bundles/org.eclipse.core.jobs/src/org/eclipse/core/runtime/jobs/ILock.java

/*******************************************************************************
 * Copyright (c) 2003, 2012 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM - Initial API and implementation
 *******************************************************************************/
package org.eclipse.core.runtime.jobs;

/**
 * A lock is used to control access to an exclusive resource.
 * <p>
 * Locks are reentrant.  That is, they can be acquired multiple times by the same thread
 * without releasing.  Locks are only released when the number of successful acquires 
 * equals the number of successful releases.
 * </p><p>
 * Locks are capable of detecting and recovering from programming errors that cause
 * circular waiting deadlocks. When a deadlock between two or more <tt>ILock</tt> 
 * instances is detected, detailed debugging information is printed to the log file.  The 
 * locks will then automatically recover from the deadlock by employing a release 
 * and wait strategy. One thread will lose control of the locks it owns, thus breaking 
 * the deadlock and  allowing other threads to proceed.  Once that thread's locks are 
 * all available, it will be given exclusive access to all its locks and allowed to proceed.  
 * A thread can only lose locks while it is waiting on an <tt>acquire()</tt> call. 
 * 
 * </p><p>
 * Successive acquire attempts by different threads are queued and serviced on
 * a first come, first served basis.
 * </p><p>
 * It is very important that acquired locks eventually get released.  Calls to release
 * should be done in a finally block to ensure they execute.  For example:
 * <pre>
 * try {
 * 	lock.acquire();
 * 	// ... do work here ...
 * } finally {
 * 	lock.release();
 * }
 * </pre>
 * Note: although <tt>lock.acquire</tt> should never fail, it is good practice to place 
 * it inside the try block anyway.  Releasing without acquiring is far less catastrophic 
 * than acquiring without releasing.
 * </p>
 * 
 * @see IJobManager#newLock()
 * @since 3.0
 * @noimplement This interface is not intended to be implemented by clients.
 * @noextend This interface is not intended to be extended by clients.
 */
public interface ILock {
	/**
	 * Attempts to acquire this lock.  If the lock is in use and the specified delay is
	 * greater than zero, the calling thread will block until one of the following happens:
	 * <ul>
	 * <li>This lock is available</li>
	 * <li>The thread is interrupted</li>
	 * <li>The specified delay has elapsed</li>
	 * </ul>
	 * <p>
	 * While a thread is waiting,  locks it already owns may be granted to other threads 
	 * if necessary to break a deadlock.  In this situation, the calling thread may be blocked
	 * for longer than the specified delay.  On returning from this call, the calling thread 
	 * will once again have exclusive access to any other locks it owned upon entering 
	 * the acquire method.
	 * 
	 * @param delay the number of milliseconds to delay
	 * @return <code>true</code> if the lock was successfully acquired, and 
	 * <code>false</code> otherwise.
	 * @exception InterruptedException if the thread was interrupted
	 */
	public boolean acquire(long delay) throws InterruptedException;

	/**
	 * Acquires this lock.  If the lock is in use, the calling thread will block until the lock 
	 * becomes available.  If the calling thread owns several locks, it will be blocked
	 * until all threads it requires become available, or until the thread is interrupted.
	 * While a thread is waiting, its locks may be granted to other threads if necessary
	 * to break a deadlock.  On returning from this call, the calling thread will 
	 * have exclusive access to this lock, and any other locks it owned upon
	 * entering the acquire method.
	 * <p>
	 * This implementation ignores attempts to interrupt the thread.  If response to
	 * interruption is needed, use the method <code>acquire(long)</code>
	 */
	public void acquire();

	/**
	 * Returns the number of nested acquires on this lock that have not been released.
	 * This is the number of times that release() must be called before the lock is
	 * freed.
	 * 
	 * @return the number of nested acquires that have not been released
	 */
	public int getDepth();

	/**
	 * Releases this lock. Locks must only be released by the thread that currently
	 * owns the lock.
	 */
	public void release();
}