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 the 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 the 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 an alternative for.
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 an alternative for.
The idea is to keep it short and simple while highlighting key
points of those useful java.util.concurrent collections.
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5 Concurrent Collections Every Java Programmer should learn
Here is my list of top 5 Concurrent Collections that I think every Java programmer should know, both beginners and experienced. These are very popular classes and you will often use them in your program. Having a good knowledge of these Concurrent collection classes will help you to write better code in Java.1. ConcurrentHashMap
ConcurrentHashMap is undoubtedly the most popular collection class introduced
in Java 5 and most of us are already using it. ConcurrentHashMap provides a concurrent alternative to Hashtable
or Synchronized Map classes with the aim to support a higher level of concurrency
by implementing fined-grained locking.
Multiple readers 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 their synchronized counterpart. Iterator of ConcurrentHashMap are fail-safe iterators that don't throw ConcurrencModificationException thus eliminates another requirement of locking during iteration which results in further scalability and performance.
Multiple readers 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 their synchronized counterpart. Iterator of ConcurrentHashMap are fail-safe iterators that don't throw ConcurrencModificationException thus eliminates another requirement of locking during iteration which results in further scalability and performance.
If you want to learn more about ConcurrentHashMap and how to use it in Java then I highly recommend you to join a comprehensive Java collections course like Java Fundamentals: Collections by Richard Warburton on Pluralsight, it's a great course to learn about essential Collection classes and interface in Java.
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2. CopyOnWriteArrayList and CopyOnWriteArraySet
CopyOnWriteArrayList is a concurrent alternative
of synchronized List. CopyOnWriteArrayList provides better concurrency
than synchronized List by allowing multiple concurrent readers and replacing the
whole list on write operation.
Yes, write operation is costly on CopyOnWriteArrayList but it performs better when there are multiple readers, and the requirement of iteration is more than writing. Since CopyOnWriteArrayList Iterator also doesn't throw ConcurrencModificationException it eliminates the need to lock the collection during iteration.
Remember both ConcurrentHashMap and CopyOnWriteArrayList doesn't provide the same level of locking as Synchronized Collection and achieves thread-safety by their locking and mutability strategy. So they perform better if requirements suit their nature.
Similarly, CopyOnWriteArraySet is a concurrent replacement to Synchronized Set. See Java Application Performance and Memory Management course on Udemy to learn how to use CopyOnWriteArrayList to write high-performance Java applications.
Yes, write operation is costly on CopyOnWriteArrayList but it performs better when there are multiple readers, and the requirement of iteration is more than writing. Since CopyOnWriteArrayList Iterator also doesn't throw ConcurrencModificationException it eliminates the need to lock the collection during iteration.
Remember both ConcurrentHashMap and CopyOnWriteArrayList doesn't provide the same level of locking as Synchronized Collection and achieves thread-safety by their locking and mutability strategy. So they perform better if requirements suit their nature.
Similarly, CopyOnWriteArraySet is a concurrent replacement to Synchronized Set. See Java Application Performance and Memory Management course on Udemy to learn how to use CopyOnWriteArrayList to write high-performance Java applications.
3. BlockingQueue
BlockingQueue is also one of the better-known collection classes in Java 5. BlockingQueue makes it
easy to implement producer-consumer
design pattern by providing inbuilt blocking support for the put() and take() method. put() method
will block if the Queue is full while the take() method
will block if the 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 elements. ArrayBlockingQueue is backed by Array and it's bounded in nature while LinkedBlockingQueue is optionally bounded.
Consider using BlockingQueue to solve the 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.
Java 5 API provides two concrete implementation of BlockingQueue in form of ArrayBlockingQueue and LinkedBlockingQueue, both of them implement FIFO ordering of elements. ArrayBlockingQueue is backed by Array and it's bounded in nature while LinkedBlockingQueue is optionally bounded.
Consider using BlockingQueue to solve the 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.
If you want to learn more about BlockingQueue from an interview point of view then you can also check out Java Interview Guide: 200+ Interview Questions and Answers course on Udemy to not just learn about BlockingQueue but also other useful Java concept for interviews.
4. Deque and BlockingDeque
Deque interface is added in Java 6 and it extends Queue interface
to support insertion and removal from both ends of Queue referred to as head and tail. Java6 also provides a concurrent
implementation of Deque like ArrayDeque and LinkedBlockingDeque.
Deque Can be used efficiently to increase parallelism in the program by allowing a set of worker thread to help each other by taking some of the workloads from other thread by utilizing Deque double end consumption property.
Deque Can be used efficiently to increase parallelism in the program by allowing a set of worker thread to help each other by taking some of the workloads from other thread by utilizing Deque double end consumption property.
So if all Thread
has their own set of task Queue and they are consuming from the head; helper the thread can also share some workload via consumption from the 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.
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 methods we have seen in our post How
to use NavigableMap in Java.
That’s all on this list of the important concurrent Collection classes for Java developers. They have been added on java.util.concurrent package as a concurrent alternative of their synchronized counterpart. It’s a good idea to learn these
Collection classes along with other popular classes from the Java Collection
Framework to become a better Java developer this year.
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Nice Stuff ..could you provide some programs using the above concurrent collection classes like producer & consumer by using BlockingQue.
ReplyDelete@Chiranjib, please see Producer Consumer using BlockingQueue
ReplyDeleteConcurrentHashMaps 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/).
ReplyDeleteAlso worth mentioning the Guava library set that adds a few more thread safe collections
ReplyDeleteAre 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?
ReplyDeleteexplained very well, but take care of spelling mistakes.
ReplyDelete