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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang. The code is tested using the latest revision of the JDK for supported LTS releases: 8, 11, 17 and 21 currently. See https://github.com/apache/commons-lang/blob/master/.github/workflows/maven.yml Please ensure your build environment is up-to-date and kindly report any build issues.

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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.commons.lang3.concurrent;

import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.util.concurrent.atomic.AtomicReference;

/**
 * Base class for circuit breakers.
 *
 * @param  the type of the value monitored by this circuit breaker
 * @since 3.5
 */
public abstract class AbstractCircuitBreaker implements CircuitBreaker {

    /**
     * An internal enumeration representing the different states of a circuit
     * breaker. This class also contains some logic for performing state
     * transitions. This is done to avoid complex if-conditions in the code of
     * {@link CircuitBreaker}.
     */
    protected enum State {

        /** The closed state. */
        CLOSED {
            /**
             * {@inheritDoc}
             */
            @Override
            public State oppositeState() {
                return OPEN;
            }
        },

        /** The open state. */
        OPEN {
            /**
             * {@inheritDoc}
             */
            @Override
            public State oppositeState() {
                return CLOSED;
            }
        };

        /**
         * Returns the opposite state to the represented state. This is useful
         * for flipping the current state.
         *
         * @return the opposite state
         */
        public abstract State oppositeState();
    }

    /**
     * The name of the open property as it is passed to registered
     * change listeners.
     */
    public static final String PROPERTY_NAME = "open";

    /**
     * Converts the given state value to a boolean open property.
     *
     * @param state the state to be converted
     * @return the boolean open flag
     */
    protected static boolean isOpen(final State state) {
        return state == State.OPEN;
    }

    /** The current state of this circuit breaker. */
    protected final AtomicReference state = new AtomicReference<>(State.CLOSED);

    /** An object for managing change listeners registered at this instance. */
    private final PropertyChangeSupport changeSupport;

    /**
     * Creates an {@link AbstractCircuitBreaker}. It also creates an internal {@link PropertyChangeSupport}.
     */
    public AbstractCircuitBreaker() {
        changeSupport = new PropertyChangeSupport(this);
    }

    /**
     * Adds a change listener to this circuit breaker. This listener is notified whenever
     * the state of this circuit breaker changes. If the listener is
     * null, it is silently ignored.
     *
     * @param listener the listener to be added
     */
    public void addChangeListener(final PropertyChangeListener listener) {
        changeSupport.addPropertyChangeListener(listener);
    }

    /**
     * Changes the internal state of this circuit breaker. If there is actually a change
     * of the state value, all registered change listeners are notified.
     *
     * @param newState the new state to be set
     */
    protected void changeState(final State newState) {
        if (state.compareAndSet(newState.oppositeState(), newState)) {
            changeSupport.firePropertyChange(PROPERTY_NAME, !isOpen(newState), isOpen(newState));
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public abstract boolean checkState();

    /**
     * {@inheritDoc}
     */
    @Override
    public void close() {
        changeState(State.CLOSED);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public abstract boolean incrementAndCheckState(T increment);

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean isClosed() {
        return !isOpen();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean isOpen() {
        return isOpen(state.get());
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void open() {
        changeState(State.OPEN);
    }

    /**
     * Removes the specified change listener from this circuit breaker.
     *
     * @param listener the listener to be removed
     */
    public void removeChangeListener(final PropertyChangeListener listener) {
        changeSupport.removePropertyChangeListener(listener);
    }

}