    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.lang3.concurrent;
    018
    019import java.beans.PropertyChangeListener;
    020import java.beans.PropertyChangeSupport;
    021import java.util.concurrent.atomic.AtomicReference;
    022
    023/**
    024 * Base class for circuit breakers.
    025 *
    026 * @param <T> the type of the value monitored by this circuit breaker
    027 * @since 3.5
    028 */
    029public abstract class AbstractCircuitBreaker<T> implements CircuitBreaker<T> {
    030
    031    /**
    032     * An internal enumeration representing the different states of a circuit
    033     * breaker. This class also contains some logic for performing state
    034     * transitions. This is done to avoid complex if-conditions in the code of
    035     * {@link CircuitBreaker}.
    036     */
    037    protected enum State {
    038
    039        /** The closed state. */
    040        CLOSED {
    041            /**
    042             * {@inheritDoc}
    043             */
    044            @Override
    045            public State oppositeState() {
    046                return OPEN;
    047            }
    048        },
    049
    050        /** The open state. */
    051        OPEN {
    052            /**
    053             * {@inheritDoc}
    054             */
    055            @Override
    056            public State oppositeState() {
    057                return CLOSED;
    058            }
    059        };
    060
    061        /**
    062         * Returns the opposite state to the represented state. This is useful
    063         * for flipping the current state.
    064         *
    065         * @return the opposite state
    066         */
    067        public abstract State oppositeState();
    068    }
    069
    070    /**
    071     * The name of the <em>open</em> property as it is passed to registered
    072     * change listeners.
    073     */
    074    public static final String PROPERTY_NAME = "open";
    075
    076    /**
    077     * Converts the given state value to a boolean <em>open</em> property.
    078     *
    079     * @param state the state to be converted
    080     * @return the boolean open flag
    081     */
    082    protected static boolean isOpen(final State state) {
    083        return state == State.OPEN;
    084    }
    085
    086    /** The current state of this circuit breaker. */
    087    protected final AtomicReference<State> state = new AtomicReference<>(State.CLOSED);
    088
    089    /** An object for managing change listeners registered at this instance. */
    090    private final PropertyChangeSupport changeSupport;
    091
    092    /**
    093     * Creates an {@link AbstractCircuitBreaker}. It also creates an internal {@link PropertyChangeSupport}.
    094     */
    095    public AbstractCircuitBreaker() {
    096        changeSupport = new PropertyChangeSupport(this);
    097    }
    098
    099    /**
    100     * Adds a change listener to this circuit breaker. This listener is notified whenever
    101     * the state of this circuit breaker changes. If the listener is
    102     * <strong>null</strong>, it is silently ignored.
    103     *
    104     * @param listener the listener to be added
    105     */
    106    public void addChangeListener(final PropertyChangeListener listener) {
    107        changeSupport.addPropertyChangeListener(listener);
    108    }
    109
    110    /**
    111     * Changes the internal state of this circuit breaker. If there is actually a change
    112     * of the state value, all registered change listeners are notified.
    113     *
    114     * @param newState the new state to be set
    115     */
    116    protected void changeState(final State newState) {
    117        if (state.compareAndSet(newState.oppositeState(), newState)) {
    118            changeSupport.firePropertyChange(PROPERTY_NAME, !isOpen(newState), isOpen(newState));
    119        }
    120    }
    121
    122    /**
    123     * {@inheritDoc}
    124     */
    125    @Override
    126    public abstract boolean checkState();
    127
    128    /**
    129     * {@inheritDoc}
    130     */
    131    @Override
    132    public void close() {
    133        changeState(State.CLOSED);
    134    }
    135
    136    /**
    137     * {@inheritDoc}
    138     */
    139    @Override
    140    public abstract boolean incrementAndCheckState(T increment);
    141
    142    /**
    143     * {@inheritDoc}
    144     */
    145    @Override
    146    public boolean isClosed() {
    147        return !isOpen();
    148    }
    149
    150    /**
    151     * {@inheritDoc}
    152     */
    153    @Override
    154    public boolean isOpen() {
    155        return isOpen(state.get());
    156    }
    157
    158    /**
    159     * {@inheritDoc}
    160     */
    161    @Override
    162    public void open() {
    163        changeState(State.OPEN);
    164    }
    165
    166    /**
    167     * Removes the specified change listener from this circuit breaker.
    168     *
    169     * @param listener the listener to be removed
    170     */
    171    public void removeChangeListener(final PropertyChangeListener listener) {
    172        changeSupport.removePropertyChangeListener(listener);
    173    }
    174
    175}