JAVA 实现tail -f 日志文件监控功能

工具：

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.4</version>
</dependency>

定义接口

package com.snow.tailer;

public interface TailerListener {
    /**
     * The tailer will call this method during construction,
     * giving the listener a method of stopping the tailer.
     * @param tailer the tailer.
     */
    void init(Tailer tailer);

    /**
     * This method is called if the tailed file is not found.
     * <p>
     * <b>Note:</b> this is called from the tailer thread.
     */
    void fileNotFound();

    /**
     * Called if a file rotation is detected.
     *
     * This method is called before the file is reopened, and fileNotFound may
     * be called if the new file has not yet been created.
     * <p>
     * <b>Note:</b> this is called from the tailer thread.
     */
    void fileRotated();

    /**
     * Handles a line from a Tailer.
     * <p>
     * <b>Note:</b> this is called from the tailer thread.
     * @param line the line.
     */
    void handle(String line);

    /**
     * Handles an Exception .
     * <p>
     * <b>Note:</b> this is called from the tailer thread.
     * @param ex the exception.
     */
    void handle(Exception ex);
}

接口实现

package com.snow.tailer;

public class TailerListenerAdapter implements TailerListener {
    /**
     * The tailer will call this method during construction,
     * giving the listener a method of stopping the tailer.
     * @param tailer the tailer.
     */
    public void init(Tailer tailer) {
    }

    /**
     * This method is called if the tailed file is not found.
     */
    public void fileNotFound() {
    }

    /**
     * Called if a file rotation is detected.
     *
     * This method is called before the file is reopened, and fileNotFound may
     * be called if the new file has not yet been created.
     */
    public void fileRotated() {
    }

    /**
     * Handles a line from a Tailer.
     * @param line the line.
     */
    public void handle(String line) {
    }

    /**
     * Handles an Exception .
     * @param ex the exception.
     */
    public void handle(Exception ex) {
    }

}

定义Tailer.java

/*
 * 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 com.snow.tailer;

import org.apache.commons.io.FileUtils;
import org.apache.commons.io.IOUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

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

/**
 * Simple implementation of the unix "tail -f" functionality.
 * <p>
 * <h2>1. Create a TailerListener implementation</h3>
 * <p>
 * First you need to create a {@link TailerListener} implementation
 * ({@link TailerListenerAdapter} is provided for convenience so that you don't have to
 * implement every method).
 * </p>
 *
 * <p>For example:</p>
 * <pre>
 *  public class MyTailerListener extends TailerListenerAdapter {
 *      public void handle(String line) {
 *          System.out.println(line);
 *      }
 *  }
 * </pre>
 * 
 * <h2>2. Using a Tailer</h2>
 *
 * You can create and use a Tailer in one of three ways:
 * <ul>
 *   <li>Using one of the static helper methods:
 *     <ul>
 *       <li>{@link Tailer#create(File, TailerListener)}</li>
 *       <li>{@link Tailer#create(File, TailerListener, long)}</li>
 *       <li>{@link Tailer#create(File, TailerListener, long, boolean)}</li>
 *     </ul>
 *   </li>
 *   <li>Using an {@link java.util.concurrent.Executor}</li>
 *   <li>Using an {@link Thread}</li>
 * </ul>
 *
 * An example of each of these is shown below.
 * 
 * <h3>2.1 Using the static helper method</h3>
 *
 * <pre>
 *      TailerListener listener = new MyTailerListener();
 *      Tailer tailer = Tailer.create(file, listener, delay);
 * </pre>
 *      
 * <h3>2.2 Use an Executor</h3>
 * 
 * <pre>
 *      TailerListener listener = new MyTailerListener();
 *      Tailer tailer = new Tailer(file, listener, delay);
 *
 *      // stupid executor impl. for demo purposes
 *      Executor executor = new Executor() {
 *          public void execute(Runnable command) {
 *              command.run();
 *           }
 *      };
 *
 *      executor.execute(tailer);
 * </pre>
 *      
 *      
 * <h3>2.3 Use a Thread</h3>
 * <pre>
 *      TailerListener listener = new MyTailerListener();
 *      Tailer tailer = new Tailer(file, listener, delay);
 *      Thread thread = new Thread(tailer);
 *      thread.setDaemon(true); // optional
 *      thread.start();
 * </pre>
 *
 * <h2>3. Stop Tailing</h3>
 * <p>Remember to stop the tailer when you have done with it:</p>
 * <pre>
 *      tailer.stop();
 * </pre>
 *
 * @see TailerListener
 * @see TailerListenerAdapter
 * @version $Id: Tailer.java 1348698 2012-06-11 01:09:58Z ggregory $
 * @since 2.0
 */
public class Tailer implements Runnable {
    private static final Logger logger = LoggerFactory.getLogger(Tailer.class);

    private static final int DEFAULT_DELAY_MILLIS = 1000;

    private static final String RAF_MODE = "r";

    private static final int DEFAULT_BUFSIZE = 4096;

    /**
     * Buffer on top of RandomAccessFile.
     */
    private final byte inbuf[];

    /**
     * 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 delayMillis;

    /**
     * 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;

    /**
     * Whether to close and reopen the file whilst waiting for more input.
     */
    private final boolean reOpen;

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

    /**
     * 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, DEFAULT_DELAY_MILLIS);
    }

    /**
     * Creates a Tailer for the given file, starting from the beginning.
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis the delay between checks of the file for new content in milliseconds.
     */
    public Tailer(File file, TailerListener listener, long delayMillis) {
        this(file, listener, delayMillis, 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 delayMillis 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 delayMillis, boolean end) {
        this(file, listener, delayMillis, end, DEFAULT_BUFSIZE);
        logger.info("Tailer inited from customer class.");
    }

    /**
     * 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 delayMillis 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.
     * @param reOpen if true, close and reopen the file between reading chunks 
     */
    public Tailer(File file, TailerListener listener, long delayMillis, boolean end, boolean reOpen) {
        this(file, listener, delayMillis, end, reOpen, DEFAULT_BUFSIZE);
    }

    /**
     * Creates a Tailer for the given file, with a specified buffer size.
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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.
     * @param bufSize Buffer size
     */
    public Tailer(File file, TailerListener listener, long delayMillis, boolean end, int bufSize) {
        this(file, listener, delayMillis, end, false, bufSize);
    }

    /**
     * Creates a Tailer for the given file, with a specified buffer size.
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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.
     * @param reOpen if true, close and reopen the file between reading chunks 
     * @param bufSize Buffer size
     */
    public Tailer(File file, TailerListener listener, long delayMillis, boolean end, boolean reOpen, int bufSize) {
        this.file = file;
        this.delayMillis = delayMillis;
        this.end = end;

        this.inbuf = new byte[bufSize];

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

    /**
     * Creates and starts a Tailer for the given file.
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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.
     * @param bufSize buffer size.
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener, long delayMillis, boolean end, int bufSize) {
        Tailer tailer = new Tailer(file, listener, delayMillis, end, bufSize);
        Thread thread = new Thread(tailer);
        thread.setDaemon(true);
        thread.start();
        return tailer;
    }

    /**
     * Creates and starts a Tailer for the given file.
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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.
     * @param reOpen whether to close/reopen the file between chunks
     * @param bufSize buffer size.
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener, long delayMillis, boolean end, boolean reOpen, int bufSize) {
        Tailer tailer = new Tailer(file, listener, delayMillis, end, reOpen, bufSize);
        Thread thread = new Thread(tailer);
        thread.setDaemon(true);
        thread.start();
        return tailer;
    }

    /**
     * Creates and starts a Tailer for the given file with default buffer size.
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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 delayMillis, boolean end) {
        return create(file, listener, delayMillis, end, DEFAULT_BUFSIZE);
    }

    /**
     * Creates and starts a Tailer for the given file with default buffer size.
     * 
     * @param file the file to follow.
     * @param listener the TailerListener to use.
     * @param delayMillis 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.
     * @param reOpen whether to close/reopen the file between chunks
     * @return The new tailer
     */
    public static Tailer create(File file, TailerListener listener, long delayMillis, boolean end, boolean reOpen) {
        return create(file, listener, delayMillis, end, reOpen, DEFAULT_BUFSIZE);
    }

    /**
     * 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 delayMillis 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 delayMillis) {
        return create(file, listener, delayMillis, 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, DEFAULT_DELAY_MILLIS, false);
    }

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

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

    /**
     * Follows changes in the file, calling the TailerListener's handle method for each new line.
     */
    @Override
    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, RAF_MODE);
                } catch (FileNotFoundException e) {
                    listener.fileNotFound();
                }

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

            while (run) {

                boolean newer = FileUtils.isFileNewer(file, last); // IO-279, must be done first

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

                if (length < position) {
                    logger.info(String.format("rotated, legth=%s, position=%s", 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, RAF_MODE);
                        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
                        position = readLines(reader);
                        last = file.lastModified();

                    } else if (newer) {
                        logger.info(String.format("newer, legth=%s, position=%s", length, position));
                        if (resetFilePositionIfOverwrittenWithTheSameLength) {
                            /*
                             * 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
                            position = readLines(reader);
                        }
                        last = file.lastModified();
                    }
                }
                if (reOpen) {
                    IOUtils.closeQuietly(reader);
                }
                try {
                    Thread.sleep(delayMillis);
                } catch (InterruptedException e) {
                }
                if (run && reOpen) {
                    reader = new RandomAccessFile(file, RAF_MODE);
                    reader.seek(position);
                    logger.info(String.format("reopen, legth=%s, position=%s", length, position));
                }
            }

        } 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 IOException if an I/O error occurs.
     */
    private long readLines(RandomAccessFile reader) throws IOException {
        StringBuilder sb = new StringBuilder();

        long pos = reader.getFilePointer();
        long rePos = pos; // position to re-read

        int num;
        boolean seenCR = false;
        while (run && ((num = reader.read(inbuf)) != -1)) {
            for (int i = 0; i < num; i++) {
                byte ch = inbuf[i];
                switch (ch) {
                    case '\n':
                        seenCR = false; // swallow CR before LF
                        listener.handle(sb.toString());
                        sb.setLength(0);
                        rePos = pos + i + 1;
                        break;
                    case '\r':
                        if (seenCR) {
                            sb.append('\r');
                        }
                        seenCR = true;
                        break;
                    default:
                        if (seenCR) {
                            seenCR = false; // swallow final CR
                            listener.handle(sb.toString());
                            sb.setLength(0);
                            rePos = pos + i + 1;
                        }
                        sb.append((char) ch); // add character, not its ascii value
                }
            }

            pos = reader.getFilePointer();
        }

        reader.seek(rePos); // Ensure we can re-read if necessary
        return rePos;
    }

}

封装使用函数：

/**
 * @param inputFile  监控文件
 * @param sleepInterval  当文件没有日志时sleep间隔
 */
private static void monitor(String inputFile, int sleepInterval) {
    TailerListener listener = new TailerListenerAdapter() {
        @Override
        public void handle(String line) {
            if (++count % 100000 == 0) {
                log.info("{} lines sent since the program up.", count);
            }
            if (StringUtils.isEmpty(line)) {
                log.warn("should not read empty line.");
                return;
            } else {
                // do something ... 
            }
        }
    };
    Tailer tailer = new Tailer(new File(inputFile), listener, sleepInterval, true);
    tailer.run();
}

调用该函数即可。