Synchronized vs. Concurrent Collections in Java MultiThreading

In Java multithreading, managing shared resources efficiently is crucial to avoid data inconsistency and race conditions. Two important types of collections come into play when dealing with concurrent access: synchronized collections and concurrent collections. This blog post will explore the differences between these two types of collections, their usage methods, common practices, and best practices.

Table of Contents

  1. Fundamental Concepts
  2. Synchronized Collections
  3. Concurrent Collections
  4. Best Practices
  5. Conclusion
  6. References

Fundamental Concepts

Synchronized Collections

Synchronized collections are traditional collections that have been made thread - safe by synchronizing each individual method call. When a thread accesses a synchronized collection, it acquires a lock on the collection object. This ensures that only one thread can access the collection at a time, preventing data corruption due to concurrent access. However, this also means that other threads have to wait for the lock to be released, which can lead to performance bottlenecks.

Concurrent Collections

Concurrent collections, on the other hand, are designed to handle concurrent access more efficiently. They use advanced techniques such as lock - striping and non - blocking algorithms. Instead of locking the entire collection, they allow multiple threads to access different parts of the collection simultaneously, reducing contention and improving performance.

Synchronized Collections

Synchronized Collections Usage Method

Java provides several ways to create synchronized collections. One common way is to use the Collections.synchronizedXXX() methods. For example, to create a synchronized List, we can do the following:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class SynchronizedListExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        List<String> synchronizedList = Collections.synchronizedList(list);
        synchronizedList.add("Element 1");
        System.out.println(synchronizedList.get(0));
    }
}

Synchronized Collections Common Practices

  • Iteration: When iterating over a synchronized collection, it is important to synchronize the iteration externally. Otherwise, another thread may modify the collection during the iteration, leading to a ConcurrentModificationException.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

public class SynchronizedListIteration {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        List<String> synchronizedList = Collections.synchronizedList(list);
        synchronizedList.add("Item 1");
        synchronizedList.add("Item 2");

        synchronized (synchronizedList) {
            Iterator<String> iterator = synchronizedList.iterator();
            while (iterator.hasNext()) {
                System.out.println(iterator.next());
            }
        }
    }
}

Concurrent Collections

Concurrent Collections Usage Method

Java’s java.util.concurrent package provides a variety of concurrent collections. For example, ConcurrentHashMap can be used as a thread - safe alternative to HashMap.

import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapExample {
    public static void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.put("Key 1", 1);
        Integer value = map.get("Key 1");
        System.out.println(value);
    }
}

Concurrent Collections Common Practices

  • Non - blocking operations: Concurrent collections support non - blocking operations, which means that threads can perform operations without waiting for a lock. For example, the putIfAbsent() method in ConcurrentHashMap can be used to add a key - value pair only if the key does not already exist.
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapPutIfAbsent {
    public static void main(String[] args) {
        ConcurrentHashMap<String, Integer> map = new ConcurrentHashMap<>();
        map.putIfAbsent("Key 1", 1);
        Integer value = map.get("Key 1");
        System.out.println(value);
    }
}

Best Practices

  • Use concurrent collections when possible: If your application requires high concurrency, concurrent collections are usually a better choice as they offer better performance.
  • Use synchronized collections for simple scenarios: If the level of concurrency is low and the code is simple, synchronized collections can be used as they are easier to understand and implement.
  • Understand the limitations: Both synchronized and concurrent collections have their limitations. For example, synchronized collections may cause performance issues under high concurrency, while concurrent collections may have more complex behavior in some edge cases.

Conclusion

In Java multithreading, both synchronized and concurrent collections play important roles in managing shared resources. Synchronized collections provide a simple way to make traditional collections thread - safe, but they may suffer from performance bottlenecks under high concurrency. Concurrent collections, on the other hand, are designed to handle concurrent access more efficiently using advanced techniques. By understanding the differences between these two types of collections and following the best practices, developers can write more efficient and reliable multithreaded applications.

References