Tuesday, February 7, 2017

Top 5 Concurrent Collections from JDK 5 and 6 Java Programmer Should Know

Several new Collection classes are added in Java 5 and Java 6 especially concurrent alternatives of standard synchronized ArrayList, Hashtable and  synchronized HashMap collection classes. Many Java programmer still not familiar with these new collection classes from java.util.concurrent package and misses a whole new set of functionality which can be utilized to build more scalable and high-performance Java application. In this Java tutorial, we will some of useful collection classes like ConcurrentHashMap, BlockingQueue which provides some of the very useful functionalities to build concurrent Java applications.

By the way, this is not a comprehensive article explaining each feature of all these concurrent collections, Instead, I will just try to list out why they are there, which Collection class they replace or provides alternative for. The idea is to keep it short and simple while highlighting key points of those useful java.util.concurrent collections.

1. ConcurrentHashMap

Java Concurrent Collections from JDK 5 and 6 Example TutorialConcurrentHashMap is undoubtedly most popular collection class introduced in Java 5 and most of us are already using it. ConcurrentHashMap provides a concurrent alternative of Hashtable or Synchronized Map classes with aim to support higher level of concurrency by implementing fined grained locking. 

Multiple reader can access the Map concurrently  while a portion of Map gets locked for write operation depends upon concurrency level of Map.

The ConcurrentHashMap provides better scalability than there synchronized counter part. Iterator of ConcurrentHashMap are fail-safe iterators which doesn't throw ConcurrencModificationException thus eliminates another requirement of locking during iteration which result in further scalability and performance.

2. CopyOnWriteArrayList and CopyOnWriteArraySet

CopyOnWriteArrayList is a concurrent alternative of synchronized List. CopyOnWriteArrayList provides better concurrency than synchronized List by allowing multiple concurrent reader and replacing the whole list on write operation. 

Yes, write operation is costly on CopyOnWriteArrayList but it performs better when there are multiple reader and requirement of iteration is more than writing. Since CopyOnWriteArrayList Iterator also don't throw ConcurrencModificationException it eliminates need to lock the collection during iteration. 

Remember both ConcurrentHashMap and CopyOnWriteArrayList doesn't provides same level of locking as Synchronized Collection and achieves thread-safety by there locking and mutability strategy. So they perform better if requirements suits there nature.

Similarly, CopyOnWriteArraySet is a concurrent replacement to Synchronized Set. See What is CopyOnWriteArrayList in Java for more details

3. BlockingQueue

BlockingQueue is also one of better known collection class in Java 5. BlockingQueue makes it easy to implement producer-consumer design pattern by providing inbuilt blocking support for put() and take() method. put() method will block if Queue is full while take() method will block if Queue is empty. 

Java 5 API provides two concrete implementation of BlockingQueue in form of ArrayBlockingQueue and LinkedBlockingQueue, both of them implement FIFO ordering of element. ArrayBlockingQueue is backed by Array and its bounded in nature while LinkedBlockingQueue is optionally bounded. 

Consider using BlockingQueue to solve producer Consumer problem in Java instead of writing your won wait-notify code. Java 5 also provides PriorityBlockingQueue, another implementation of BlockingQueue which is ordered on priority and useful if you want to process elements on order other than FIFO.

4. Deque and BlockingDeque

Deque interface is added in Java 6 and it extends Queue interface to support insertion and removal from both end of Queue referred as head and tail. Java6 also provides concurrent implementation of Deque like ArrayDeque and LinkedBlockingDeque

Deque Can be used efficiently to increase parallelism in program by allowing set of worker thread to help each other by taking some of work load from other thread by utilizing Deque double end consumption property. So if all Thread has there own set of task Queue and they are consuming from head; helper thread can also share some work load via consumption from tail.

5. ConcurrentSkipListMap and ConcurrentSkipListSet

Just like ConcurrentHashMap provides a concurrent alternative of synchronized HashMap. ConcurrentSkipListMap and ConcurrentSkipListSet provide concurrent alternative for synchronized version of SortedMap and SortedSet

For example instead of using TreeMap or TreeSet wrapped inside synchronized Collection, You can consider using ConcurrentSkipListMap or ConcurrentSkipListSet from java.util.concurrent package. They also implement NavigableMap and NavigableSet to add additional navigation method we have seen in our post How to use NavigableMap in Java.

That’s all on this list of concurrent Collection classes from Java 5 and 6. They are added on java.util.concurrent package as concurrent alternative of there synchronized counterpart. It’s good idea to learn these Collection classes along with other popular classes from Java Collection Framework.

Further Learning
Java In-Depth: Become a Complete Java Engineer
Java Fundamentals: Collections
Data Structures and Algorithms: Deep Dive Using Java

Related Java Collection Tutorials from Javarevisited Blog


Unknown said...

Nice Stuff ..could you provide some programs using the above concurrent collection classes like producer & consumer by using BlockingQue.

Javin @ ClassLoader in Java said...

@Chiranjib, please see Producer Consumer using BlockingQueue

Anonymous said...

ConcurrentHashMaps also consume a lot of memory compared to a regular HashMap (1,7kb for an empty CCH as described here: http://www.javatuning.com/concurrenthashmap-fat-memory-footprint/).

Yair Blog said...

Also worth mentioning the Guava library set that adds a few more thread safe collections

Anonymous said...

Are concurrent collections are same as thread safe collection in Java? What is different between older thread safe collection e.g. Vector and Hashtable and new thread-safe collection classes like CopyOnWriteArrayList and ConcurrentHashMap?

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