jpa/org.eclipse.persistence.jpa.jpql/src/main/java/org/eclipse/persistence/jpa/jpql/utility/iterable/SnapshotCloneListIterable.java

/*
 * Copyright (c) 2012, 2021 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0,
 * or the Eclipse Distribution License v. 1.0 which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
 */

// Contributors:
//     Oracle - initial API and implementation
//
package org.eclipse.persistence.jpa.jpql.utility.iterable;

import java.util.Arrays;
import java.util.List;
import java.util.ListIterator;
import org.eclipse.persistence.jpa.jpql.utility.iterator.CloneListIterator;

/**
 * A <code>SnapshotCloneListIterable</code> returns a list iterator on a
 * "snapshot" of a list, allowing for concurrent access to the original list.
 * A copy of the list is created when the list iterable is constructed.
 * As a result, the contents of the list will be the same with
 * every call to {@link #iterator()}, even if the original list is modified via
 * the list iterator's mutation methods.
 * <p>
 * The original list passed to the <code>SnapshotCloneListIterable</code>'s
 * constructor should be thread-safe (e.g. {@link java.util.Vector});
 * otherwise you run the risk of a corrupted list.
 * <p>
 * By default, the list iterator returned by a <code>SnapshotCloneListIterable</code> does not
 * support the {@link ListIterator} mutation operations; this is because it does not
 * have access to the original list. But if the <code>SnapshotCloneListIterable</code>
 * is supplied with a {@link CloneListIterator.Mutator}
 * it will delegate the
 * {@link ListIterator} mutation operations to the <code>Mutator</code>.
 * Alternatively, a subclass can override the list iterable's mutation
 * methods.
 * <p>
 * This list iterable is useful for multiple passes over a list that should not
 * be changed (e.g. by another thread) between passes.
 *
 * @param <E> the type of elements returned by the list iterable's list iterator
 *
 * @see CloneListIterator
 * @see <a href="http://git.eclipse.org/c/dali/webtools.dali.git/tree/common/plugins/org.eclipse.jpt.common.utility/src/org/eclipse/jpt/common/utility/internal/iterable/LiveCloneListIterable.java">LiveCloneListIterable</a>
 * @see <a href="http://git.eclipse.org/c/dali/webtools.dali.git/tree/common/plugins/org.eclipse.jpt.common.utility/src/org/eclipse/jpt/common/utility/internal/iterable/SnapshotCloneListIterable.java">SnapshotCloneListIterable</a>
 */
public class SnapshotCloneListIterable<E>
    extends CloneListIterable<E>
{
    private final Object[] array;


    // ********** constructors **********

    /**
     * Construct a "snapshot" list iterable for the specified list.
     * The {@link ListIterator} modify operations will not be supported
     * by the list iterator returned by {@link #iterator()}
     * unless a subclass overrides the list iterable's modify
     * method.
     */
    public SnapshotCloneListIterable(List<? extends E> list) {
        super();
        this.array = list.toArray();
    }

    /**
     * Construct a "snapshot" list iterable for the specified list.
     * The specified mutator will be used by any generated list iterators to
     * modify the original list.
     */
    public SnapshotCloneListIterable(List<? extends E> list, CloneListIterator.Mutator<E> mutator) {
        super(mutator);
        this.array = list.toArray();
    }


    // ********** ListIterable implementation **********

    @Override
    public ListIterator<E> iterator() {
        return new LocalCloneListIterator<>(this.mutator, this.array);
    }

    @Override
    public String toString() {
        return Arrays.toString(this.array);
    }


    // ********** clone iterator **********

    /**
     * provide access to "internal" constructor
     */
    protected static class LocalCloneListIterator<E> extends CloneListIterator<E> {
        protected LocalCloneListIterator(Mutator<E> mutator, Object[] array) {
            super(mutator, array);
        }
    }
}