ConcurrentHashMap in Java

Java ConcurrentHashMap class is part of the Concurrency Collection Classes. It’s a hash table implementation, which supports concurrent retrieval and updates. It’s used in a multi-threaded environment to avoid ConcurrentModificationException.

ConcurrentHashMap

If we try to modify the collection while iterating over it, we get ConcurrentModificationException. Java 1.5 introduced Concurrent classes in the java.util.concurrent package to overcome this scenario. ConcurrentHashMap is the Map implementation that allows us to modify the Map while iteration. The ConcurrentHashMap operations are thread-safe. ConcurrentHashMap doesn’t allow null for keys and values.

Java ConcurrentHashMap Example

The ConcurrentHashMap class is similar to HashMap, except that it’s thread-safe and allows modification while iteration.

package com.journaldev.util;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapExample {

	public static void main(String[] args) {

		//ConcurrentHashMap
		Map<String,String> myMap = new ConcurrentHashMap<String,String>();
		myMap.put("1", "1");
		myMap.put("2", "1");
		myMap.put("3", "1");
		myMap.put("4", "1");
		myMap.put("5", "1");
		myMap.put("6", "1");
		System.out.println("ConcurrentHashMap before iterator: "+myMap);
		Iterator<String> it = myMap.keySet().iterator();

		while(it.hasNext()){
			String key = it.next();
			if(key.equals("3")) myMap.put(key+"new", "new3");
		}
		System.out.println("ConcurrentHashMap after iterator: "+myMap);

		//HashMap
		myMap = new HashMap<String,String>();
		myMap.put("1", "1");
		myMap.put("2", "1");
		myMap.put("3", "1");
		myMap.put("4", "1");
		myMap.put("5", "1");
		myMap.put("6", "1");
		System.out.println("HashMap before iterator: "+myMap);
		Iterator<String> it1 = myMap.keySet().iterator();

		while(it1.hasNext()){
			String key = it1.next();
			if(key.equals("3")) myMap.put(key+"new", "new3");
		}
		System.out.println("HashMap after iterator: "+myMap);
	}

}

Output:

ConcurrentHashMap before iterator: {1=1, 5=1, 6=1, 3=1, 4=1, 2=1}
ConcurrentHashMap after iterator: {1=1, 3new=new3, 5=1, 6=1, 3=1, 4=1, 2=1}
HashMap before iterator: {3=1, 2=1, 1=1, 6=1, 5=1, 4=1}
Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at com.test.ConcurrentHashMapExample.main(ConcurrentHashMapExample.java:44)

It’s clear from the output that ConcurrentHashMap takes care of the new entry in the map while iteration whereas HashMap throws ConcurrentModificationException. Let’s look at the exception stack trace closely. The following statement has thrown Exception.

String key = it1.next();

It means that the new entry got inserted in the HashMap but Iterator is failing. Actually, Iterator on Collection objects is fail-fast i.e any modification in the structure or the number of entries in the collection object will trigger the exception.

How does iterator know about the modification in the Collection?

We have taken the set of keys from HashMap and then iterating over it. HashMap contains a variable to count the number of modifications and iterator use it when you call its next() function to get the next entry. HashMap.java:

/**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient volatile int modCount;

Let’s change the code a little bit to come out of the iterator loop when we insert the new entry. All we need to do is add a break statement after the put call.

if(key.equals("3")){
	myMap.put(key+"new", "new3");
	break;
}

The output with the above code:

ConcurrentHashMap before iterator: {1=1, 5=1, 6=1, 3=1, 4=1, 2=1}
ConcurrentHashMap after iterator: {1=1, 3new=new3, 5=1, 6=1, 3=1, 4=1, 2=1}
HashMap before iterator: {3=1, 2=1, 1=1, 6=1, 5=1, 4=1}
HashMap after iterator: {3=1, 2=1, 1=1, 3new=new3, 6=1, 5=1, 4=1}

What happens if the key value is modified?

What if we don’t add a new entry but update the existing key-value pair? Will it throw exception? Let’s change the code in the original program and check it out.

//myMap.put(key+"new", "new3");
myMap.put(key, "new3");

There won’t be any exception because the collection is modified but its structure remains the same.

Further Reading

Did you notice those angle brackets while creating our collection object and Iterator? It’s called generics and it’s very powerful when it comes to type-checking at compile time to remove ClassCastException at runtime. Learn more about generics in Java Generics Example. You should also read Java Collections Interview Questions and Iterator Design Pattern in Java.

You can checkout more Java collection examples from our GitHub Repository.

Reference: API Doc