Related
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?
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.
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)).
...
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).
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.
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.
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
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.
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.
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
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
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)).
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
Ability to lock interruptibly.
Ability to timeout while waiting for lock.
Power to create fair lock.
API to get list of waiting thread for lock.
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.
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.
Major difference between lock and synchronized:
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.
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..."
Lock makes programmers' life easier. Here are a few situations that can be achieved easily with lock.
Lock in one method, and release the lock in another method.
If You have two threads working on two different pieces of code, however, in the first thread has a pre-requisite on a certain piece of code in the second thread (while some other threads also working on the same piece of code in the second thread simultaneously). A shared lock can solve this problem quite easily.
Implementing monitors. For example, a simple queue where the put and get methods are executed from many other threads. However, you do not want multiple put (or get) methods running simultaneously, neither the put and get method running simultaneously. A private lock makes your life a lot easier to achieve this.
While, the lock, and conditions build on the synchronized mechanism. Therefore, can certainly be able to achieve the same functionality that you can achieve using the lock. However, solving complex scenarios with synchronized may make your life difficult and can deviate you from solving the actual problem.
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
The way Lock interface with class Reentrant(true) lock works is that it uses BlockingQueue to store Threads that want to acquire the lock. In that way thread that 'came first, go out first'-FIFO. All clear about that.
But where do 'unfair locks' go, or ReentrantLock(false). What is their internal implementation? How does OS decide which thread now to pick? And most importantly are now these threads also stored in a queue or where? (they must be somewhere)
The class ReentrantLock does not use a BlockingQueue. It uses a non-public subclass of AbstractQueuedSynchronizer behind the scenes.
The AbstractQueuedSynchronizer class, as its documentation states, maintains “a first-in-first-out (FIFO) wait queue”. This data structure is the same for fair and unfair locks. The unfairness doesn’t imply that the lock would change the order of enqueued waiting threads, as there would be no advantage in doing that.
The key difference is that an unfair lock allows a lock attempt to succeed immediately when the lock just has been released, even when there are other threads waiting for the lock for a longer time. In that scenario, the queue is not even involved for the overtaking thread. This is more efficient than adding the current thread to the queue and putting it into the wait state while removing the longest waiting thread from the queue and changing its state to “runnable”.
When the lock is not available by the time, a thread tries to acquire it, the thread will be added to the queue and at this point, there is no difference between fair and unfair locks for it (except that other threads may overtake it without getting enqueued). Since the order has not been specified for an unfair lock, it could use a LIFO data structure behind the scenes, but it’s obviously simpler to have just one implementation code for both.
For synchronized, on the other hand, which does not support fair acquisition, there are some JVM implementations using a LIFO structure. This may change from one version to another (or even with the same, as a side effect of some JVM options or environmental aspects).
Another interesting point in this regard, is that the parameterless tryLock() of the ReentrantLock implementation will be unfair, even when the lock is otherwise in fair mode. This demonstrates that being unfair is not a property of the waiting queue here, but the treatment of the arriving thread that makes a new lock attempt.
Even when this lock has been set to use a fair ordering policy, a call to tryLock() will immediately acquire the lock if it is available, whether or not other threads are currently waiting for the lock. This "barging" behavior can be useful in certain circumstances, even though it breaks fairness.
Monitor = mutex(lock) + condition variable
Each Java object has a monitor, holding above principle.
synchronized key word claim a monitor(lock + conditionvar) of an object.
My understanding is, for atomicity, conditionvar is not required, lock(mutex) would suffice.
To maintain atomicity of a memory area, Java provides Lock , atomic package and binary semaphore.
For atomicity, Which approach is better in terms of performance?
It depends on the access pattern: synchronized(var) { ... } is the simplest to use as it doesn't require explicit unlock, unlike ReentrantLock. Those two are the same: synchronized(var) will grab a lock on var, while Lock will grab a lock on "itself" (so to say). But the ReentrantLock allows you to get extended information (see its javadoc for more information: isHeldByCurrentThread() and getHoldCount()).
Performance-wise, ReentrantReadWriteLock will improve performance when you have few writes and many reads (as you don't need to lock when you're only reading), but you should take extra care when taking and releasing locks, as "ownable synchronizers" can deadlock your threads (read and write locks are not treated in the same manner).
But if the data you want to read/write is a "simple type" (as described in the atomic package javadoc), you will get the best performance by using the AtomicInteger and the likes, as they use specific, optimized CPU instruction set such as compare-and-swap in SSE* set.
I'm starting with Threads, Semaphores, volatile variables, etc.
I wonder if when I'm using Semaphores it is necessary to define the variable as volatile, I mean:
Having 2 Threads, one increases and the other decreases the variable for example and obviously, before each access I have a mutex that controls at any time only one thread is "playing" with the variable.
It would be necessary to define as volatile?
From API doc of Semaphore:
Memory consistency effects: Actions in a thread prior to calling a
"release" method such as release() happen-before actions following a
successful "acquire" method such as acquire() in another thread.
So it is safe to read/write variables that are guarded by a semaphore. No need to declare them as volatile.
Semaphores should not be used in the place of synchronized because semaphores does not hold exclusive mutual lock even if it is initialized to one, like synchronized on some object. It is true that the semaphore initialized to one, allows only one thread at a time to access the object, which holds the permit. But the thread which holds the permit does not own it, any other thread could release that permit and acquire the permit. So, two threads could get access to the same object at the same time, and if both threads manipulates that object, there will be multi-threading issues like lost update, stale read etc.
In your example of having 2 threads, one increasing and one decreasing the same variable. mutual exclusion is sufficient and volatile declaration is not needed. here I assume mutual exclusion is achieved via synchronized and not by semaphores.
volatile is less strict than synchronized, you may want to use volatile when the operations performed are atomic (read or write). volatile should not be used when the performing read-update-write operation.
I wonder if when I'm using Semaphores it is necessary to define the
variable as volatile,
I dont think there is any such restriction. A mutex is a mutual exclusion semaphore, a special variant of a semaphore that only allows one locker at a time. It's equivalent to a normal counting semaphore with a count of one and the requirement that it can only be released by the same thread that locked it.
If we talk specifically for Semaphore in Java: A semaphore is a counter of permits and acquire is like decrement which waits rather than go below zero. It has no upper limit. As mentioned in CIP:
The implementation has no actual permit objects, and Semaphore does
not associate dispensed permits with threads, so a permit acquired in
one thread can be released from another thread. You can think of
acquire as consuming a permit and release as creating one; a
Semaphore is not limited to the number of permits it was created with.
For your scenario you can share a counter and make it volatile or better use AtomicInteger as they use CAS mechanism which performs really well under low contention.
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?
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.
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)).
...
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).
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.
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.
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
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.
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.
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
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
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)).
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
Ability to lock interruptibly.
Ability to timeout while waiting for lock.
Power to create fair lock.
API to get list of waiting thread for lock.
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.
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.
Major difference between lock and synchronized:
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.
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..."
Lock makes programmers' life easier. Here are a few situations that can be achieved easily with lock.
Lock in one method, and release the lock in another method.
If You have two threads working on two different pieces of code, however, in the first thread has a pre-requisite on a certain piece of code in the second thread (while some other threads also working on the same piece of code in the second thread simultaneously). A shared lock can solve this problem quite easily.
Implementing monitors. For example, a simple queue where the put and get methods are executed from many other threads. However, you do not want multiple put (or get) methods running simultaneously, neither the put and get method running simultaneously. A private lock makes your life a lot easier to achieve this.
While, the lock, and conditions build on the synchronized mechanism. Therefore, can certainly be able to achieve the same functionality that you can achieve using the lock. However, solving complex scenarios with synchronized may make your life difficult and can deviate you from solving the actual problem.
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