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Locking APIs and utilities for Neo4j.
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/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [https://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
package org.neo4j.lock;

import java.util.Objects;

/**
 * Abstract implementation to keep all the boiler-plate code separate from actual locking logic.
 *
 * Diagram of how classes inter-relate:
 *  * ({@link LockService}) --------[returns]--------> ({@link Lock})
 *  ^                                            ^
 *  |                                            |
 *  [implements]                                 [extends]
 *  |                                            |
 * ({@link AbstractLockService}) -[contains]-> ({@link LockReference}) -[holds]-> ({@link LockedEntity})
 *  ^      |                                     |                                     ^
 *  |      |                                     [references]                          |
 *  |      |                                     |                                     [extends]
 *  |      |                                     V                                     |
 *  |      +-----------[type param]-----------> (HANDLE)                              ({@link LockedNode})
 *  |                                            ^
 *  [extends]                                    |
 *  |                                            [satisfies]
 *  |                                            |
 * ({@link ReentrantLockService})-[type param]->({@link ReentrantLockService.OwnerQueueElement})
 * 
 *
 * @param  A handle that the concrete implementation used for releasing the lock.
 */
abstract class AbstractLockService implements LockService {
    @Override
    public Lock acquireNodeLock(long nodeId, LockType type) {
        return lock(new LockedNode(nodeId));
    }

    @Override
    public Lock acquireRelationshipLock(long relationshipId, LockType type) {
        return lock(new LockedRelationship(relationshipId));
    }

    @Override
    public Lock acquireCustomLock(int resourceType, long id, LockType type) {
        return lock(new CustomLockedEntity(resourceType, id));
    }

    private Lock lock(LockedEntity key) {
        return new LockReference(key, acquire(key));
    }

    protected abstract HANDLE acquire(LockedEntity key);

    protected abstract void release(LockedEntity key, HANDLE handle);

    protected abstract static class LockedEntity {
        final long id;

        private LockedEntity(long id) {
            this.id = id;
        }

        @Override
        public final String toString() {
            StringBuilder repr = new StringBuilder(getClass().getSimpleName()).append('[');
            toString(repr);
            return repr.append(']').toString();
        }

        void toString(StringBuilder repr) {
            repr.append("id=").append(id);
        }

        @Override
        public int hashCode() {
            return (int) (id ^ (id >>> 32));
        }

        @Override
        public boolean equals(Object obj) {
            if (obj != null && obj.getClass().equals(getClass())) {
                LockedEntity that = (LockedEntity) obj;
                return this.id == that.id;
            }
            return false;
        }
    }

    private class LockReference extends Lock {
        private final LockedEntity key;
        private HANDLE handle;

        LockReference(LockedEntity key, HANDLE handle) {
            this.key = key;
            this.handle = handle;
        }

        @Override
        public String toString() {
            StringBuilder repr = new StringBuilder(key.getClass().getSimpleName()).append('[');
            key.toString(repr);
            if (handle != null) {
                repr.append("; HELD_BY=").append(handle);
            } else {
                repr.append("; RELEASED");
            }
            return repr.append(']').toString();
        }

        @Override
        public void release() {
            if (handle == null) {
                return;
            }
            try {
                AbstractLockService.this.release(key, handle);
            } finally {
                handle = null;
            }
        }
    }

    static final class LockedNode extends LockedEntity {
        LockedNode(long nodeId) {
            super(nodeId);
        }
    }

    static final class LockedRelationship extends LockedEntity {
        LockedRelationship(long relationshipId) {
            super(relationshipId);
        }
    }

    private static class CustomLockedEntity extends LockedEntity {
        private final int type;

        private CustomLockedEntity(int type, long id) {
            super(id);
            this.type = type;
        }

        @Override
        public int hashCode() {
            return Objects.hash(type, id);
        }

        @Override
        public boolean equals(Object obj) {
            if (!super.equals(obj)) {
                return false;
            }
            CustomLockedEntity that = (CustomLockedEntity) obj;
            return this.type == that.type;
        }
    }
}