JVM.GC.Why do we need weak references ?

Why Do We Need Weak References in Java?

Weak references in Java allow objects to be garbage collected while still being referenced, which is useful for caching, memory-sensitive applications, and preventing memory leaks.

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1️⃣ The Problem with Strong References

• In Java, objects are normally referenced using strong references.
• If a strong reference exists, the object is not eligible for garbage collection.
• This can lead to memory leaks when objects are held in memory longer than necessary.

Example: Strong References Causing Memory Leaks

import java.util.HashMap;
import java.util.Map;

public class StrongReferenceExample {
    static Map<Object, String> cache = new HashMap<>();

    public static void main(String[] args) {
        Object key = new Object();
        cache.put(key, "Some Data");

        key = null; // Remove strong reference

        System.gc(); // Suggest GC

        System.out.println(cache); // Object is still in memory (not garbage collected)
    }
}

Even though key = null, the object is still stored in cache, preventing GC from reclaiming it.This is a memory leak!

2️⃣ Weak References Solve This Problem

A weak reference allows an object to be garbage collected as soon as no strong references exist.

Example: Using WeakReference

import java.lang.ref.WeakReference;

public class WeakReferenceExample {
    public static void main(String[] args) {
        MyObject strongRef = new MyObject();
        WeakReference<MyObject> weakRef = new WeakReference<>(strongRef);

        strongRef = null; // Remove strong reference

        System.gc(); // Suggest GC

        if (weakRef.get() == null) {
            System.out.println("Object has been garbage collected.");
        } else {
            System.out.println("Object is still available.");
        }
    }
}

Since only a WeakReference exists, the object is garbage collected when GC runs.

3️⃣ Where Are Weak References Used?

✅ 1. Weak References in Caching

• In caches, we may want to keep objects only if memory allows.
• If memory is low, the cache should be cleared.

Using WeakHashMap for Caching

import java.util.WeakHashMap;

public class WeakHashMapExample {
    public static void main(String[] args) {
        WeakHashMap<Object, String> cache = new WeakHashMap<>();
        Object key = new Object();
        cache.put(key, "Cached Data");

        key = null; // Remove strong reference

        System.gc(); // Suggest GC

        System.out.println(cache); // The entry may be removed!
    }
}

If GC runs, the key may be removed automatically from WeakHashMap, preventing memory leaks.

✅ 2. Preventing Memory Leaks in Listeners & Event Handlers

• Listeners (e.g., UI components, observers) often retain references to objects.
• If they are not manually removed, they can cause memory leaks.

Using WeakReference for Listeners

import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.List;

class EventListener {
    void onEvent() {
        System.out.println("Event triggered!");
    }
}

public class WeakListenerExample {
    public static void main(String[] args) {
        List<WeakReference<EventListener>> listeners = new ArrayList<>();

        EventListener listener = new EventListener();
        listeners.add(new WeakReference<>(listener));

        listener = null; // Remove strong reference

        System.gc(); // Suggest GC

        for (WeakReference<EventListener> ref : listeners) {
            if (ref.get() == null) {
                System.out.println("Listener has been garbage collected.");
            }
        }
    }
}

• The listener automatically gets garbage collected when no strong references exist.

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✅ 3. Avoiding Memory Leaks in Large Collections

• Holding strong references in large collections (like HashMap) prevents GC from reclaiming unused objects.
• Weak references allow GC to remove unused elements, freeing up memory.

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4️⃣ Summary: Why Do We Need Weak References?

Problem	Solution with WeakReference
Memory leaks in caches	Use WeakReference or WeakHashMap to allow GC to remove unused entries.
Event listeners & callbacks causing leaks	Use WeakReference to avoid retaining unused objects.
Large collections holding unnecessary objects	Use weak references to allow GC to clean up unused data.

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Conclusion

• Weak references help manage memory more efficiently by allowing garbage collection of unused objects.
• They are useful in caching, listeners, and collections where objects should not be kept forever.
• Use WeakReference, WeakHashMap, and weak listener patterns to prevent memory leaks.

📌 In short: Weak references prevent memory leaks by allowing GC to collect objects when no strong references exist! 🚀