org.apache.ode.utils.WatchDog Maven / Gradle / Ivy
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* Licensed to the Apache Software Foundation (ASF) under one
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* 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;
import java.io.File;
import java.util.Map;
import java.util.HashMap;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
/**
* This class is based on {@link org.apache.log4j.helpers.FileWatchdog}.
* 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.
* Now two use cases coexist:
*
* - 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.
* - 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.
*
*
* @author Alexis Midon
*/
public class WatchDog implements Runnable {
static final public long DEFAULT_DELAY = 30000;
final Log log = LogFactory.getLog(WatchDog.class);
private long expire;
private T lastModif;
private long delay = DEFAULT_DELAY;
private boolean existedBefore, warnedAlready, interrupted;
protected Mutable mutable;
protected C observer;
public WatchDog() {
}
/**
* @param mutable the object to watch closely
* @param delay between two checks
*/
public WatchDog(Mutable mutable, long delay) {
this(mutable);
this.delay = delay;
}
public WatchDog(Mutable mutable, C observer) {
this.mutable = mutable;
this.observer = observer;
}
/**
* @see #WatchDog(org.apache.ode.utils.WatchDog.Mutable, long)
*/
public WatchDog(Mutable mutable) {
this.mutable = mutable;
}
public Mutable getMutable() {
return mutable;
}
public C getObserver() {
return observer;
}
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) {
/* get a lock on the observer right now
It would be overkilled to lock before testing the dates.
By locking after the comparaison the worst scenario is that 2 threads get in the "if",
thread A gets the lock, thread B waits for it. Once the lock is released, thread B acquires it and do the checks on the mutable.
So this scenario is *harmless*.
*/
observer.getLock().lock();
expire = now + delay;
try {
if (log.isDebugEnabled()) log.debug("[" + mutable + "]" + " check for changes");
if (mutable.exists()) {
existedBefore = true;
if (lastModif == null || mutable.hasChangedSince(lastModif)) {
lastModif = mutable.lastModified();
observer.onUpdate();
if (log.isInfoEnabled()) log.info("[" + mutable + "]" + " updated");
warnedAlready = false;
} else {
if (log.isDebugEnabled()) log.debug("[" + mutable + "]" + " has not changed");
}
} else if (!observer.isInitialized()) {
// no resource and first time
observer.init();
if (log.isInfoEnabled()) log.info("[" + mutable + "]" + " initialized");
} else {
if (existedBefore) {
existedBefore = false;
lastModif = null;
observer.onDelete();
if (log.isInfoEnabled()) log.info("[" + mutable + "]" + " deleted");
}
if (!warnedAlready) {
warnedAlready = true;
if (log.isInfoEnabled()) log.info("[" + mutable + "]" + " does not exist.");
}
}
}catch(Exception e){
if (log.isDebugEnabled()) log.debug("[" + mutable + "]" + " exception occurred during check.", e);
// reset so that the next check retries right away
expire = 0;
lastModif = null;
existedBefore = false;
warnedAlready = false;
observer.reset();
if (log.isInfoEnabled()) log.info("[" + mutable + "] resetted.");
throw new RuntimeException(e);
} finally {
observer.getLock().unlock();
}
} else {
if (log.isTraceEnabled()) log.trace("[" + mutable + "]" + " wait period is not over");
}
}
public static WatchDog watchFile(File file, C handler) {
return new WatchDog(new FileMutable(file), handler);
}
public static WatchDog, C> watchFiles(List files, C handler) {
return new WatchDog, C>(new FileSetMutable(files), handler);
}
/**
* have you said that duck typing would be nice?
*/
public interface Mutable {
boolean exists();
boolean hasChangedSince(T since);
T lastModified();
}
static public class FileMutable implements WatchDog.Mutable {
File file;
public FileMutable(File file) {
this.file = file;
}
public boolean exists() {
return file.exists();
}
public boolean hasChangedSince(Long since) {
// do use 'greater than' to handle file deletion. The timestamp of a non-exising file is 0L.
return lastModified().longValue() != since.longValue();
}
public Long lastModified() {
return Long.valueOf(file.lastModified());
}
public String toString() {
return file.toString();
}
}
static public class FileSetMutable implements WatchDog.Mutable> {
File[] files;
public FileSetMutable(Collection files) {
this.files = new File[files.size()];
files.toArray(this.files);
}
public FileSetMutable(File[] files) {
this.files = files;
}
public boolean exists() {
return true;
}
public boolean hasChangedSince(Map since) {
Map snapshot = lastModified();
return !CollectionUtils.equals(snapshot, since);
}
public Map lastModified() {
Map m = new HashMap(files.length * 15 / 10);
for (File f : files) m.put(f, Long.valueOf(f.lastModified()));
return m;
}
}
public interface Observer {
boolean isInitialized();
/**
* Called by {@link WatchDog#check()} if the underlying object is not {@link #isInitialized initialized} and the {@link WatchDog.Mutable#exists()} resource does not exist}.
*
This method might called to reset the underlying object.
*
* @throws Exception
*/
void init();
void reset();
/**
* Called only if the resource previously existed and now does not exist.
*
The default implementation invokes {@link #init()} .
*
* @throws Exception
*/
void onDelete();
/**
* Called only if the resource previously existed but the {@link WatchDog.Mutable#lastModified()} timestamp has changed (greater than the previous value).
*
The default implementation invokes {@link #init()} .
*
* @throws Exception
*/
void onUpdate();
Lock getLock();
A get();
}
/**
* A default implementation of #ChangeHandler. Delete and Update will both invoke the #init method which satifies most use cases.
* So subclasses may simply override the #init method to fit their own needs.
*/
public static class DefaultObserver implements Observer {
protected final ReadWriteLock lock = new ReentrantReadWriteLock();
protected A object;
/**
* @return true if the wrapped if not null
*/
public boolean isInitialized() {
return object != null;
}
/**
* empty implementation
*/
public void init() {
}
public void reset() {
object = null;
}
/**
* delegate to #init
*/
public void onDelete() {
init();
}
/**
* delegate to #init
*/
public void onUpdate() {
init();
}
public Lock getLock() {
return lock.writeLock();
}
public A get() {
lock.readLock().lock();
try {
return object;
} finally {
lock.readLock().unlock();
}
}
}
}
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