Challenges of Java Synchronized List in Multithreaded Environments

Introduction

Multithreading in Java introduces complexities in managing shared resources among multiple threads. While synchronized collections, like Collections.synchronizedList(), are intended to provide a level of thread safety, they come with their own set of challenges, particularly when it comes to compound operations such as iteration. In this article, I will explore how a synchronized list can fail in a multithreaded environment and why developers should be cautious about its limitations.

Synchronization Basics

In Java, the Collections.synchronizedList() method is commonly used to create a synchronized version of a list. This method returns a synchronized wrapper around the specified list, ensuring that all methods that modify the list are synchronized. This helps prevent data corruption that can occur when multiple threads attempt to modify the list simultaneously.

The Problem with Iteration:

One common scenario where a synchronized list may fail is during iteration. While individual operations like add() or remove() are synchronized, the synchronization does not extend to compound operations like iteration. Consider the following example

List<Integer> synchronizedList = Collections.synchronizedList(new ArrayList<>());

// Thread 1: Add elements to the list
Thread thread1 = new Thread(() -> {
    for (int i = 1; i <= 5; i++) {
        synchronizedList.add(i);
        try {
            Thread.sleep(10); // Simulating some processing time
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
});

// Thread 2: Iterate over the list
Thread thread2 = new Thread(() -> {
    synchronized (synchronizedList) {
        for (Integer num : synchronizedList) {
            System.out.println(num);
            try {
                Thread.sleep(10); // Simulating some processing time
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
});

thread1.start();
thread2.start();

In this example, thread1 is adding elements to the list, and thread2 is iterating over it. The synchronization provided by Collections.synchronizedList only guards individual operations, not the entire iteration process.
ConcurrentModificationException
As a consequence, the iteration in thread2 may throw a “ConcurrentModificationException”. This exception occurs when the list is modified during iteration, leading to an inconsistency in the internal state of the synchronized list.

Mitigating the Issue

Developers need to be aware of the limitations of synchronized collections and choose appropriate alternatives based on their specific requirements. One solution is to use thread-safe alternatives like CopyOnWriteArrayList, which creates a new copy of the underlying array whenever a modification is made. This allows safe iteration over the original array while modifications occur on a separate copy.

Conclusion
While synchronized collections like Collections.synchronizedList() provide basic thread safety for individual operations, they may fall short in scenarios involving compound operations like iteration. Understanding these limitations is crucial for writing robust multithreaded applications. Developers should carefully choose thread-safe alternatives and adopt best practices to mitigate potential issues, ensuring the integrity of shared data in a concurrent environment.