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/*
* Copyright (C) 2016 The CyanogenMod Project
*
* Licensed 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 com.android.contacts.common.util;
/**
* Encapsulates a threadsafe singleton pattern.
*
* This class is designed to be used as a public constant, living within a class that has a private constructor.
* It defines a {@link #create(I)} method that will only ever be called once, upon the first call of {@link #get(I)}.
* That method is responsible for creating the actual singleton instance, and that instance will be returned for all
* future calls of {@link #get(I)}.
*
* Example:
* <code>
* public class FooSingleton {
* public static final SingletonHolder<FooSingleton, ParamObject> HOLDER =
* new SingletonHolder<FooSingleton, ParamObject>() {
* @Override
* protected FooSingleton create(ParamObject param) {
* return new FooSingleton(param);
* }
* };
*
* private FooSingleton(ParamObject param) {
*
* }
* }
*
* // somewhere else
* FooSingleton.HOLDER.get(params).doStuff();
* </code>
* @param <E> The type of the class to hold as a singleton.
* @param <I> A parameter object to use during creation of the singleton object.
*/
public abstract class SingletonHolder<E, I> {
E mInstance;
final Object LOCK = new Object();
public E get(I initializer) {
if (null == mInstance) {
synchronized (LOCK) {
if (null == mInstance) {
mInstance = create(initializer);
}
}
}
return mInstance;
}
protected abstract E create(I initializer);
/**
* Specialized version of {@link SingletonHolder} which will keep a count of referring
* objects, and clear the instance when all references have been removed.
*
* @param <E> The type of the class to hold as a singleton.
* @param <I> A parameter object to use during creation of the singleton object.
*/
public static abstract class RefCountedSingletonHolder<E, I> extends SingletonHolder<E, I> {
private int mRefCount;
public E get(I initializer) {
synchronized (LOCK) {
mRefCount++;
}
return super.get(initializer);
}
public void release() {
synchronized (LOCK) {
if(--mRefCount == 0) {
destroy(mInstance);
mInstance = null;
}
}
}
protected abstract void destroy(E instance);
}
}
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