How to Implement a Thread-Safe LinkedHashMap in Java?

Does Java provide a thread-safe variant of LinkedHashMap?

// There is no standard LinkedConcurrentHashMap in Java. Use Collections.synchronizedMap or ConcurrentHashMap.

Java does not provide a built-in `LinkedConcurrentHashMap`. However, you can achieve a similar effect by combining `ConcurrentHashMap` with a `LinkedHashMap`. This combination allows you to maintain the insertion order while providing thread-safety for concurrent access and modifications.

import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Map;

public class ThreadSafeLinkedHashMap {
    public static void main(String[] args) {
        // Creating a thread-safe LinkedHashMap
        Map<Integer, String> linkMap = Collections.synchronizedMap(new LinkedHashMap<>());

        // Example usage
        linkMap.put(1, "One");
        linkMap.put(2, "Two");
        
        synchronized(linkMap) {
            for (Map.Entry<Integer, String> entry : linkMap.entrySet()) {
                System.out.println(entry.getKey() + " : " + entry.getValue());
            }
        }
    }
}

Causes

• Java's standard library does not include a direct implementation of a thread-safe linked hash map.
• The `LinkedHashMap` is not synchronized, making it unsafe for concurrent access.

Solutions

• Use `Collections.synchronizedMap` on a `LinkedHashMap` for basic synchronization, though this does not maintain the concurrent features.
• Implement your own thread-safe linked hash map using a combination of `ConcurrentHashMap` and `LinkedHashMap` or use external libraries like Guava or Apache Commons Collections that provide similar functionality.

Mistake: Using `synchronizedMap` without synchronizing the iteration process can lead to `ConcurrentModificationException`.

Solution: Always synchronize on the map during iteration.

Mistake: Ignoring the performance trade-offs when using `Collections.synchronizedMap`, which can lead to contention in high-concurrency situations.

Solution: Evaluate using `ConcurrentHashMap` in situations where high concurrency is a factor.