org.apache.jackrabbit.oak.spi.whiteboard.AbstractServiceTracker Maven / Gradle / Ivy
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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.
*/
package org.apache.jackrabbit.oak.spi.whiteboard;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static java.util.Collections.emptyList;
import java.util.List;
import javax.annotation.Nonnull;
/**
* {@code AbstractServiceTracker} is a base class for composite components
* that dynamically look up the available component services from the
* whiteboard.
*/
public abstract class AbstractServiceTracker {
/**
* Sentinel object used as the {@link #tracker} value of an already
* stopped instance.
*/
private final Tracker stopped = new Tracker() {
@Override
public List getServices() {
return emptyList();
}
@Override
public void stop() {
// do nothing
}
};
/**
* The type of services tracked by this instance.
*/
private final Class type;
/**
* The underlying {@link Tracker}, or the {@link #stopped} sentinel
* sentinel object when this instance is not active. This variable
* is {@code volatile} so that the {@link #getServices()} method will
* always see the latest state without having to be synchronized.
*/
private volatile Tracker tracker = stopped;
protected AbstractServiceTracker(@Nonnull Class type) {
this.type = checkNotNull(type);
}
public synchronized void start(Whiteboard whiteboard) {
checkState(tracker == stopped);
tracker = whiteboard.track(type);
}
public synchronized void stop() {
if (tracker != stopped) {
Tracker t = tracker;
tracker = stopped;
t.stop();
}
}
/**
* Returns all services of type {@code T} that are currently available.
* This method is intentionally not synchronized to prevent lock
* contention when accessed frequently in highly concurrent code.
*
* @return currently available services
*/
protected List getServices() {
return tracker.getServices();
}
}