Android uses Java as a platform for development. This helps us with many low level issues including memory management, platform type dependencies, and so on. However we still sometimes get crashes with OutOfMemory. So where’s the garbage collector?

I’m going to focus on one of the cases where big objects in memory can’t be cleared for a lengthy period of time. This case is not ultimately a memory leak - objects will be collected at some point - so we sometimes ignore it. This is not advisable as it can sometimes lead to OOM errors.

The case I’m describing is the Handler leak, which is usually detected as a warning by Lint.

Basic Example

Basic Code Sample

This is a very basic activity. Notice that this anonymous Runnable has been posted to the Handler with a very long delay. We’ll run it and rotate the phone couple of times, then dump memory and analyze it.

Analyse HPROF

We have seven activities in memory now. This is definitely not good. Let’s find out why GC is not able to clear them.

The query I made to get a list of all Activities remaining in memory was created in OQL (Object Query Language), which is very simple, yet powerful.

Analyse HPROF

As you can see, one of the activities is referenced by this$0. This is an indirect reference from the anonymous class to the owner class. This$0 is referenced by callback, which is then referenced by a chain of next’s of Message back to the main thread.

Any time you create a non-static class inside the owner class, Java creates an indirect reference to the owner

Once you post Runnable or Message into Handler, it’s then stored in list of Message commands referenced from LooperThread until the message is executed. Posting delayed messages is a clear leak for at least the time of the delay value. Posting without delay may cause a temporary leak as well if the queue of messages is large.

Static Runnable Solution

Let’s try to overcome a memory leak by getting rid of this$0, by converting the anonymous class to static.

Static runnable code

Run, rotate and get the memory dump.

Analyse static runnable HPROF

What, again? Let’s see who keeps referring to Activities.

Analyse static runnable HPROF

Take a look at the bottom of the tree - activity is kept as a reference to mContext inside mTextView of our DoneRunnable class. Using static inner classes is not enough to overcome memory leaks, however. We need to do more.

Static Runnable With WeakReference

Let’s continue using iterative fixes and get rid of the reference to TextView, which keeps activity from being destroyed.

Static runnable with weak reference

Note that we are keeping WeakReference to TextView, and let’s run, rotate and dump memory.

Be careful with WeakReferences. They can be null at any moment, so resolve them first to a local variable (hard reference) and then check to null before use.

Analyse static runnable with weak reference HPROF

Hooray! Only one activity instance. This solves our memory problem.

So for this approach we should:

  • Use static inner classes (or outer classes)
  • Use WeakReference to all objects manipulated from Handler/Runnable

If you compare this code to the initial code, you might find a big difference in readability and code clearance. The initial code is much shorter and much clearer, and you’ll see that eventually, text in textView will be changed to ‘Done’. No need to browse the code to realise that.

Writing this much boilerplate code is very tedious, especially if postDelayed is set to a short time, such as 50ms. There are better and clearer solutions.

Cleanup All Messages onDestroy

Handler class has an interesting feature - removeCallbacksAndMessages - which can accept null as argument. It will remove all Runnables and Messages posted to a particular handler. Let’s use it in onDestroy.

Remove callbacks code

Let’s run, rotate and dump memory.

Analise remove callbacks HPROF

Good! Only one instance.

This approach is way better than the previous one, as it keeps code clear and readable. The only overhead is to remember to clear all messages on activity/fragment destroy.

I have one more solution which, if you’re lazy like me, you might like even more. :)

Use WeakHandler

The Badoo team came up with the interesting idea of introducing WeakHandler - a class that behaves as Handler, but is way safer.

It takes advantage of hard and weak references to get rid of memory leaks. I will describe the idea in detail a bit later, but let’s look at the code first:

WeakHandler code

Very similar to the original code apart from one small difference - instead of using android.os.Handler, I’ve used WeakHandler. Let’s run, rotate and dump memory:

Analise remove callbacks HPROF

Nice, isn’t it? The code is cleaner than ever, and memory is clean as well! :)

To use it, just add dependency to your build.gradle:

repositories {
    maven {
        repositories {
            url 'https://oss.sonatype.org/content/repositories/releases/'
        }
    }
}

dependencies {
    compile 'com.badoo.mobile:android-weak-handler:1.0'
}

And import it in your java class:

import com.badoo.mobile.util.WeakHandler;

Visit Badoo’s github page, where you can fork it, or study it’s source code https://github.com/badoo/android-weak-handler

WeakHandler. How it works

The main aim of WeakHandler is to keep Runnables/Messages hard-referenced while WeakHandler is also hard-referenced. Once it can be GC-ed, all messages should go away as well.

Here is a simple diagram that demonstrates differences between using normal Handler and WeakHandler to post anonymous runnables:

WeakHandler diagram

Looking at the top diagram, Activity keeps a reference to Handler, which posts Runnable (puts it into queue of Messages referenced from Thread). Everything is fine except the indirect reference from Runnable to Activity. While Message is in the queue, all graphs can’t be garbage-collected.

By comparison, in the bottom diagram Activity holds WeakHandler, which keeps Handler inside. When we ask it to post Runnable, it is wrapped into WeakRunnable and posted. So the Message queue keeps reference only to WeakRunnable. WeakRunnable keeps weak reference to the desired Runnable, so the Runnable can be garbage-collected.

Another little trick is that WeakHandler still keeps a hard reference to the desired Runnable, to prevent it from being garbage-collected while WeakRunnable is active.

The side-effect of using WeakHandler is that all messages and runnables may not be executed if WeakHandler has been garbage-collected. To prevent that, just keep a reference to it from Activity. Once Activity is ready to be collected, all graphs with WeakHandler will collected as well.

Conclusions

Using postDelayed in Android requires additional effort. To achieve it we came up with three different methods:

  • Use a static inner Runnable/Handler with WeakReference to owner class
  • Clear all messages from Handler in onDestroy of Activity/Fragment
  • Use WeakHandler from Badoo as a silver bullet

It’s up to you to choose your preferred technique. The second seems very reasonable, but needs some extra work. The third is my favourite, obviously, but it require some attention as well - WeakHandler should not be used without hard reference from outside.