How to use WeakReference in Java and Android development?
I have been a java developer for 2 years.
But I have never wrote a WeakReference in my code. How to use WeakReference to make my application more efficient especially the Android application?
Using a WeakReference
in Android isn't any different than using one in plain old Java.
You should think about using one whenever you need a reference to an object, but you don't want that reference to protect the object from the garbage collector. A classic example is a cache that you want to be garbage collected when memory usage gets too high (often implemented with WeakHashMap
).
Be sure to check out SoftReference
and PhantomReference
as well.
EDIT: Tom has raised some concerns over implementing a cache with WeakHashMap
. Here is an article laying out the problems: WeakHashMap is not a cache!
Tom is right that there have been complaints about poor Netbeans performance due to WeakHashMap
caching.
I still think it would be a good learning experience to implement a cache with WeakHashMap
and then compare it against your own hand-rolled cache implemented with SoftReference
. In the real world, you probably wouldn't use either of these solutions, since it makes more sense to use a 3rd party library like Apache JCS.
[EDIT2] I found another good example of WeakReference
. Processing Bitmaps Off the UI Thread page in Displaying Bitmaps Efficiently training guide, shows one usage of WeakReference
in AsyncTask.
class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> {
private final WeakReference<ImageView> imageViewReference;
private int data = 0;
public BitmapWorkerTask(ImageView imageView) {
// Use a WeakReference to ensure the ImageView can be garbage collected
imageViewReference = new WeakReference<ImageView>(imageView);
}
// Decode image in background.
@Override
protected Bitmap doInBackground(Integer... params) {
data = params[0];
return decodeSampledBitmapFromResource(getResources(), data, 100, 100));
}
// Once complete, see if ImageView is still around and set bitmap.
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
}
It says,
The WeakReference to the ImageView ensures that the AsyncTask does not prevent the ImageView and anything it references from being garbage collected. There’s no guarantee the ImageView is still around when the task finishes, so you must also check the reference in onPostExecute(). The ImageView may no longer exist, if for example, the user navigates away from the activity or if a configuration change happens before the task finishes.
Happy coding!
[EDIT] I found a really good example of WeakReference
from facebook-android-sdk. ToolTipPopup class is nothing but a simple widget class that shows tooltip above anchor view. I captured a screenshot.
The class is really simple(about 200 lines) and worthy to look at. In that class, WeakReference
class is used to hold reference to anchor view, which makes perfect sense, because it makes possible for anchor view to be garbage collected even when a tooltip instance lives longer than its anchor view.
Happy coding! :)
Let me share one working example of WeakReference
class. It's a little code snippet from Android framework widget called AutoCompleteTextView
.
In short, WeakReference
class is used to hold View
object to prevent memory leak in this example.
I'll just copy-and-paste PopupDataSetObserver class, which is a nested class of AutoCompleteTextView
. It's really simple and the comments explains the class well. Happy coding! :)
/**
* Static inner listener that keeps a WeakReference to the actual AutoCompleteTextView.
* <p>
* This way, if adapter has a longer life span than the View, we won't leak the View, instead
* we will just leak a small Observer with 1 field.
*/
private static class PopupDataSetObserver extends DataSetObserver {
private final WeakReference<AutoCompleteTextView> mViewReference;
private PopupDataSetObserver(AutoCompleteTextView view) {
mViewReference = new WeakReference<AutoCompleteTextView>(view);
}
@Override
public void onChanged() {
final AutoCompleteTextView textView = mViewReference.get();
if (textView != null && textView.mAdapter != null) {
// If the popup is not showing already, showing it will cause
// the list of data set observers attached to the adapter to
// change. We can't do it from here, because we are in the middle
// of iterating through the list of observers.
textView.post(updateRunnable);
}
}
private final Runnable updateRunnable = new Runnable() {
@Override
public void run() {
final AutoCompleteTextView textView = mViewReference.get();
if (textView == null) {
return;
}
final ListAdapter adapter = textView.mAdapter;
if (adapter == null) {
return;
}
textView.updateDropDownForFilter(adapter.getCount());
}
};
}
And the PopupDataSetObserver
is used in setting adapter.
public <T extends ListAdapter & Filterable> void setAdapter(T adapter) {
if (mObserver == null) {
mObserver = new PopupDataSetObserver(this);
} else if (mAdapter != null) {
mAdapter.unregisterDataSetObserver(mObserver);
}
mAdapter = adapter;
if (mAdapter != null) {
//noinspection unchecked
mFilter = ((Filterable) mAdapter).getFilter();
adapter.registerDataSetObserver(mObserver);
} else {
mFilter = null;
}
mPopup.setAdapter(mAdapter);
}
One last thing. I also wanted to know working example of WeakReference
in Android application, and I could find some samples in its official sample applications. But I really couldn't understand some of them's usage. For example, ThreadSample and DisplayingBitmaps applications use WeakReference
in its code, but after running several tests, I found out that the get() method never returns null
, because referenced view object is recycled in adapters, rather then garbage collected.
Some of the other answers seem incomplete or overly long. Here is a general answer.
How to use WeakReference in Java and Android
You can do the following steps:
- Create a
WeakReference
variable - Set the weak reference
- Use the weak reference
Code
MyClass
has a weak reference to AnotherClass
.
public class MyClass {
// 1. Create a WeakReference variable
private WeakReference<AnotherClass> mAnotherClassReference;
// 2. Set the weak reference (nothing special about the method name)
void setWeakReference(AnotherClass anotherClass) {
mAnotherClassReference = new WeakReference<>(anotherClass);
}
// 3. Use the weak reference
void doSomething() {
AnotherClass anotherClass = mAnotherClassReference.get();
if (anotherClass == null) return;
// do something with anotherClass
}
}
AnotherClass
has a strong reference to MyClass
.
public class AnotherClass {
// strong reference
MyClass mMyClass;
// allow MyClass to get a weak reference to this class
void someMethod() {
mMyClass = new MyClass();
mMyClass.setWeakReference(this);
}
}
Notes
- The reason you need a weak reference is so that the Garbage Collector can dispose of the objects when they are no longer needed. If two objects retain a strong reference to each other, then they can't be garbage collected. This is a memory leak.
- If two objects need to reference each other, object A (generally the shorter lived object) should have a weak reference to object B (generally the longer lived object), while B has a strong reference to A. In the example above,
MyClass
was A andAnotherClass
was B. - An alternative to using a
WeakReference
is to have another class implement an interface. This is done in the Listener/Observer Pattern.
Practical example
- Static inner AsyncTask class