'Can anyone explain how this memory leak in Java/Android works in detail?

I have been doing Java development for a long time, but I am ashamed to say I still don't have a good grasp on how memory leaks actually takes place. I however have no experience with Android development.

Take the following code for example taken from this presentation:

enter image description here

I have the following questions (please note that I am new to Android, but this example is not primarily focused on Android)

  1. How is memory leak even possible here ? The author says that its because the inner class is not marked as static and it will hold a reference to the outer class.

So if I keep creating objects of the outer class like so:

MainActivity one = new MainActivity();
one.onCreate(bundle);

MainActivity two = new MainActivity();
two.onCreate(bundle);

MainActivity three = new MainActivity();
three.onCreate(bundle);

How does it matter ? If I am not wrong about how static references work, then only one object of LeakClass is created, right ? Unless a separate classloader is involved, right ?

  1. Is the memory leak only possible here because of how Android works ? If I were to manually create these objects then this would not be a problem right ?

Please help me with an ELI5 explanation here, I am really unable to understand how this works.

Thanks.

javaandroidmemory-leaksclassloader


Solution 1:[1]

Because LeakClass is an inner class, it has an inherent reference to the instance of the outer class that owns it. Since you're storing that value in a static variable, that means you're storing a reference to the outer class in that static transitively. Storing an Activity in a static is ALWAYS a leak, and ALWAYS a bad idea. Activities aren't singletons, you cannot assume there will only be one of them.

If you don't need that parent reference, you can make it a static inner class. Static inner classes do not have an implicit outer reference, but cannot access member variables of functions of the owning class. Or you can make it a standalone, non-inner class (the two are equally good solutions).

If you do need that parent reference, you cannot store it in a static variable without a leak. Figure out another way to make that data available.

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Gabe Sechan

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