How to Synchronize a Collection in Java?

Java collections are central to the Java programming language. These collections allow you to manage data in an organized manner, and can be modified to suit your application’s needs. However, when working with multiple threads that access or modify these collections, ensuring thread safety becomes crucial. This article will walk you through the different methods to synchronize a collection in Java, with explanations and code examples.

What Does Synchronizing a Collection Mean?

Synchronization in Java refers to making a collection thread-safe, meaning multiple threads can interact with the collection without causing data inconsistencies or errors. If two or more threads try to access a shared collection simultaneously, they might corrupt the collection unless proper synchronization is ensured. In simple terms, synchronization guarantees that only one thread can access a resource at a time.

In Java, you can synchronize a collection in the following ways:

• Using the Collections.synchronizedList() and other similar methods.
• Manually synchronizing code blocks or methods.
• Using concurrent collections that are thread-safe by design.

1. Synchronizing a Collection Using Collections.synchronizedXXX() Methods

Java provides the Collections.synchronizedXXX() utility methods to wrap a collection in a synchronized version. This ensures that all operations on the collection are thread-safe. These methods return a collection backed by a synchronized wrapper, which ensures that only one thread can access the collection at a time.

Example: Synchronizing a List

import java.util.*;

public class SynchronizedListExample {
    public static void main(String[] args) {
        List synchronizedList = Collections.synchronizedList(new ArrayList<>());

        // Adding elements to the synchronized list
        synchronizedList.add("Java");
        synchronizedList.add("Python");
        synchronizedList.add("JavaScript");

        // Iterating over the synchronized list (needs explicit synchronization)
        synchronized (synchronizedList) {
            for (String language : synchronizedList) {
                System.out.println(language);
            }
        }
    }
}
            

In the example above, we wrap an ArrayList inside a synchronized list using Collections.synchronizedList(). Note that when iterating over the synchronized collection, we need to manually synchronize the iteration to avoid concurrent modification exceptions.

Available Methods for Synchronization

The following methods are available in the Collections class to synchronize different types of collections:

• Collections.synchronizedList(List<E> list) — Synchronizes a List.
• Collections.synchronizedSet(Set<E> set) — Synchronizes a Set.
• Collections.synchronizedMap(Map<K, V> map) — Synchronizes a Map.
• Collections.synchronizedSortedSet(SortedSet<E> s) — Synchronizes a SortedSet.
• Collections.synchronizedSortedMap(SortedMap<K, V> m) — Synchronizes a SortedMap.

2. Manual Synchronization Using synchronized Keyword

While using Collections.synchronizedXXX() is a convenient way to synchronize a collection, it may not always be sufficient for complex use cases. In such cases, you can manually synchronize blocks of code or methods using the synchronized keyword.

Example: Synchronizing Code Blocks

import java.util.*;

public class ManualSynchronizationExample {
    private static List list = new ArrayList<>();

    public static void main(String[] args) {
        // Adding elements in a synchronized block
        addItem("Java");
        addItem("Python");

        // Iterating over the list in a synchronized block
        synchronized (list) {
            for (String item : list) {
                System.out.println(item);
            }
        }
    }

    private static void addItem(String item) {
        synchronized (list) {
            list.add(item);
        }
    }
}
            

In this example, we explicitly synchronize the methods and code blocks that modify or access the shared list. This ensures that only one thread can modify the list at any given time.

3. Using Concurrent Collections for Thread Safety

In many scenarios, you don’t need to manually synchronize collections. Java provides built-in concurrent collections that are designed to be thread-safe from the ground up. These classes reside in the java.util.concurrent package and provide high-performance alternatives to the traditional synchronized collections.

Example: Using CopyOnWriteArrayList

import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListExample {
    public static void main(String[] args) {
        CopyOnWriteArrayList list = new CopyOnWriteArrayList<>();

        // Adding elements
        list.add("Java");
        list.add("Python");

        // Iterating over the list
        for (String language : list) {
            System.out.println(language);
        }
    }
}
            

CopyOnWriteArrayList is a concurrent collection that offers thread-safety without needing explicit synchronization. It achieves this by creating a new copy of the underlying array every time a write operation is performed, ensuring safe reads while updates are happening. This collection is ideal for scenarios where read operations are more frequent than write operations.

Other Concurrent Collections

• CopyOnWriteArraySet
• ConcurrentLinkedQueue
• ConcurrentSkipListMap
• BlockingQueue (e.g., ArrayBlockingQueue, LinkedBlockingQueue)

4. When to Use Synchronized Collections

Choosing the right synchronization approach depends on your specific use case. Here are some general guidelines:

• If you have a relatively small application and don’t need high concurrency, Collections.synchronizedXXX() methods are easy and effective.
• If you have a large-scale multi-threaded application with frequent read and write operations, consider using concurrent collections like CopyOnWriteArrayList or ConcurrentHashMap.
• If you need to control synchronization granularity or synchronization is required for specific methods, consider using explicit synchronized blocks or methods.

Conclusion

Synchronizing collections in Java is essential for ensuring thread safety in concurrent applications. Java provides various mechanisms for synchronization, including the Collections.synchronizedXXX() methods, explicit synchronization using the synchronized keyword, and concurrent collections like CopyOnWriteArrayList and ConcurrentHashMap.

By choosing the right synchronization strategy, you can ensure that your applications remain reliable and performant even in multi-threaded environments.