/*
* 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.ode.utils;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
/**
* This class is based on {@link org.apache.log4j.helpers.FileWatchdog}.<p/>
* Modifications have been made to support additional abstract ressource and more events (creation, deletion and updates), and to allow "manual"
* invocations of {@link #check()} (i.e wihtout having to use a thread) while preserving time checking.<p/>
* Now two use cases coexist:
* <ol>
* <li>Pass an instance of {@link WatchDog} to a new thread ({@link WatchDog} is a {@link Runnable}).
* So that {@link WatchDog# check ()} will be called automatically every {@code delay} milliseconds.</li>
* <li>Invoke {@link WatchDog# check ()} only when you feel like it. If the expiration date previously set is lower than NOW then event
* callback methods will be invoked accordingly.</li>
* </ol>
*
* @author <a href="mailto:midon@intalio.com">Alexis Midon</a>
*/
public class WatchDog<T> implements Runnable {
static final public long DEFAULT_DELAY = 30000;
final Log log = LogFactory.getLog(getClass());
private long expire;
private T lastModif;
private long delay;
private boolean existedBefore, warnedAlready, interrupted;
protected final Mutable<T> mutable;
/**
* @param mutable the object to watch closely
* @param delay between two checks
*/
public WatchDog(Mutable<T> mutable, long delay) {
this(mutable);
this.delay = delay;
}
/**
* @see #WatchDog(org.apache.ode.utils.WatchDog.Mutable, long)
*/
public WatchDog(Mutable<T> mutable) {
this.mutable = mutable;
this.delay = DEFAULT_DELAY;
}
protected boolean isInitialized() {
return true;
}
/**
* Called by {@link #check()} if the object is not {@link #isInitialized initialized} and the {@link WatchDog.Mutable#exists()} resource does not exist}.
* <br/> This method might called to reset the object.
*
* @throws Exception
*/
protected void init() {
}
/**
* Called only if the resource previously existed and now does not exist.
* <br/>The default implementation invokes {@link #init()} .
*
* @throws Exception
*/
protected void doOnDelete() {
init();
}
/**
* Called only if the resource previously existed but the {@link WatchDog.Mutable#lastModified()} timestamp has changed (greater than the previous value).
* <br/>The default implementation invokes {@link #init()} .
*
* @throws Exception
*/
protected void doOnUpdate() {
init();
}
public long getDelay() {
return delay;
}
public void setDelay(long delay) {
this.delay = delay;
}
public void run() {
try {
while (!interrupted) {
try {
Thread.sleep(delay);
} catch (InterruptedException e) {
// no interruption expected
}
check();
}
} catch (Exception e) {
log.warn("Exception occured. Thread will stop", e);
}
}
public final void check() {
long now = System.currentTimeMillis();
if (expire <= now) {
expire = now + delay;
if (mutable.exists()) {
existedBefore = true;
if (lastModif==null || mutable.hasChangedSince(lastModif)) {
lastModif = mutable.lastModified();
if (log.isDebugEnabled())
log.debug(mutable + " has been modified");
doOnUpdate();
warnedAlready = false;
}
} else if (!isInitialized()) {
// no resource and first time
init();
} else {
if (existedBefore) {
existedBefore = false;
lastModif = null;
doOnDelete();
}
if (!warnedAlready) {
warnedAlready = true;
if (log.isDebugEnabled()) log.debug(mutable + "] does not exist.");
}
}
}
}
/**
* have you said that duck typing would be nice?
*/
public interface Mutable<T> {
boolean exists();
boolean hasChangedSince(T since);
T lastModified();
}
}