A soft reference in Java is a type of reference that allows an object to be garbage-collected only when the JVM runs low on memory. It is used to implement memory-sensitive caches.

Characteristics of Soft Reference

  • Memory-sensitive: The object referred to by a soft reference is not immediately garbage collected. It will only be collected if the JVM needs memory and is running low on resources.
  • Garbage Collection: Unlike regular (strong) references, soft references can be cleared by the garbage collector when memory is needed, making them useful for caching data that can be recomputed or reloaded if necessary.

Use Case

  • Soft references are often used in caching scenarios where you want to keep objects in memory as long as there’s enough memory available. If the system starts running low on memory, these objects can be safely discarded to free up space.

Example

import java.lang.ref.SoftReference;

public class Main {
    public static void main(String[] args) {
        // Create a large object
        String largeString = new String("This is a large string");

        // Create a soft reference to the object
        SoftReference<String> softRef = new SoftReference<>(largeString);
        
        // The original object is no longer strongly referenced
        largeString = null;

        // Attempt to retrieve the object from the soft reference
        String retrievedString = softRef.get();

        if (retrievedString != null) {
            System.out.println("Soft Reference object: " + retrievedString);
        } else {
            System.out.println("Object has been garbage collected");
        }
    }
}

Key Points

  1. Soft Reference Behavior: Objects referenced by soft references are kept in memory unless the JVM needs to reclaim memory.
  2. Garbage Collection: If the JVM runs low on memory, soft references are eligible for garbage collection, unlike strong references.
  3. Common Use: Soft references are typically used for implementing memory-sensitive caches.

Conclusion

Soft references are a way to allow objects to stay in memory as long as possible but still be eligible for garbage collection when the system needs memory. They are useful for cache implementations where objects can be discarded if the system requires resources.