lazyitemiterator.java

来自「jsr170接口的java实现。是个apache的开源项目。」· Java 代码 · 共 216 行

/*
 * 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.jackrabbit.core;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import javax.jcr.Item;
import javax.jcr.Node;
import javax.jcr.NodeIterator;
import javax.jcr.Property;
import javax.jcr.PropertyIterator;
import javax.jcr.RepositoryException;
import javax.jcr.ItemNotFoundException;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.ArrayList;

/**
 * <code>LazyItemIterator</code> is an id-based iterator that instantiates
 * the <code>Item</code>s only when they are requested.
 * <p/>
 * <strong>Important:</strong> <code>Item</code>s that appear to be nonexistent
 * for some reason (e.g. because of insufficient access rights or because they
 * have been removed since the iterator has been retrieved) are silently
 * skipped. As a result the size of the iterator as reported by
 * {@link #getSize()} might appear to be shrinking while iterating over the
 * items.
 * todo should getSize() better always return -1?
 *
 * @see #getSize()
 */
class LazyItemIterator implements NodeIterator, PropertyIterator {

    /** Logger instance for this class */
    private static Logger log = LoggerFactory.getLogger(LazyItemIterator.class);

    /** the item manager that is used to lazily fetch the items */
    private final ItemManager itemMgr;

    /** the list of item ids */
    private final List idList;

    /** the position of the next item */
    private int pos;

    /** prefetched item to be returned on <code>{@link #next()}</code> */
    private Item next;

    /**
     * Creates a new <code>LazyItemIterator</code> instance.
     *
     * @param itemMgr item manager
     * @param idList  list of item id's
     */
    public LazyItemIterator(ItemManager itemMgr, List idList) {
        this.itemMgr = itemMgr;
        this.idList = new ArrayList(idList);
        // prefetch first item
        pos = 0;
        prefetchNext();
    }

    /**
     * Prefetches next item.
     * <p/>
     * {@link #next} is set to the next available item in this iterator or to
     * <code>null</code> in case there are no more items.
     */
    private void prefetchNext() {
        // reset
        next = null;
        while (next == null && pos < idList.size()) {
            ItemId id = (ItemId) idList.get(pos);
            try {
                next = itemMgr.getItem(id);
            } catch (ItemNotFoundException e) {
                log.debug("ignoring nonexistent item " + id);
                // remove invalid id
                idList.remove(pos);
                // try next
            } catch (RepositoryException e) {
                log.error("failed to fetch item " + id + ", skipping...", e);
                // remove invalid id
                idList.remove(pos);
                // try next
            }
        }
    }

    //---------------------------------------------------------< NodeIterator >
    /**
     * {@inheritDoc}
     */
    public Node nextNode() {
        return (Node) next();
    }

    //-----------------------------------------------------< PropertyIterator >
    /**
     * {@inheritDoc}
     */
    public Property nextProperty() {
        return (Property) next();
    }

    //--------------------------------------------------------< RangeIterator >
    /**
     * {@inheritDoc}
     */
    public long getPosition() {
        return pos;
    }

    /**
     * {@inheritDoc}
     * <p/>
     * Note that the size of the iterator as reported by {@link #getSize()}
     * might appear to be shrinking while iterating because items that for
     * some reason cannot be retrieved through this iterator are silently
     * skipped, thus reducing the size of this iterator.
     *
     * todo better to always return -1?
     */
    public long getSize() {
        return idList.size();
    }

    /**
     * {@inheritDoc}
     */
    public void skip(long skipNum) {
        if (skipNum < 0) {
            throw new IllegalArgumentException("skipNum must not be negative");
        }
        if (skipNum == 0) {
            return;
        }
        if (next == null) {
            throw new NoSuchElementException();
        }

        // reset
        next = null;
        // skip the first (skipNum - 1) items without actually retrieving them
        while (--skipNum > 0) {
            pos++;
            if (pos >= idList.size()) {
                // skipped past last item
                throw new NoSuchElementException();
            }
            ItemId id = (ItemId) idList.get(pos);
            // eliminate invalid items from this iterator
            while (!itemMgr.itemExists(id)) {
                log.debug("ignoring nonexistent item " + id);
                // remove invalid id
                idList.remove(pos);
                if (pos >= idList.size()) {
                    // skipped past last item
                    throw new NoSuchElementException();
                }
                id = (ItemId) idList.get(pos);
                // try next
                continue;
            }
        }
        // prefetch final item (the one to be returned on next())
        pos++;
        prefetchNext();
    }

    //-------------------------------------------------------------< Iterator >
    /**
     * {@inheritDoc}
     */
    public boolean hasNext() {
        return next != null;
    }

    /**
     * {@inheritDoc}
     */
    public Object next() {
        if (next == null) {
            throw new NoSuchElementException();
        }
        Item item = next;
        pos++;
        prefetchNext();
        return item;
    }

    /**
     * {@inheritDoc}
     *
     * @throws UnsupportedOperationException always since not implemented
     */
    public void remove() {
        throw new UnsupportedOperationException("remove");
    }
}