Synchronization vs Lock

问题:

java.util.concurrent API provides a class called as Lock, which would basically serialize the control in order to access the critical resource. It gives method such as park() and unpark().

We can do similar things if we can use synchronized keyword and using wait() and notify() notifyAll() methods.

I am wondering which one of these is better in practice and why?

回答1:

If you\'re simply locking an object, I\'d prefer to use synchronized

Example:

Lock.acquire();
doSomethingNifty(); // Throws a NPE!
Lock.release(); // Oh noes, we never release the lock!

You have to explicitly do try{} finally{} everywhere.

Whereas with synchronized, it\'s super clear and impossible to get wrong:

synchronized(myObject) {
    doSomethingNifty();
}

That said, Locks may be more useful for more complicated things where you can\'t acquire and release in such a clean manner. I would honestly prefer to avoid using bare Locks in the first place, and just go with a more sophisticated concurrency control such as a CyclicBarrier or a LinkedBlockingQueue, if they meet your needs.

I\'ve never had a reason to use wait() or notify() but there may be some good ones.

回答2:

I am wondering which one of these is better in practice and why?

I\'ve found that Lock and Condition (and other new concurrent classes) are just more tools for the toolbox. I could do most everything I needed with my old claw hammer (the synchronized keyword), but it was awkward to use in some situations. Several of those awkward situations became much simpler once I added more tools to my toolbox: a rubber mallet, a ball-peen hammer, a prybar, and some nail punches. However, my old claw hammer still sees its share of use.

I don\'t think one is really \"better\" than the other, but rather each is a better fit for different problems. In a nutshell, the simple model and scope-oriented nature of synchronized helps protect me from bugs in my code, but those same advantages are sometimes hindrances in more complex scenarios. Its these more complex scenarios that the concurrent package was created to help address. But using this higher level constructs requires more explicit and careful management in the code.

===

I think the JavaDoc does a good job of describing the distinction between Lock and synchronized (the emphasis is mine):

Lock implementations provide more extensive locking operations than can be obtained using synchronized methods and statements. They allow more flexible structuring, may have quite different properties, and may support multiple associated Condition objects.

...

The use of synchronized methods or statements provides access to the implicit monitor lock associated with every object, but forces all lock acquisition and release to occur in a block-structured way: when multiple locks are acquired they must be released in the opposite order, and all locks must be released in the same lexical scope in which they were acquired.

While the scoping mechanism for synchronized methods and statements makes it much easier to program with monitor locks, and helps avoid many common programming errors involving locks, there are occasions where you need to work with locks in a more flexible way. For example, **some algorithms* for traversing concurrently accessed data structures require the use of \"hand-over-hand\" or \"chain locking\": you acquire the lock of node A, then node B, then release A and acquire C, then release B and acquire D and so on. Implementations of the Lock interface enable the use of such techniques by allowing a lock to be acquired and released in different scopes, and allowing multiple locks to be acquired and released in any order.

With this increased flexibility comes additional responsibility. The absence of block-structured locking removes the automatic release of locks that occurs with synchronized methods and statements. In most cases, the following idiom should be used:

...

When locking and unlocking occur in different scopes, care must be taken to ensure that all code that is executed while the lock is held is protected by try-finally or try-catch to ensure that the lock is released when necessary.

Lock implementations provide additional functionality over the use of synchronized methods and statements by providing a non-blocking attempt to acquire a lock (tryLock()), an attempt to acquire the lock that can be interrupted (lockInterruptibly(), and an attempt to acquire the lock that can timeout (tryLock(long, TimeUnit)).

...

回答3:

You can achieve everything the utilities in java.util.concurrent do with the low-level primitives like synchronized, volatile, or wait / notify

However, concurrency is tricky, and most people get at least some parts of it wrong, making their code either incorrect or inefficient (or both).

The concurrent API provides a higher-level approach, which is easier (and as such safer) to use. In a nutshell, you should not need to use synchronized, volatile, wait, notify directly anymore.

The Lock class itself is on the lower-level side of this toolbox, you may not even need to use that directly either (you can use Queues and Semaphore and stuff, etc, most of the time).

回答4:

There are 4 main factors into why you would want to use synchronized or java.util.concurrent.Lock.

Note: Synchronized locking is what I mean when I say intrinsic locking.

1. When Java 5 came out with ReentrantLocks, they proved to have quite a noticeble throughput difference then intrinsic locking. If youre looking for faster locking mechanism and are running 1.5 consider j.u.c.ReentrantLock. Java 6\'s intrinsic locking is now comparable.

2. j.u.c.Lock has different mechanisms for locking. Lock interruptable - attempt to lock until the locking thread is interrupted; timed lock - attempt to lock for a certain amount of time and give up if you do not succeed; tryLock - attempt to lock, if some other thread is holding the lock give up. This all is included aside from the simple lock. Intrinsic locking only offers simple locking

3. Style. If both 1 and 2 do not fall into categories of what you are concerned with most people, including myself, would find the intrinsic locking semenatics easier to read and less verbose then j.u.c.Lock locking.
4. Multiple Conditions. An object you lock on can only be notified and waited for a single case. Lock\'s newCondition method allows for a single Lock to have mutliple reasons to await or signal. I have yet to actually need this functionality in practice, but is a nice feature for those who need it.

回答5:

I would like to add some more things on top of Bert F answer.

Locks support various methods for finer grained lock control, which are more expressive than implicit monitors (synchronized locks)

A Lock provides exclusive access to a shared resource: only one thread at a time can acquire the lock and all access to the shared resource requires that the lock be acquired first. However, some locks may allow concurrent access to a shared resource, such as the read lock of a ReadWriteLock.

Advantages of Lock over Synchronization from documentation page

1. The use of synchronized methods or statements provides access to the implicit monitor lock associated with every object, but forces all lock acquisition and release to occur in a block-structured way

2. Lock implementations provide additional functionality over the use of synchronized methods and statements by providing a non-blocking attempt to acquire a lock (tryLock()), an attempt to acquire the lock that can be interrupted (lockInterruptibly(), and an attempt to acquire the lock that can timeout (tryLock(long, TimeUnit)).

3. A Lock class can also provide behavior and semantics that is quite different from that of the implicit monitor lock, such as guaranteed ordering, non-reentrant usage, or deadlock detection

ReentrantLock: In simple terms as per my understanding, ReentrantLock allows an object to re-enter from one critical section to other critical section . Since you already have lock to enter one critical section, you can other critical section on same object by using current lock.

ReentrantLock key features as per this article

1. Ability to lock interruptibly.
2. Ability to timeout while waiting for lock.
3. Power to create fair lock.
4. API to get list of waiting thread for lock.
5. Flexibility to try for lock without blocking.

You can use ReentrantReadWriteLock.ReadLock, ReentrantReadWriteLock.WriteLock to further acquire control on granular locking on read and write operations.

Apart from these three ReentrantLocks, java 8 provides one more Lock

StampedLock:

Java 8 ships with a new kind of lock called StampedLock which also support read and write locks just like in the example above. In contrast to ReadWriteLock the locking methods of a StampedLock return a stamp represented by a long value.

You can use these stamps to either release a lock or to check if the lock is still valid. Additionally stamped locks support another lock mode called optimistic locking.

Have a look at this article on usage of different type of ReentrantLock and StampedLock locks.

回答6:

The main difference is fairness, in other words are requests handled FIFO or can there be barging? Method level synchronization ensures fair or FIFO allocation of the lock. Using

synchronized(foo) {
}

or

lock.acquire(); .....lock.release();

does not assure fairness.

If you have lots of contention for the lock you can easily encounter barging where newer requests get the lock and older requests get stuck. I\'ve seen cases where 200 threads arrive in short order for a lock and the 2nd one to arrive got processed last. This is ok for some applications but for others it\'s deadly.

See Brian Goetz\'s \"Java Concurrency In Practice\" book, section 13.3 for a full discussion of this topic.

回答7:

Brian Goetz\'s \"Java Concurrency In Practice\" book, section 13.3: \"...Like the default ReentrantLock, intrinsic locking offers no deterministic fairness guarantees, but the statistical fairness guarantees of most locking implementations are good enough for almost all situations...\"

回答8:

Lock makes programmers life easier. Here are few situations that can be achieved easier with lock.

1. Lock in one method, and release the lock in other method.
2. You have two threads working on two different pieces of code, however first thread there is dependency on there second thread to complete certain piece of code before it proceed any further (while some other threads also working simultaneously). A shared lock can solve this problem quite easily.
3. Implementing monitors. For example, a simple queue where the put and get methods are executed from many different threads. However, do you want to over lap neither the same methods one on another, nor both put and get methods can overlap. In such case a private lock makes the life very easier.

While, the lock, and conditions are build on the synchronized. So certainly you can achiever the same goal with that. However, that might make your life difficult and can deviate you from solving the actual problem.

回答9:

The major difference between lock and synchronized is - with locks, you can release and acquire the locks in any order. - with synchronized, you can release the locks only in the order it was acquired.

回答10:

Lock and synchronize block both serves the same purpose but it depends on the usage. Consider the below part

void randomFunction(){
.
.
.
synchronize(this){
//do some functionality
}

.
.
.
synchronize(this)
{
// do some functionality
}


} // end of randomFunction

In the above case , if a thread enters the synchronize block, the other block is also locked. If there are multiple such synchronize block on the same object, all the blocks are locked. In such situations , java.util.concurrent.Lock can be used to prevent unwanted locking of blocks