org.apereo.portal.utils.threading.SingletonDoubleCheckedCreator.java Source code

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Here is the source code for org.apereo.portal.utils.threading.SingletonDoubleCheckedCreator.java

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/**
 * Licensed to Apereo under one or more contributor license
 * agreements. See the NOTICE file distributed with this work
 * for additional information regarding copyright ownership.
 * Apereo 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 the following location:
 *
 *   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.apereo.portal.utils.threading;

import java.util.concurrent.atomic.AtomicBoolean;

import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

/**
 * Provides a DoubleCheckedCreator impl that tracks the singleton instance internally
 * 
 * @author Eric Dalquist
 */
public abstract class SingletonDoubleCheckedCreator<T> extends DoubleCheckedCreator<T> {
    private final AtomicBoolean creating = new AtomicBoolean(false);
    private final AtomicBoolean created = new AtomicBoolean(false);
    private T instance;

    /**
     * Called only once as long as it returns successfully
     * 
     * @see DoubleCheckedCreator#create(Object...)
     */
    protected abstract T createSingleton(Object... args);

    /* (non-Javadoc)
     * @see org.apereo.portal.utils.threading.DoubleCheckedCreator#create(java.lang.Object[])
     */
    @Override
    protected final T create(Object... args) {
        if (this.creating.get()) {
            throw new IllegalStateException(
                    "Singleton creator has been called again while creation is in progress, this is indicative of a creation loop in a single thread");
        }

        this.creating.set(true);
        try {
            final T instance = this.createSingleton(args);
            this.instance = instance;
            this.created.set(true);
            return instance;
        } finally {
            this.creating.set(false);
        }
    }

    /* (non-Javadoc)
     * @see org.apereo.portal.utils.threading.DoubleCheckedCreator#retrieve(java.lang.Object[])
     */
    @Override
    protected final T retrieve(Object... args) {
        return this.instance;
    }

    /**
     * @return true if the singleton has been created as of this call
     */
    public final boolean isCreated() {
        return this.created.get();
    }

    /**
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        return new ToStringBuilder(this, ToStringStyle.SHORT_PREFIX_STYLE).append("instance", this.instance)
                .toString();
    }
}