Avoid memory leaks using Weak&Soft references


Some Java developers believe that there is no such a thing as memory leak in Java (thanks to the fabulous automatic Garbage Collection concept)

Some others had met the OutOfMemoryError and understood that the JVM has encountered some memory issue but they are not sure if it’s all about the code or maybe even an OS issue…

The OutOfMemoryError API docs reveals that it “Thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector. ”

As we know, the JVM has a parameter that represents the maximum heap size(-Xmx), hence we can defiantly try to increase the heap size. yet some code can generate new instances all the time, if those instances are accessible(being referenced by the main program – in a recursive manner) for the entire program life span, then the GC won’t reclaim those instances. hence the heap will keep increasing and eventually a OutOfMemoryError will be thrown <- we call that memory leak.

Our job as Java developers is to release references (that are accessible by the main program) that we won’t use in the future. by doing that we are making sure that the GC will reclaim those instances (free the memory that those instances occupying in the heap).

In some cases we reference an instance from 2 different roots. one root represent a fast-retrieval space(e.g. HashMap) and the other manages the real lifespan of that instance. Sometimes we would like to remove the reference of that instance from one root and get the other root(fast retrieval) reference removed automatically.

We wouldn’t want to do it manually due to the fact that we are not C++ developers and we wouldn’t like to manage the memory manually..

Weak references

In order to solve that we can use WeakReference.

Instances that are being referenced by only Weak references will get collected on the next collection! (Weakly reachable), in other words those references don’t protect their value from the garbage collector.

Hence if we would like to manage the life span of an instance by one reference only, we will use the WeakReference object to create all the other references. ( usage: WeakReference wr = new WeakReference(someObject);)

In some apps we would like to add all our existing references to some static list, those references should not be strong, otherwise we would have to clean those references manually, we would add those references to the list using this code.

public static void addWeakReference(Object o){
 refList.add(new WeakReference(o));
}

since most of the WeakReferences use cases needs a Map data structure, there is an implementation of Map that add a WeakReference automatically for you – WeakHashMap

Soft References

I saw few implementations of Cache using weak references (e.g. the cache is just a WeakHashMap => the GC is cleaning old objects in the cahce), without WeakReferences naive cache can easily cause memory leaks and therefor weak references might be a solution for that.

The main problem is that the GC will clean the cached-object probably and most-likely faster then you need.

Soft references solve that, those references are exactly like weak references, yet the GC won’t claim them as fast. we can be sure that the JVM won’t throw an OutOfMemory before it will claim all the soft and weak references!

using a soft references in order to cache considered the naive generic cache solution. (poor’s men cache)

( usage:SoftReference sr = new SoftReference(someObject);)

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One thought on “Avoid memory leaks using Weak&Soft references

  1. Pingback: Java Job Interview Question & Answer 1 « רעיונות פשוטים

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