I have a Data Transfer Object (DTO) in a Java application that is being read from and written to in several different threads across the application. Up until now I have been able to use synchronized(dto.class) for synchronization. There is now one instance where I need to hold the lock outside of the method that it is called in, so I will use the ReentrantLock() class.
Is there a thread safe way to use a reentrant lock for its functionality in the one instance while keeping the synchronized blocks as is in the rest of the code? Or, is it the case that the use of a reentrant lock means all related synchronized blocks have to be removed?
Is there a thread safe way to use a reentrant lock for its functionality in the one instance while keeping the synchronized blocks as is in the rest of the code?
What data do the synchronized blocks protect? What data do you want to protect with the ReentrantLock? If they're different data then there should be no problem using different means to protect them. But it doesn't make any sense to use synchronized in one place and ReentrantLock in a different place if you're trying to protect the same data in both places.
Locking a ReentrantLock will not prevent some other thread from entering a synchronized block and vice versa.
Related
Effective Java - Item 66 (2nd Edition) states
In fact, synchronization has no effect unless both read and write operations are synchronized.
Suppose I have an object which is being updated by one thread and read by another thread. Synchronization on the read thread is needed for communication purpose, because as stated
In other words, the synchronization on these methods is used solely for its communication effects, not for mutual exclusion.
But if the write is performed by a single thread, why can't we get away without using synchronised during write?
Because of how happens-before works in Java Memory Model. To allow thread2 to see a new value of variable written by thread1 you need to establish happens-before relationship between this write and read (https://www.logicbig.com/tutorials/core-java-tutorial/java-multi-threading/happens-before.html). One of the ways to do it is to use synchronization (synchronized blocks/methods, volatile, AtomicXXX) because releasing a lock always happens before acquiring of the same lock, write to volatile variable happens before subsequent read of the same volatile variable. And AtomicXXX.set() happens before AtomicXXX.get() for the same variable.
Actually, I am a bit confused in regards of several explanation from website or blog about synchronization and thread-safe. I've done some research on different class of Core Java Api or Java Framework (Collections). And i've often noticed that some class are synchronize and thread-safe which means, at a time, only one thread can access the code.
But i need some precision :
A class is synchronize so its thread-safe ?
Or synchronize and thread-safe have two different meaning ?
Best regards
A class is synchronize so its thread-safe ?
A class is not synchronized. Rather a method, or a block of code is synchronized.
Synchronization (using synchronized) is one way to make code thread-safe. There are other ways.
Or synchronize and thread-safe have two different meaning ?
Yes. They have different meanings.
And i've often noticed that some class are synchronize and thread-safe which means, at a time, only one thread can access the code.
Actually, if you "noticed" that, you were not paying attention!
With a synchronized method, only one thread can access the code while holding a given lock; i.e. you get mutual exclusion. If two threads use different locks, then you won't get mutual exclusion.
The other thing to note is that merely using synchronized does not guarantee thread-safety. You need to use it in the right way:
threads need to synchronize on the appropriate objects / locks
threads need to synchronize in all appropriate code
if the code entails acquiring multiple locks, the locks need to be acquired in an order that avoids deadlocks.
I'm new to java.
I'm little bit confused between Threadsafe and synchronized.
Thread safe means that a method or class instance can be used by multiple threads at the same time without any problems occurring.
Where as Synchronized means only one thread can operate at single time.
So how they are related to each other?
The definition of thread safety given in Java Concurrency in Practice is:
A class is thread-safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.
For example, a java.text.SimpleDateFormat object has internal mutable state that is modified when a method that parses or formats is called. If multiple threads call the methods of the same dateformat object, there is a chance a thread can modify the state needed by the other threads, with the result that the results obtained by some of the threads may be in error. The possibility of having internal state get corrupted causing bad output makes this class not threadsafe.
There are multiple ways of handling this problem. You can have every place in your application that needs a SimpleDateFormat object instantiate a new one every time it needs one, you can make a ThreadLocal holding a SimpleDateFormat object so that each thread of your program can access its own copy (so each thread only has to create one), you can use an alternative to SimpleDateFormat that doesn't keep state, or you can do locking using synchronized so that only one thread at a time can access the dateFormat object.
Locking is not necessarily the best approach, avoiding shared mutable state is best whenever possible. That's why in Java 8 they introduced a date formatter that doesn't keep mutable state.
The synchronized keyword is one way of restricting access to a method or block of code so that otherwise thread-unsafe data doesn't get corrupted. This keyword protects the method or block by requiring that a thread has to acquire exclusive access to a certain lock (the object instance, if synchronized is on an instance method, or the class instance, if synchronized is on a static method, or the specified lock if using a synchronized block) before it can enter the method or block, while providing memory visibility so that threads don't see stale data.
Thread safety is a desired behavior of the program, where the synchronized block helps you achieve that behavior. There are other methods of obtaining Thread safety e.g immutable class/objects. Hope this helps.
Thread safety: A thread safe program protects it's data from memory consistency errors. In a highly multi-threaded program, a thread safe program does not cause any side effects with multiple read/write operations from multiple threads on shared data (objects). Different threads can share and modify object data without consistency errors.
synchronized is one basic method of achieving ThreadSafe code.
Refer to below SE questions for more details:
What does 'synchronized' mean?
You can achieve thread safety by using advanced concurrency API. This documentation page provides good programming constructs to achieve thread safety.
Lock Objects support locking idioms that simplify many concurrent applications.
Concurrent Collections make it easier to manage large collections of data, and can greatly reduce the need for synchronization.
Atomic Variables have features that minimize synchronization and help avoid memory consistency errors.
ThreadLocalRandom (in JDK 7) provides efficient generation of pseudorandom numbers from multiple threads.
Refer to java.util.concurrent and java.util.concurrent.atomic packages too for other programming constructs.
Related SE question:
Synchronization vs Lock
Synchronized: only one thread can operate at same time.
Threadsafe: a method or class instance can be used by multiple threads at the same time without any problems occurring.
If you relate this question as, Why synchronized methods are thread safe? than you can get better idea.
As per the definition this appears to be confusive. But not,if you understand it analytically.
Synchronized means: sequentially one by one in an order,Not concurrently [Not at the same time].
synchronized method not allows to act another thread on it, While a thread is already working on it.This avoids concurrency.
example of synchronization: If you want to buy a movie ticket,and stand in a queue. you will get the ticket only after the person in front of you get the ticket.
Thread safe means: method becomes safe to be accessed by multiple threads without any problem at the same time.synchronized keyword is one of the way to achieve 'thread safe'. But Remember:Actually while multiple threads tries to access synchronized method they follow the order so becomes safe to access. Actually, Even they act at the same time, but cannot access the same resource(method/block) at the same time, because of synchronized behavior of the resource.
Because If a method becomes synchronized, so this is becomes safe to allow multiple threads to act on it, without any problem. Remember:: multiple threads "not act on it at the same time" hence we call synchronized methods thread safe.
Hope this helps to understand.
After patiently reading through a lot of answers and not being too technical at the same time, I could say something definite but close to what Nayak had already replied to fastcodejava above, which comes later on in my answer but look
synchronization is not even close to brute-forcing thread-safety; it's just making a piece of code (or method) safe and incorruptible for a single authorized thread by preventing it from being used by any other threads.
Thread safety is about how all threads accessing a certain element behave and get their desired results in the same way if they would have been sequential (or even not so), without any form of undesired corruption (sorry for the pleonasm) as in an ideal world.
One of the ways of achieving proximity to thread-safety would be using classes in java.util.concurrent.atomic.
Sad, that they don't have final methods though!
Nayak, when we declare a method as synchronized, all other calls to it from other threads are locked and can wait indefinitely. Java also provides other means of locking with Lock objects now.
You can also declare an object to be final or volatile to guarantee its availability to other concurrent threads.
ref: http://www.javamex.com/tutorials/threads/thread_safety.shtml
In practice, performance wise, Thread safe, Synchronised, non-thread safe and non-synchronised classes are ordered as:
Hashtable(slower) < Collections.SynchronizedMap < HashMap(fastest)
While reading concurrency in Java, I have following doubts:
Does Java provides lower level construct then synchronized for synchronization?
In what circumstances will we use semaphore over synchronized (which provides monitor behaviour in Java)
Synchronized allows only one thread of execution to access the resource at the same time. Semaphore allows up to n (you get to choose n) threads of execution to access the resource at the same time.
There is also volatile keyword, according to http://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html volatile variable access is more efficient than accessing these variables through synchronized code
java.util.concurrent.Semaphore is used to restrict the number of threads that can access a resource. That is, while synchronized allows only one thread to aquire lock and execute the synchonized block / method, Semaphore gives permission up to n threads to go and blocks the others.
There is also atomics. This gives access to the basic hardware compare-and-swap command that's the basis of all synchronization. It allows you, for example, to increment a number safely. If you ++ a volatile field, another thread executing the same instruction could read the field before your thread writes to it, then write back to it after your thread. So one increment gets lost. Atomics do the read and write "atomically" and so avoid the problem.
Actually, volatiles, synchronized statements, and atomics tend to force all thread data to be refreshed from main memory and/or written to main memory as appropriate, so none of them are really that low level. (I'm simplifying here. Unlike C#, Java does not really have a concept of "main memory".)
Is using Locks (java.util.concurrent.locks.Lock) instead of keyword synchronized + method wait() and method notify() totally the same?
Can I thread-safely program using locks (explicit locks) rather than implicit locks (synchronized)?
As of know I have always been using implicit locks. I am aware of the advantages given by the Lock interface implementation like methods: isLocked(), getLockQueueLength(), getHoldCount(), etc... however still the old school way (wait() and notify()) would have other limits other than not having those methods?
I am also aware of the possibility of constructing a lock with a (boolean fairness) parameter which allows lack of starvation.
Yes, absolutely you can write thread-safe program using java.util.concurrent.locks.Lock. If you see any implementation of java.util.concurrent.locks.Lock like ReentrantLock internal implementation uses old synchronized blocks.
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.
Adding to my difference the synchronized keyword has naturally built in language support. This can mean the JIT can optimise synchronised blocks in ways it cannot with Locks. e.g. it can combine synchronized blocks.synchronized is best for a small number of threads accessing a lock and Lock may be best for a high number of threads accessing the same locks . Also synchronized block makes no guarantees about the sequence in which threads waiting to entering it are granted access.
Locks and synchronized blocks have the same semantics and provide the same guarantees from a Java Memory Model perspective. The main difference is that Locks provide more control (such as with tryLock or when asking a lock to be fair etc.) which allow for a more flexible and fine-grained lock management.
However, when you don't need those additional features, it is better to use a plain old synchronized block as it reduces the room for error (e.g. you can't "forget" to unlock it).