View source for AllOrNothingLocks

credit for this assignment: [http://jcip.net/ Java Concurrency in Practice], Ben Choi, Miles Cushing, and Dennis Cosgrove

=Motivation=
In addition to [[Ordered_Locks|Ordered Locks]] we can employ an all-or-nothing strategy for acquiring locks for mutual exclusion.

We gain experience with explicit locks and trying to acquire them and responding appropriately to success and failure.

=Background=
Check out the [[Lock_Ordering#Background|lock ordering]] exercise.

In Java, locks are a synchronization tool that exists to ensure whatever is protected by the lock is only accessed one thread at a time, as to avoid a possible data race. In this exercise we will explore the second type of locks: explicitly created <code>Lock</code> objects from the <code>java.util.concurrent.locks</code> package.

Every object has an intrinsic lock associated with the object, but <code>Lock</code> objects are explicitly defined and instantiated locks which can be called to lock or unlock. Lock objects are acted upon just like any other object, but we are mostly interested in the <code>lock()</code> method, the <code>tryLock()</code> method, and the <code>unlock()</code> method. For this exercise, we will specifically use the <code>ReentrantLock</code> object, a class which implements the general <code>Lock</code> interface.

To demonstrate how to avoid both data races and deadlock issues, we will create a method designed to transfer money between two bank accounts. Two parties should be able to asynchronously and continuously transfer money between each other without a data race (courtesy of locks) or deadlock (something you will address).

For this approach, we will make use of the <code>tryLock()</code> method in order to attempt to acquire an object’s lock. If the object’s lock is not available at the time, we will continue to try to acquire the lock until it finally succeeds, after which we can successfully transfer funds from one bank account to another. In this approach, an asynchronous pair of transfers from A to B and B to A will not lead to deadlock as the two processes will give up on acquiring the lock until it becomes available, at which point it will finally acquire the lock. This approach does not make use of ordering, but simply relies on making lock attempts give up if the desired lock is not immediately available (and then try again later).

==Java Util Concurrent==
Refer to Oracle's official documentation for more information:
* [https://docs.oracle.com/javase/tutorial/essential/concurrency/newlocks.html Lock Objects Tutorial] 
* [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/package-summary.html package java.util.concurrent.locks]

*[https://docs.oracle.com/javase/tutorial/java/nutsandbolts/while.html while loops]
*[https://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html finally]
*[https://docs.oracle.com/javase/8/docs/api/java/lang/Thread.html#yield-- Thread yield]

* [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/Lock.html-- Lock]
** [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/Lock.html#tryLock-- tryLock]
** [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/Lock.html#unlock--- unlock]

* [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/ReentrantLock.html--- ReentrantLock]
** [https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/ReentrantLock.html#isHeldByCurrentThread--- isHeldByCurrentThread ]

* [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html ListIterator<E>]
** [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html#hasNext-- boolean hasNext()]
** [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html#next-- E next()]
** [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html#hasPrevious-- boolean hasPrevious()]
** [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html#previous-- E previous()]

=Somewhat Dated Demo Videos=
{{CollapsibleYouTube|All Or Nothing Locks|<youtube>0MLAdhm1RCA</youtube>}}

{{CollapsibleYouTube|Bank Account Try Lock|<youtube>yaB2f8CS5wQ</youtube>}}

{{CollapsibleYouTube|AllOrNothingLockUtils|<youtube>cI_wrhfT7Vw</youtube>}}

{{CollapsibleYouTube|Chef|<youtube>rKIEDf1nHVg</youtube>}}


=Code To Implement=
As mentioned in the background section, this implementation will avoid deadlock by giving up on acquiring a lock if it is not immediately available before coming back to it later (when it might actually be available). This implementation will need to make use of the ReentrantLocks associated with the sender and recipient bank account objects. More specifically, it will have to make use of the <code>tryLock()</code> method. For an example of how to format a tryLock, look below:

 <nowiki>Lock lock = ...
if (lock.tryLock()) {
	try {
		doSomething();
	} finally {
		lock.unlock();
	}
}</nowiki>

===BankAccountLockTrying===

{{CodeToImplement|BankAccountLockTrying|attemptToTransferMoney|lock.allornothing.bank.exercise}}

{{ThreadSafe|public static TransferResult attemptToTransferMoney(AccountWithReentrantLock sender, AccountWithReentrantLock recipient, int amount)}}

A couple of notes and common issues:

*<code>tryLock()</code> will grab the lock if it can. There's no need to call <code>lock()</code> after <code>tryLock()</code>.
*We recommend you call the <code>TransferUtils.checkBalanceAndTransfer(sender, receiver, amount)</code> method for the sake of simplicity. This method will check that the sender and recipient are not the same people and that the sender has enough money in her account to send the specified amount to the recipient.
*Anytime you lock a lock object, you should/must use a <code>try/finally</code> block to unlock the lock.
*Your implementation will have to make use of a nested <code>try/finally</code> block (one <code>try/finally</code> call within another).
*If at any point, you are unable to complete the transaction, your program should first, give up any locks it's currently holding (via the finally's), and then return the UNABLE_TO_ATTEMPT_AT_THIS_TIME TransferResult enum.

===AllOrNothingLockUtils===
{{CodeToImplement|AllOrNothingLockUtils|tryLockAll<br>unlockAll<br>runWithAllLocksOrDontRunAtAll|lock.allornothing.util.exercise}}

tryLockAll and unlockAll are private methods which will be used in the implementation of runWithAllLocksOrDontRunAtAll.

====tryLockAll====
{{Sequential|private static boolean tryLockAll(List<ReentrantLock> locks)}}

tryLockAll should... well... [https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/Lock.html#tryLock() try to acquire] all of the locks.

* if all of the locks are successfully locked, true should be returned (with all of the locks held).
* if all of the locks cannot be successfully locked, the locks that were acquired should be unlocked.

{{Alert|A lock cannot be unlocked from a thread that doesn't hold the lock.<br/>Consider using the [https://docs.oracle.com/javase/8/docs/api/java/util/ListIterator.html ListIterator] interface to move forward and backward through the provided list of ReentrantLocks.}}

====unlockAll====
{{Sequential|private static void unlockAll(List<ReentrantLock> locks)}}

unlockAll should... well... [https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantLock.html#unlock() unlock] all of the locks.  This method should only be called when all of the locks are held by the current thread.  Thus, it should throw an [https://docs.oracle.com/javase/7/docs/api/java/lang/IllegalMonitorStateException.html IllegalMonitorStateException] if one of the locks is not held by the current thread.

* If a lock is held by the current thread, it should be unlocked.
* If a lock is not held by the current thread, an [https://docs.oracle.com/javase/7/docs/api/java/lang/IllegalMonitorStateException.html IllegalMonitorStateException] should be thrown.  Note: this perhaps sounds more complicated than it is.  Calling an [https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReentrantLock.html#unlock() unlock] on an unheld Lock will throw an [https://docs.oracle.com/javase/7/docs/api/java/lang/IllegalMonitorStateException.html IllegalMonitorStateException].  Therefore, a simple for each loop that unlocks all of the locks without checking to see if they are held will produce the correct behavior.

====runWithAllLocksOrDontRunAtAll====
{{ThreadSafe|public static boolean runWithAllLocksOrDontRunAtAll(List<ReentrantLock> locks, Runnable body)}}

runWithAllLocksOrDontRunAtAll should use tryLockAll and unlockAll to produce the desired behavior.

* If all of the locks are successfully acquired, the [https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html Runnable] body should be [https://docs.oracle.com/javase/7/docs/api/java/lang/Runnable.html#run() run] while the locks are being held, and then the locks should be released before returning true indicating success.
* If all of the locks cannot be acquired, false should be returned indicating failure to run the specified body.

====runWithAllLocks====
This version of runWithAllLocks is a convenience method which repeatedly tries to runWithAllLocksOrDontRunAtAll until it works, invoking the failureAction each time it fails.

{{CollapsibleCode|runWithAllLocks(locks, body, failureAction)|
<syntaxhighlight lang="java">
public static void runWithAllLocks(List<ReentrantLock> locks, Runnable body, InterruptibleRunnable failureAction) throws InterruptedException {
	while (true) {
		if (runWithAllLocksOrDontRunAtAll(locks, body)) {
			break;
		} else {
			failureAction.run();
		}
	}
}
</syntaxhighlight>}}

====runWithAllLocks====
This version of runWithAllLocks is a convenience method which calls the more general version of runWithAllLocks, specifying to [https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#yield() yield] the current Thread as its failure action.

{{CollapsibleCode|runWithAllLocks(locks, body)|
<syntaxhighlight lang="java">
public static void runWithAllLocks(List<ReentrantLock> locks, Runnable body) throws InterruptedException {
	runWithAllLocks(locks, body, () -> {
		if (Thread.currentThread().isInterrupted()) {
			throw new InterruptedException();
		}
		Thread.yield();
	});
}
</syntaxhighlight>}}

===Chef===

{{CodeToImplement|Chef|attemptToPrepare|lock.allornothing.kitchen.exercise}}

{{Sequential|public static boolean attemptToPrepare(Recipe recipe)}}

A couple of notes:

*An object of class Recipe contains a list of Appliances that it needs in order to complete the item. An object of class Appliance has a lock that you can use to make sure the method remains thread-safe.
*Use the <code>recipe.prepare()</code> method in order to actually execute the recipe. Caution: this method will throw an <code>IllegalThreadStateException()</code> if it is called without all the locks being held by the same thread that called the create method.
* Since it is unknown how many locks each recipe will need beforehand, make use of the <code>AllOrNothingLockUtils.runWithAllLocksOrDontRunAtAll(List<ReentrantLock> locks, Runnable body)</code> method you wrote earlier.

Perhaps a higher-order function from earlier in the semester will be useful here.

{{Spoiler|[[Higher-order_Functions_Map_And_Reduce_Assignment#map|map higher-order function]]}}

=Testing Your Solution=
{{TestSuite|_TryLockingTestSuite|lock.allornothing.exercise}}

=Pledge, Acknowledgments, Citations=
{{Pledge|all-or-nothing-locks}}