Android Handler Memory Leak

转载自https://techblog.badoo.com/blog/2014/08/28/android-handler-memory-leaks

Android Handler Memory Leaks

By
Dmytro Voronkevych on 28 Aug 2014 -

android

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 becleared 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 awarning by Lint.

Basic Example

This is a very basic activity. Notice that this anonymous

Runnable

has beenposted to the

Handler

with a very long delay. We’ll run it and rotate thephone couple of times, then dump memory and analyze it.



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 createdin OQL (Object Query Language), which is very simple, yet powerful.



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 bya 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 listof

Message

commands referenced from

LooperThread

untilthe message is executed. Posting delayed messages is a clear leak for at leastthe time of the delay value. Posting without delay may cause a temporary leakas 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 convertingthe anonymous class to static.



Run, rotate and get the memory dump.



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

Activities

.



Take a look at the bottom of the tree - activity is kept as a referenceto

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.



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

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



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 inreadability and code clearance. The initial code is much shorter and muchclearer, and you’ll see that eventually, text in

textView

will bechanged 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

.



Let’s run, rotate and dump memory.



Good! Only one instance.

This approach is way better than the previous one, as it keeps code clear andreadable. 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:



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:



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’ssource 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 beGC-ed, all messages should go away as well.

Here is a simple diagram that demonstrates differencesbetween using normal

Handler

and

WeakHandler

to post anonymous runnables:



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 thedesired

Runnable

, to prevent it from being garbage-collectedwhile

WeakRunnable

is active.

The side-effect of using WeakHandler is that all messages and runnablesmay 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.