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Learn CopyOnWriteArrayList tutorials with examples in java


CopyOnWriteArrayList is a class in java.util.Concurrent package. It introduces in Java 5 as part of java concurrency API changes.

It uses in multi-threaded safe applications. It is a concurrent version of ArrayList.

Why CopyOnWriteArrayList introduced?

ArrayList is a List implementation in java collections. ArrayList is not threaded safe and can not be used in multi-threaded. applications. See the below example. Created ArrayList object. Iteration of each element loop using the Iterate method.

import java.util.ArrayList;  
import java.util.Iterator;  
import java.util.List;  
  
public class ArrayListDemo {  
 public static void main(String args[]) {  
  List names = new ArrayList<>();  
  names.add("one");  
  names.add("two");  
  names.add("three");  
  names.add("four");  
  Iterator it = names.iterator();  
  while (it.hasNext()) {  
   String value = it.next();  
   System.out.println(value);  
   if (value.equals("three")) {  
    names.remove("four");  
    names.add("five");  
    names.add("four");  
   }  
  }  
  System.out.println(names.size());  
  
 }  
}

During iteration, when an element is removed during iterator(), iterator.next() methods throws ConcurrentModifiedException

one  
two  
three  
Exception in thread "main" java.util.ConcurrentModificationException  
 at java.util.ArrayList$Itr.checkForComodification(Unknown Source)  
 at java.util.ArrayList$Itr.next(Unknown Source)  
 at ArrayListDemo.main(ArrayListDemo.java:14)  

ArrayList is fail fast, which means if the list is added or removed if any thread is doing iteration, next() method throws ConcurrentModifiedException ArrayList will not work in multithreaded applications as expected.

How CopyOnWriteArrayList object works?

CopyOnWriteArrayList is a multithreaded application that is the same or similar class like ArrayList in applications When iterator() is called for the first time on this list, It creates a snapshot copy of the list.

It will create a cloned version of the array list for every modify(add, set remove) operation, and synchronization will be done by JVM. Performance-wise is costly as it creates duplicate copies for every modified operation.

Example

import java.util.Iterator;  
import java.util.List;  
import java.util.concurrent.CopyOnWriteArrayList;  
  
public class ListDemo {  
 public static void main(String args[]) {  
  List myList = new CopyOnWriteArrayList();  
  myList.add("ten");  
  myList.add("nine");  
  myList.add("eight");  
  myList.add("seven");  
  myList.add("six");  
  Iterator it = myList.iterator();  
  while (it.hasNext()) {  
   String element = it.next();  
   System.out.println(element);  
   if (element.equals("eight")) {  
    myList.remove("seven");  
    myList.add("five");  
    myList.add("four");  
   }  
  }  
  System.out.println(myList.size());  
  
 }  
}

Advantages

  • Safely iteration of a list when list modified/updated by other thread
  • It is best for a Read operation
  • suitable for a few modifications and many read operations

Disadvantages

It is costly if you are doing frequently add elements.

Fix for ConcurrentModifiedException in Multithreaded Applications

  • Using the Synchronized block to have a lock during iteration
  • Use CopyOnWriteArrayList to avoid this exception

Difference between ArrayList and CopyOnWriteArrayList

We will see the difference between this list.

ArrayList CopyOnWriteArrayList
This class is not synchronized. Synchronized for thread safety operations
Performance is good performance is less as there is duplicate copy that needs to be created for every modified operation during iteration
It throws ConcurrentModifiedException when the thread modifies the list during iteration. This will never throw an error in multi-thread applications.
The iterator on this list is fail-fast The iterator on this list is fail-safe.
This was introduced in Java 1.2 version. This was introduced in Java 1.5 version.
This was Used in Multi-thread applications for concurrent Usage. It is not stable for multi-thread applications.
It defines in java.util package. Defined in java.util.concurrent package
CopyOnWriteArraySet This is useful for set for multi-threaded applications.

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