jgit/org.eclipse.jgit/src/org/eclipse/jgit/lib/AsyncObjectLoaderQueue.java

/*
 * Copyright (C) 2010, Google Inc.
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
 * accompanies this distribution, is reproduced below, and is
 * available at http://www.eclipse.org/org/documents/edl-v10.php
 *
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials provided
 *   with the distribution.
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 * - Neither the name of the Eclipse Foundation, Inc. nor the
 *   names of its contributors may be used to endorse or promote
 *   products derived from this software without specific prior
 *   written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
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 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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package org.eclipse.jgit.lib;

import java.io.IOException;

import org.eclipse.jgit.errors.MissingObjectException;

/**
 * Queue to open objects asynchronously.
 *
 * A queue may perform background decompression of objects and supply them
 * (possibly out-of-order) to the application.
 *
 * @param <T>
 *            type of identifier supplied to the call that made the queue.
 */
public interface AsyncObjectLoaderQueue<T extends ObjectId> extends
		AsyncOperation {

	/**
	 * Position this queue onto the next available result.
	 *
	 * Even if this method returns true, {@link #open()} may still throw
	 * {@link MissingObjectException} if the underlying object database was
	 * concurrently modified and the current object is no longer available.
	 *
	 * @return true if there is a result available; false if the queue has
	 *         finished its input iteration.
	 * @throws MissingObjectException
	 *             the object does not exist. If the implementation is retaining
	 *             the application's objects {@link #getCurrent()} will be the
	 *             current object that is missing. There may be more results
	 *             still available, so the caller should continue invoking next
	 *             to examine another result.
	 * @throws IOException
	 *             the object store cannot be accessed.
	 */
	public boolean next() throws MissingObjectException, IOException;

	/**
	 * @return the current object, null if the implementation lost track.
	 *         Implementations may for performance reasons discard the caller's
	 *         ObjectId and provider their own through {@link #getObjectId()}.
	 */
	public T getCurrent();

	/** @return the ObjectId of the current object. Never null. */
	public ObjectId getObjectId();

	/**
	 * Obtain a loader to read the object.
	 *
	 * This method can only be invoked once per result
	 *
	 * Due to race conditions with a concurrent modification of the underlying
	 * object database, an object may be unavailable when this method is
	 * invoked, even though next returned successfully.
	 *
	 * @return the ObjectLoader to read this object. Never null.
	 * @throws MissingObjectException
	 *             the object does not exist. If the implementation is retaining
	 *             the application's objects {@link #getCurrent()} will be the
	 *             current object that is missing. There may be more results
	 *             still available, so the caller should continue invoking next
	 *             to examine another result.
	 * @throws IOException
	 *             the object store cannot be accessed.
	 */
	public ObjectLoader open() throws IOException;
}