        return file;
    }

    /**
     * Return the delay.
     *
     * @return the delay
     */
    public long getDelay() {
        return delay;
    }

    /**
     * Follows changes in the file, calling the TailerListener's handle method for each new line.
     */
    public void run() {
        RandomAccessFile reader = null;
        try {
            long last = 0; // The last time the file was checked for changes
            long position = 0; // position within the file
            // Open the file
            while (run && reader == null) {
                try {
                    reader = new RandomAccessFile(file, "r");
                } catch (FileNotFoundException e) {
                    listener.fileNotFound();
                }

                if (reader == null) {
                    try {
                        Thread.sleep(delay);
                    } catch (InterruptedException e) {
                    }
                } else {
                    // The current position in the file
                    position = end ? file.length() : 0;
                    last = System.currentTimeMillis();
                    reader.seek(position);                    
                }
            }


            while (run) {

                // Check the file length to see if it was rotated
                long length = file.length();

                if (length < position) {

                    // File was rotated
                    listener.fileRotated();

                    // Reopen the reader after rotation
                    try {
                        // Ensure that the old file is closed iff we re-open it successfully
                        RandomAccessFile save = reader;
                        reader = new RandomAccessFile(file, "r");
                        position = 0;
                        // close old file explicitly rather than relying on GC picking up previous RAF
                        IOUtils.closeQuietly(save);
                    } catch (FileNotFoundException e) {
                        // in this case we continue to use the previous reader and position values
                        listener.fileNotFound();
                    }
                    continue;
                } else {

                    // File was not rotated

                    // See if the file needs to be read again
                    if (length > position) {

                        // The file has more content than it did last time
                        last = System.currentTimeMillis();
                        position = readLines(reader);

                    } else if (FileUtils.isFileNewer(file, last)) {

                        /* This can happen if the file is truncated or overwritten
                         * with the exact same length of information. In cases like
                         * this, the file position needs to be reset
                         */
                        position = 0;
                        reader.seek(position); // cannot be null here

                        // Now we can read new lines
                        last = System.currentTimeMillis();
                        position = readLines(reader);
                    }
                }
                try {
                    Thread.sleep(delay);
                } catch (InterruptedException e) {
                }
            }

        } catch (Exception e) {

            listener.handle(e);

        } finally {
            IOUtils.closeQuietly(reader);
        }
    }

    /**
     * Allows the tailer to complete its current loop and return.
     */
    public void stop() {
        this.run = false;
    }

    /**
     * Read new lines.
     *
     * @param reader The file to read
     * @return The new position after the lines have been read
     * @throws java.io.IOException if an I/O error occurs.
     */
    private long readLines(RandomAccessFile reader) throws IOException {
        String line = reader.readLine();
        while (line != null) {
            listener.handle(line);
            line = reader.readLine();
        }
        return reader.getFilePointer();
    }

}

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