This article provides a deep dive into how references function in Android and Java, helping developers understand the importance of memory management in mobile applications. As an Android developer, failing to use WeakReferences can lead to serious issues, especially when it comes to avoiding memory leaks.
A few months ago, I co-authored a book titled "Android High Performance" with Diego Gincini. One of the most popular chapters focuses on memory management in Android. In this section, we explain how memory works on mobile devices, what causes memory leaks, why they are important, and how to avoid them. Over the years, I've often noticed that many developers tend to ignore or downplay memory-related problems, even though these issues can have long-term consequences.
It's easy to prioritize new features over clean code, especially when the immediate goal is to deliver something visible in a demo. But this mindset leads to technical debt—unseen but costly issues that can cause frustration, lower code quality, and even team conflicts. These problems are hard to measure through unit tests, much like political decisions that look good in the short term but have long-term consequences.
While there are plenty of resources on software design principles, this article aims to provide a concise yet thorough explanation of the different types of references in Java and how they apply to Android development. Understanding these concepts is crucial for building efficient and stable apps.
First, what exactly is a reference in Java?
In Java, a reference is a way to access an object. There are four built-in types of references: StrongReference, SoftReference, WeakReference, and PhantomReference. Some people simplify this by grouping them into strong and weak references, with weak references having different levels of strength. While classification can vary, understanding all types is essential before deciding on your own approach.
So, what do these references mean? A StrongReference is the default type in Java. Whenever you create an object, it’s automatically assigned a strong reference. For example:
MyObject object = new MyObject();
Here, a new MyObject instance is created, and a strong reference is stored in the variable 'object'. The key point is that as long as a strong reference exists, the garbage collector will not reclaim the object. This is usually what we want, but it can also lead to memory leaks if not managed properly.
Take a moment to spot where the problem might occur.
What now?
The AsyncTask is created and executed within the Activity's onCreate() method. However, since it's an inner class, it holds a reference to the outer Activity throughout its lifecycle. If the Activity is destroyed, the AsyncTask still holds a reference to it, preventing it from being garbage-collected. This is a classic example of a memory leak.
Side note: I used to ask interview candidates to demonstrate how to create a memory leak instead of asking theoretical questions. It was always more engaging!
Memory leaks don’t only happen when an Activity is destroyed. They can also occur during configuration changes, such as when a device rotates from landscape to portrait. If the AsyncTask holds a reference to a View, it may cause a crash when the View becomes null. For example, if an AsyncTask references a ProgressDialog, and the screen orientation changes, the ProgressDialog may no longer exist, leading to a crash.
So, how can we prevent this from happening? We'll now introduce another type of reference: WeakReference.
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