This is only for my reference gathered from

http://crunchify.com/hashmap-vs-concurrenthashmap-vs-synchronizedmap-how-a-hashmap-can-be-synchronized-in-java/

• What’s the difference between ConcurrentHashMap and Collections.synchronizedMap(Map)?
• What’s the difference between ConcurrentHashMap and Collections.synchronizedMap(Map) in term of performance?
• ConcurrentHashMap vs Collections.synchronizedMap()
• Popular HashMap and ConcurrentHashMap interview questions

In this tutorial we will go over all above queries and reason why and how we could Synchronize Hashmap?

Why?

The Map object is an associative containers that store elements, formed by a combination of a uniquely identify key and a mapped value. If you have very highly concurrent application in which you may want to modify or read key value in different threads then it’s ideal to use Concurrent Hashmap. Best example is Producer Consumer which handles concurrent read/write.

So what does the thread-safe Map means? If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally to avoid an inconsistent view of the contents.

How?

There are two ways we could synchronized HashMap

1. Java Collections synchronizedMap() method
2. Use ConcurrentHashMap

HashMap Vs. synchronizedMap Vs. ConcurrentHashMap

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//Hashtable Map<String, String> normalMap = new Hashtable<String, String>();  //synchronizedMap synchronizedHashMap = Collections.synchronizedMap(new HashMap<String, String>());  //ConcurrentHashMap concurrentHashMap = new ConcurrentHashMap<String, String>();

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.
• The locking is at a much finer granularity at a hashmap bucket level.
• ConcurrentHashMap doesn’t throw a ConcurrentModificationException if one thread tries to modify it while another is iterating over it.
• ConcurrentHashMap uses multitude of locks.

SynchronizedHashMap

• Synchronization at Object level.
• Every read/write operation needs to acquire lock.
• Locking the entire collection is a performance overhead.
• This essentially gives access to only one thread to the entire map & blocks all the other threads.
• It may cause contention.
• SynchronizedHashMap returns Iterator, which fails-fast on concurrent modification.

Now let’s take a look at code

1. Create class CrunchifyConcurrentHashMapVsSynchronizedHashMap.java
2. Create object for each HashTable, SynchronizedMap and CrunchifyConcurrentHashMap
3. Add and retrieve 500k entries from Map
4. Measure start and end time and display time in milliseconds
5. We will use ExecutorService to run 5 threads in parallel

CrunchifyConcurrentHashMapVsSynchronizedMap.java

Java

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package crunchify.com.tutorials; import java.util.Collections; import java.util.HashMap; import java.util.Hashtable; import java.util.Map; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.TimeUnit; /** * @author Crunchify.com * */ public class CrunchifyConcurrentHashMapVsSynchronizedMap { public final static int THREAD_POOL_SIZE = 5; public static Map<String, Integer> crunchifyHashTableObject = null; public static Map<String, Integer> crunchifySynchronizedMapObject = null; public static Map<String, Integer> crunchifyConcurrentHashMapObject = null; public static void main(String[] args) throws InterruptedException { // Test with Hashtable Object crunchifyHashTableObject = new Hashtable<String, Integer>(); crunchifyPerformTest(crunchifyHashTableObject); // Test with synchronizedMap Object crunchifySynchronizedMapObject = Collections.synchronizedMap(new HashMap<String, Integer>()); crunchifyPerformTest(crunchifySynchronizedMapObject); // Test with ConcurrentHashMap Object crunchifyConcurrentHashMapObject = new ConcurrentHashMap<String, Integer>(); crunchifyPerformTest(crunchifyConcurrentHashMapObject); } public static void crunchifyPerformTest(final Map<String, Integer> crunchifyThreads) throws InterruptedException { System.out.println("Test started for: " + crunchifyThreads.getClass()); long averageTime = 0; for (int i = 0; i < 5; i++) { long startTime = System.nanoTime(); ExecutorService crunchifyExServer = Executors.newFixedThreadPool(THREAD_POOL_SIZE); for (int j = 0; j < THREAD_POOL_SIZE; j++) { crunchifyExServer.execute(new Runnable() { @SuppressWarnings("unused") @Override public void run() { for (int i = 0; i < 500000; i++) { Integer crunchifyRandomNumber = (int) Math.ceil(Math.random() * 550000); // Retrieve value. We are not using it anywhere Integer crunchifyValue = crunchifyThreads.get(String.valueOf(crunchifyRandomNumber)); // Put value crunchifyThreads.put(String.valueOf(crunchifyRandomNumber), crunchifyRandomNumber); } } }); } // Make sure executor stops crunchifyExServer.shutdown(); // Blocks until all tasks have completed execution after a shutdown request crunchifyExServer.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS); long entTime = System.nanoTime(); long totalTime = (entTime - startTime) / 1000000L; averageTime += totalTime; System.out.println("500K entried added/retrieved in " + totalTime + " ms"); } System.out.println("For " + crunchifyThreads.getClass() + " the average time is " + averageTime / 5 + " ms\n"); } }

Result

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Test started for: class java.util.Hashtable 500K entried added/retrieved in 1432 ms 500K entried added/retrieved in 1425 ms 500K entried added/retrieved in 1373 ms 500K entried added/retrieved in 1369 ms 500K entried added/retrieved in 1438 ms For class java.util.Hashtable the average time 1407 ms Test started for: class java.util.Collections$SynchronizedMap 500K entried added/retrieved in 1431 ms 500K entried added/retrieved in 1460 ms 500K entried added/retrieved in 1387 ms 500K entried added/retrieved in 1456 ms 500K entried added/retrieved in 1406 ms For class java.util.Collections$SynchronizedMap the average time 1428 ms Test started for: class java.util.concurrent.ConcurrentHashMap 500K entried added/retrieved in 413 ms 500K entried added/retrieved in 351 ms 500K entried added/retrieved in 427 ms 500K entried added/retrieved in 337 ms 500K entried added/retrieved in 339 ms For class java.util.concurrent.ConcurrentHashMap the average time 373 ms  <== Much faster

Property	ConcurrentHashMap	HashMap
1.  Thread -Safe :	ConcurrentHashMap is thread-safe that is the code can be accessed by single thread at a time .	while HashMap is not thread-safe .
2. Synchronization Method	ConcurrentHashMap synchronizes or locks on the certain portion of the Map . To optimize    the performance of ConcurrentHashMap , Map is divided into different partitions depending    upon the Concurrency level . So that we do not need to synchronize the whole Map Object.	HashMap can be synchronized by using        synchronizedMap(HashMap)  method .  By using this      method we get a HashMap object which is equivalent     to the HashTable object . So every modification  is performed        on  Map is locked on Map object.
3.  Null Key	ConcurrentHashMap does not allow NULL values . So the key can not be null in      ConcurrentHashMap	HashMap there can only be one null key .
4.  Performance	In multiple threaded environment HashMap is usually faster than ConcurrentHashMap . As         only single thread can access the certain portion of the Map and thus reducing the  performance .	in HashMap any number of threads can     access the code at the same time .