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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.
 */

// Contributors:  Mathias Bogaert

package org.apache.log4j.helpers;

import java.io.File;

/**
 * Check every now and then that a certain file has not changed. If it has, then
 * call the {@link #doOnChange} method.
 * 
 * 
 * @author Ceki Gülcü
 * @since version 0.9.1
 */
public abstract class FileWatchdog extends Thread {

    /**
     * The default delay between every file modification check, set to 60 seconds.
     */
    static final public long DEFAULT_DELAY = 60000;
    /**
     * The name of the file to observe for changes.
     */
    protected String filename;

    /**
     * The delay to observe between every check. By default set
     * {@link #DEFAULT_DELAY}.
     */
    protected long delay = DEFAULT_DELAY;

    File file;
    long lastModif = 0;
    boolean warnedAlready = false;
    boolean interrupted = false;

    protected FileWatchdog(String filename) {
	super("FileWatchdog");
	this.filename = filename;
	file = new File(filename);
	setDaemon(true);
	checkAndConfigure();
    }

    /**
     * Set the delay to observe between each check of the file changes.
     */
    public void setDelay(long delay) {
	this.delay = delay;
    }

    abstract protected void doOnChange();

    protected void checkAndConfigure() {
	boolean fileExists;
	try {
	    fileExists = file.exists();
	} catch (SecurityException e) {
	    LogLog.warn("Was not allowed to read check file existance, file:[" + filename + "].");
	    interrupted = true; // there is no point in continuing
	    return;
	}

	if (fileExists) {
	    long l = file.lastModified(); // this can also throw a SecurityException
	    if (l > lastModif) { // however, if we reached this point this
		lastModif = l; // is very unlikely.
		doOnChange();
		warnedAlready = false;
	    }
	} else {
	    if (!warnedAlready) {
		LogLog.debug("[" + filename + "] does not exist.");
		warnedAlready = true;
	    }
	}
    }

    public void run() {
	while (!interrupted) {
	    try {
		Thread.sleep(delay);
	    } catch (InterruptedException e) {
		// no interruption expected
	    }
	    checkAndConfigure();
	}
    }
}