Commons IO example source code file (Tailer.java)
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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