AllOrNothingLocks

credit for this assignment: Java Concurrency in Practice, Ben Choi, Miles Cushing, and Dennis Cosgrove

Motivation

In addition to 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 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 Lock objects from the java.util.concurrent.locks package.

Every object has an intrinsic lock associated with the object, but Lock 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 lock() method, the tryLock() method, and the unlock() method. For this exercise, we will specifically use the ReentrantLock object, a class which implements the general Lock 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 tryLock() 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:

• Lock Objects Tutorial
• package java.util.concurrent.locks

• while loops
• finally
• Thread yield

• Lock
  • tryLock
  • unlock

• ReentrantLock
  • isHeldByCurrentThread

• ListIterator<E>
  • boolean hasNext()
  • E next()
  • boolean hasPrevious()
  • E previous()

Somewhat Dated Demo Videos

Video: All Or Nothing Locks

Video: Bank Account Try Lock

Video: AllOrNothingLockUtils

Video: Chef

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 tryLock() method. For an example of how to format a tryLock, look below:

Lock lock = ...
if (lock.tryLock()) {
	try {
		doSomething();
	} finally {
		lock.unlock();
	}
}

BankAccountLockTrying

class:	BankAccountLockTrying.java
methods:	attemptToTransferMoney
package:	lock.allornothing.bank.exercise
source folder:	student/src/main/java

method: public static TransferResult attemptToTransferMoney(AccountWithReentrantLock sender, AccountWithReentrantLock recipient, int amount) (thread-safe required)

A couple of notes and common issues:

• tryLock() will grab the lock if it can. There's no need to call lock() after tryLock().
• We recommend you call the TransferUtils.checkBalanceAndTransfer(sender, receiver, amount) 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 try/finally block to unlock the lock.
• Your implementation will have to make use of a nested try/finally block (one try/finally call within another).

AllOrNothingLockUtils

class:	AllOrNothingLockUtils.java
methods:	tryLockAll unlockAll runWithAllLocksOrDontRunAtAll
package:	lock.allornothing.util.exercise
source folder:	student/src/main/java

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

tryLockAll

method: private static boolean tryLockAll(List<ReentrantLock> locks) (sequential implementation only)

tryLockAll should... well... 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. Consider using the ListIterator interface to move forward and backward through the provided list of ReentrantLocks.

unlockAll

method: private static void unlockAll(List<ReentrantLock> locks) (sequential implementation only)

unlockAll should... well... 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 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 IllegalMonitorStateException should be thrown. Note: this perhaps sounds more complicated than it is. Calling an unlock on an unheld Lock will throw an 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

method: public static boolean runWithAllLocksOrDontRunAtAll(List<ReentrantLock> locks, Runnable body) (thread-safe required)

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

• If all of the locks are successfully acquired, the Runnable body should be 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.

Chef

class:	Chef.java
methods:	attemptToPrepare
package:	lock.allornothing.kitchen.exercise
source folder:	student/src/main/java

method: public static boolean attemptToPrepare(Recipe recipe) (sequential implementation only)

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 recipe.prepare() method in order to actually execute the recipe. Caution: this method will throw an IllegalThreadStateException() 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 AllOrNothingLockUtils.runWithAllLocksOrDontRunAtAll(List<ReentrantLock> locks, Runnable body) method you wrote earlier.

Testing Your Solution

class:	_TryLockingTestSuite.java
package:	lock.allornothing.exercise
source folder:	testing/src/test/java

Pledge, Acknowledgments, Citations

file:

all-or-nothing-locks-pledge-acknowledgments-citations.txt

More info about the Honor Pledge