log4j-1.2-api/src/main/java/org/apache/log4j/helpers/FileWatchdog.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 org.apache.log4j.helpers;

import java.io.File;

/**
 * Checks every now and then that a certain file has not changed. If it has, then call the {@link #doOnChange} method.
 *
 * @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 = 60_000;

    /**
     * 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 lastModified;
    boolean warnedAlready;
    boolean interrupted;

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

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

    abstract protected void doOnChange();

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

    /**
     * Sets the delay in milliseconds to observe between each check of the file changes.
     *
     * @param delayMillis the delay in milliseconds
     */
    public void setDelay(final long delayMillis) {
        this.delay = delayMillis;
    }
}