Fail-fast and Fail-safe in Java

Fail-fast and Fail-safe are the iterators or collections in Java. Java SE specification doesn't use the Fail-safe term. We use the Fail-safe to segregate between the Non-Fail-fast and Fail-fast iterators. The Fail-Fast system terminates the operation as-fast-as-possible that are exposing failures and stop the entire operation. Whereas, Fail-Safe system doesn't terminate the operation that are exposing failures. The Fail-safe system tries to avoid raising Failures as much as possible.

Fail-fast and Fail-safe are the concepts of concurrent modification. Concurrent modification is a process in which an object is modified concurrently when a different task is running over it. Fail-fast and Fail-safe are the iterators to iterate over the Collection objects.

Let's dive into detail about Fail-fast and Fail-safe one by one:

Fail-fast Iterator

When we use the Fail-fast iterator, it immediately throws ConcurrentModificationException when an element is added or removed from the collection while the thread is iterating over the collection. Examples: Iterator in HashMap, Iterator on ArrayList, etc.

The Fail-fast iterators use modCount, i.e., an internal flag to check whether the collection is structurally modified or not. When the Fail-fast iterator gets the next value using the next() method, it checks the modCount flag's value. When the iterator finds the modified modCount value, it throws the ConcurrentModificationException.

Let's take an example of Fail-fast iterator to understand how it works in Java.

FailFastIterator.java

//importing classes
import java.util.HashMap; 
import java.util.Iterator; 
import java.util.Map; 

//creating FailFastIterator.java class 
public class FailFastIterator {
	
	//main() method starts
    public static void main(String[] args) 
    {
		//creating map
        Map<Integer, String> students = new HashMap<Integer, String>(); 
        students.put(101, "Emma"); 
        students.put(102, "Paul"); 
        students.put(103, "Walker"); 
		
		//creating an instance of the Iterator class
        Iterator itr = students.keySet().iterator(); 
		
		//iterating map using Iterator 
        while (itr.hasNext()) {
			//printing elements of Map
            System.out.println(students.get(itr.next())); 
  
            // adding an element to Map 
            students.put(104, "Rubby"); // on the next call of next() method, exception will be thrown.
        } 
    } 
}

Output

Let's take another example of a Fail-fast iterator in which we will use the iterator on the ArrayList.

FailFastIterator2.java

//importing classes
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.Iterator;
import java.util.ArrayList;


//creating FailFastIterator2.java class 
public class FailFastIterator2 {
	
	//main() method starts
    public static void main(String[] args) 
    {
		//creating ArrayList
        ArrayList<String> students = new ArrayList<String>(); 
        students.add("Emma"); 
        students.add("Paul"); 
        students.add("Walker");
		students.add("Elanie");
		students.add("Amara");
		
		
		//creating an instance of the Iterator class
        Iterator itr = students.iterator(); 
		
		//iterating ArrayList using Iterator 
        while (itr.hasNext()) {
            if ((String)itr.next() == "Paul") 
				// It will not throw an exception 
                students.remove("Amara");
		}
		
		System.out.println(students);
		
		itr = students.iterator(); 
		
		//iterating ArrayList using Iterator 
        while (itr.hasNext()) {
            if ((String)itr.next() == "Walker") 
				// It will throw an exception on next call of next() method
                students.remove("Walker");
		}
    } 
}

Output

Fail-safe Iterator

The Java SE specification doesn't use the Fail-safe term. It is better to call it a Non-Fail-fast Iterator. The Fail-safe iterator doesn't throw the ConcurrentModificationException, and it tries to avoid raising the exception. The Fail-safe iterator creates a copy of the original collection or object array and iterates over that copied collection. Any structural modification made in the iterator affects the copied collection, not the original collection. Therefore, the original collection remains structurally unchanged.

Below are some key points related to the Fail-safe:

We can modify the collection at the time when the thread is iterate iterating over it by using the Fail-safe iterator.
The Fail-safe iterator doesn't throw an exception when a collection is modified while iterating over it.
In order to traverse the collection's element, it creates a copy of that collection object.
Extra space is required to create a copy of the collection object. Ex : Iterator on ConcurrentHashMap, Iterator on CopyOnWriteArrayList and etc.

Let's take an example Fail-safe iterator to understand how it actually works in Java.

FailSafeIterator.java

//importing classes
import java.util.concurrent.ConcurrentHashMap; 
import java.util.Iterator; 
import java.util.Map;
import java.util.HashMap;

//creating FailSafeIterator.java class 
public class FailSafeIterator {
	
	//main() method starts
    public static void main(String[] args) 
    {
		//creating map
        Map<Integer, String> students = new HashMap<Integer, String>(); 
        students.put(101, "Emma"); 
        students.put(102, "Paul"); 
        students.put(103, "Walker");
		
		//creating ConcurrentHashMap by copy students map 
        ConcurrentHashMap<Integer, String> copyStudents = new ConcurrentHashMap<Integer, String>(students); 
		
		//creating an instance of the Iterator class
        Iterator itr = copyStudents.keySet().iterator(); 
		
		//iterating map using Iterator 
        while (itr.hasNext()) {
			//printing elements of copyStudents
            System.out.println(copyStudents.get(itr.next())); 
  
            // adding an element to copyStudents 
            copyStudents.put(104, "Rubby"); // on the next call of next() method, exception will be thrown.
        } 
    } 
}

Output

Let's take another example of a Fail-safe iterator in which we will use the Iterator on CopyOnWriteArrayList.

FailSafeIterator2.java

//importing classes
import java.util.concurrent. CopyOnWriteArrayList;
import java.util.Iterator;
import java.util.ArrayList;


//creating FailSafeIterator2.java class 
public class FailSafeIterator2 {
	
	//main() method starts
    public static void main(String[] args) 
    {
		//creating ArrayList
        ArrayList<String> students = new ArrayList<String>(); 
        students.add("Emma"); 
        students.add("Paul"); 
        students.add("Walker");
		
		//creating CopyOnWriteArrayList by copy students ArrayList 
        CopyOnWriteArrayList<String> copyStudents = new CopyOnWriteArrayList<String>(students); 
		
		//creating an instance of the Iterator class
        Iterator itr = copyStudents.iterator(); 
		
		//iterating CopyOnWriteArrayList using Iterator 
        while (itr.hasNext()) {
			String name = (String)itr.next(); 
            System.out.println(name); 
            if (name == "Paul") 
				// The Rubby name will not print but it has created separate copy 
                copyStudents.add("Rubby");
		} 
    } 
}

Output

Difference between Fail-safe and Fail-fast

We define some main difference between Fail-safe and Fail-fast iterators based on some key factors:

Sr. No.	Key	Fail-Fast	Fail-Safe
1.	Exception	If we add, remove or update the collection elements at the time when a thread is iterating them, the Fail fast throw concurrent modification exception.	The fail-safe iterator tries to avoid raising failure and doesn't throw an exception.
2.	Performance and Memory	It doesn't require more memory while iterating over it because it iterates over the original collection, and all the modifications are done in the original collection.	It requires more memory due to working with copy collection. All the modifications are done in the copy collection, not in the original one.
3.	Modifications	We cannot make changes in the collection while the iterator iterates over it.	We can make changes in the collection while the iterator iterates over it.
4.	Examples	Iterator on ArrayList and Iterator on HashMap.	Iterator on CopyOnWriteArrayList and Iterator on ConcurrentHashMap.

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