What is lock striping?
Lock striping is a technique where the locking occurs on several buckets or stripes, meaning that accessing a bucket only locks that bucket and not the entire data structure.
Do you need to lock ConcurrentHashMap?
In ConcurrentHashMap, at a time any number of threads can perform retrieval operation but for updated in the object, the thread must lock the particular segment in which the thread wants to operate.
What is the use of ConcurrentHashMap?
You should use ConcurrentHashMap when you need very high concurrency in your project. It is thread safe without synchronizing the whole map . Reads can happen very fast while write is done with a lock. There is no locking at the object level.
How many threads can ConcurrentHashMap read?
By default ConcurrentHashMap has segment array size as 16 so simultaneously 16 Threads can put data in map considering each thread is working on separate Segment array index.
What is ReadWriteLock in Java?
A ReadWriteLock maintains a pair of associated locks , one for read-only operations and one for writing. The read lock may be held simultaneously by multiple reader threads, so long as there are no writers. The write lock is exclusive.
What is difference between lock and synchronization in Java?
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.
How ConcurrentHashMap is fail-safe?
Iterator of ConcurrentHashMap is fail-safe, it means that it doesn’t throw ConcurrentModificationException even if underlying ConcurrentHashMap is modified once Iteration begins.
How is ConcurrentHashMap thread-safe?
ConcurrentHashMap class achieves thread-safety by dividing the map into segments, the lock is required not for the entire object but for one segment, i.e one thread requires a lock of one segment. In ConcurrenHashap the read operation doesn’t require any lock.
How is ConcurrentHashMap locking implemented?
ConcurrentHashMap uses internally divides buckets into segments and one particular segment can be locked by a thread for writing purpose.
Why is ConcurrentHashMap thread-safe?
Is ConcurrentHashMap volatile?
No, you don’t. volatile means that the variable cannot be cached in a register, and so will always be “write-through” to memory. This means that one thread’s change to a variable will be visible to other threads.
Is ConcurrentHashMap synchronized?
In ConcurrentHashMap, the Object is divided into a number of segments according to the concurrency level. By default, it allows 16 thread to read and write from the Map without any synchronization….Java.
| ConcurrentHashMap | Synchronized HashMap |
|---|---|
| It locks some portion of the map. | It locks the whole map. |
Why is ReentrantLock needed?
When should you use ReentrantLock s? According to that developerWorks article… The answer is pretty simple — use it when you actually need something it provides that synchronized doesn’t, like timed lock waits, interruptible lock waits, non-block-structured locks, multiple condition variables, or lock polling.
What is a ReentrantReadWriteLock?
ReentrantReadWriteLock class of Java is an implementation of ReadWriteLock, that also supports ReentrantLock functionality. The ReadWriteLock is a pair of associated locks, one for read-only operations and one for writing.
Is synchronized or lock better?
Lock framework works like synchronized blocks except locks can be more sophisticated than Java’s synchronized blocks. Locks allow more flexible structuring of synchronized code.
Is ConcurrentHashMap fail-fast?
The concurrent map, as opposed to non concurrent hashmap, does not fast-fail if you add stuff while iterating on it. However, there is no guarantee about when and if the iterator (or the get() method, or any other read operation) will see newly added/removed elements or not.
How does ConcurrentHashMap achieve scalability?
Unlike Hashtable which achieves its thread-safety by compromising the scalability, ConcurrentHashMap uses advanced techniques e.g. dividing the map into segments to remain thread-safe and scalable at the same time.
How many locks are there in ConcurrentHashMap?
For example, the implementation of ConcurrentHashMap uses an array of 16 locks, each of which guards 1/16 of the hash buckets; bucket N is guarded by lock N mod 16. I still have problems to understand and visualize the lock striping and buckets mechanism. Can someone explain this with good understanding words 🙂 Thanks in advance.
What is lock splitting and lock striping?
Lock splitting can sometimes be extended to partition locking on a variablesized set of independent objects, in which case it is called lock striping. For example, the implementation of ConcurrentHashMap uses an array of 16 locks, each of which guards 1/16 of the hash buckets; bucket N is guarded by lock N mod 16.
What is lock striping in Java?
According to Java Concurrency in Practice, chapter 11.4.3 says: Lock splitting can sometimes be extended to partition locking on a variablesized set of independent objects, in which case it is called lock striping.
What is the difference between singlelock and stripedlock?
Our SingleLock class defines a single lock for the entire data structure using a ReentrantLock: 4.4. Concurrent Access with Striped Then, the StripedLock class defines a striped lock for each bucket: So, which strategy performs better?