Java Weak References

Reusing objects is important in Java but also it can cause memory and performance issues if the objects to be reused can't be freed out by GC. Sometimes we need to reuse objects as soon as they are still being referenced or as soon as they may have good chance to be used in the future.

Weak references give us the opportunity to achieve this by not largely affecting memory and performance of our application (In a more GC-friendly). In today's post I will go through the differences between strong and weak references in Java and the effect of them on garbage collector.

Strong References

 

Strong references are the default type/class references in Java. Objects that are strongly references aren't eligible for GC neither minor nor full GC until they are not strongly referenced anymore. 

 

All object references in Java are strongly referenced unless explicitly specified, see below sections.

StringBuilder builder = new StringBuilder();

Weak References

Weak reference objects are not default and must be explicitly specified. This type of references is to maintain references to objects that are `WEAK` meaning that if the object is eligible for GC `not strongly referenced anymore` but still `weakly referenced` the object can be collected.

StringBuilder builder = new StringBuilder();
WeakReference<StringBuilder> weakBuilder = new WeakReference<>(builder);

Soft References

 

This type of references is also a weak reference that remains in memory for longer - It resists minor GC until memory is really needed to be reclaimed (Application is reaching OOM then it will clear all soft references). Soft references are essentially one large, least recently used (LRU) object pool (cache).

The JVM keeps track of the last access to each reference and calculates if the soft reference is eligible for GC. This is controlled by the JVM `-XX:SoftRefLRUPolicyMSPerMB` flag which is by default = 1000 = One second of lifetime per free megabyte in the heap

StringBuilder builder = new StringBuilder();
SoftReference<StringBuilder> weakBuilder = new SoftReference<>(builder);

Phanotm References

 

Phantom references help us doing finalization to avoid implementing the `Object.finalize` method which could have negative implications to the application as its not deterministic and can make the object reachable again or negatively impact application performance.

Phantom references are different from Weak/Soft references in that GC will not collect a phantom reachable object until its cleared out all acquired resources.

`PhanomReference` class accepts two parameters the referent object and a `ReferenceQueue` which is then used to enqueue objects eligible for clearance.

ReferenceQueue q = new ReferenceQueue();
PhantomReference pr = new PhantomReference(object, referenceQueue);
// Later on another point
Reference r = q.remove();
// Now, clear up any thing you want

Quick Summary (From: Java Performance - The Definitive Guide)

• Indefinite (soft, weak, phantom) references alter ordinary lifecycle of java objects, allowing them to be reused in ways that may be more GC-friendly.
• Weak references should be used when an application is interested in an object only if that object is strongly referenced else where in the application.
• Soft references hold onto objects for (possibly) long periods of time, providing a simple GC-friendly LRU cache.
• Indefinite reference consume their own memory and hold onto memory of other objects for long periods of time; they should be used sparingly.