/*
* 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.geode.cache.query.internal;
/**
* This class is intended to hold a single 'observer' which will receive callbacks from the query
* subsystem when various events take place. There can be only one such observer at a time. If no
* observer is needed, this member variable should point to an object with 'do-nothing' methods,
* such as QueryObserverAdapter.
*
* Code which wishes to observe events during a query should do so using the following technique:
*
* class MyQueryObserver extends QueryObserverAdapter { // ... override methods of interest ... }
*
* QueryObserver old = QueryObserverHolder.setInstance(new MyQueryObserver()); try { //... call
* query methods here ... } finally { // reset to the original QueryObserver.
* QueryObserverHolder.setInstance(old); }
*
* The query code will call methods on this static member using the following technique:
*
* QueryObserver observer = QueryObserverHolder.getInstance(); try {
* observer.startMethod(arguments); doSomething(); } finally { observer.stopMethod(arguments); }
*
* @version $Revision: 1.1 $
*/
public final class QueryObserverHolder {
/**
* The default 'do-nothing' query observer *
*/
private static final QueryObserver NO_OBSERVER = new QueryObserverAdapter();
/**
* The current observer which will be notified of all query events.
*/
private static QueryObserver _instance = NO_OBSERVER;
/**
* Set the given observer to be notified of query events. Returns the current observer.
*/
public static final QueryObserver setInstance(QueryObserver observer) {
Support.assertArg(observer != null, "setInstance expects a non-null argument!");
QueryObserver oldObserver = _instance;
_instance = observer;
return oldObserver;
}
public static boolean hasObserver() {
return _instance != NO_OBSERVER;
}
/** Return the current QueryObserver instance */
public static final QueryObserver getInstance() {
return _instance;
}
/**
* Only for test purposes.
*/
public static final void reset() {
_instance = NO_OBSERVER;
}
}