Several new Collection classes are added in Java 5 and Java 6 specially 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 e.g. ConcurrentHashMap, BlockingQueue which provides some of the very useful functionalities to build concurrent Java application. 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. Idea is to keep it short and simple while highlighting key points of those useful java.util.concurrent collections.
ConcurrentHashMap 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. 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
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.
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