#LeakCanary

A memory leak detection library for Android and Java.

“A small leak will sink a great ship.” - Benjamin Franklin

Getting started

In your build.gradle:

 dependencies {
   debugCompile 'com.squareup.leakcanary:leakcanary-android:1.3.1'
   releaseCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.3.1'
 }

In your Application class:

public class ExampleApplication extends Application {

  @Override public void onCreate() {
    super.onCreate();
    LeakCanary.install(this);
  }
}

You're good to go! LeakCanary will automatically show a notification when an activity memory leak is detected in your debug build.

Why should I use LeakCanary?

Glad you ask! We wrote a blog post to answer precisely that question.

How do I use it?

Use a RefWatcher to watch references that should be GCed:

RefWatcher refWatcher = {...};

// We expect schrodingerCat to be gone soon (or not), let's watch it.
refWatcher.watch(schrodingerCat);

LeakCanary.install() returns a pre configured RefWatcher. It also installs an ActivityRefWatcher that automatically detects if an activity is leaking after Activity.onDestroy() has been called.

public class ExampleApplication extends Application {

  public static RefWatcher getRefWatcher(Context context) {
    ExampleApplication application = (ExampleApplication) context.getApplicationContext();
    return application.refWatcher;
  }

  private RefWatcher refWatcher;

  @Override public void onCreate() {
    super.onCreate();
    refWatcher = LeakCanary.install(this);
  }
}

You could use the RefWatcher to watch for fragment leaks:

public abstract class BaseFragment extends Fragment {

  @Override public void onDestroy() {
    super.onDestroy();
    RefWatcher refWatcher = ExampleApplication.getRefWatcher(getActivity());
    refWatcher.watch(this);
  }
}

How does it work?

1. RefWatcher.watch() creates a KeyedWeakReference to the watched object.
2. Later, in a background thread, it checks if the reference has been cleared and if not it triggers a GC.
3. If the reference is still not cleared, it then dumps the heap into a .hprof file stored on the app file system.
4. HeapAnalyzerService is started in a separate process and HeapAnalyzer parses the heap dump using HAHA.
5. HeapAnalyzer finds the KeyedWeakReference in the heap dump thanks to a unique reference key and locates the leaking reference.
6. HeapAnalyzer computes the shortest strong reference path to the GC Roots to determine if there is a leak, and then builds the chain of references causing the leak.
7. The result is passed back to DisplayLeakService in the app process, and the leak notification is shown.

How do I copy the leak trace?

You can see the leak trace in Logcat:

In com.example.leakcanary:1.0:1 com.example.leakcanary.MainActivity has leaked:
* GC ROOT thread java.lang.Thread.<Java Local> (named 'AsyncTask #1')
* references com.example.leakcanary.MainActivity$3.this$0 (anonymous class extends android.os.AsyncTask)
* leaks com.example.leakcanary.MainActivity instance

* Reference Key: e71f3bf5-d786-4145-8539-584afaecad1d
* Device: Genymotion generic Google Nexus 6 - 5.1.0 - API 22 - 1440x2560 vbox86p
* Android Version: 5.1 API: 22
* Durations: watch=5086ms, gc=110ms, heap dump=435ms, analysis=2086ms

You can also share the leak trace and the heap dump file from the action bar menu.

How do I fix a memory leak?

Once you have the leak trace, figure out which reference in the path should not exist. Then figure out why that reference still exists. Often times it's a registered listener that should have been unregistered, a close() method that wasn't called, an anonymous class that holds a reference an outer class. If you cannot figure out an issue in your code, please do not file an issue. Instead, create a Stack Overflow question (using the leakcanary tag).

My leak is caused by the Android SDK implementation!

There are a number of known memory leaks that have been fixed over time in AOSP as well as in manufacturer implementations. When such a leak occurs, there is little you can do as an app developer to fix it. For that reason, LeakCanary has a built-in list of known Android leaks to ignore: AndroidExcludedRefs.java.

If you find a new one, please create an issue and follow these steps:

1. Provide the entire leak trace information (reference key, device, etc).
2. Read the AOSP source for that version of Android, and try to figure out why it happens. You can easily navigate through SDK versions android/platform_frameworks_base.
3. Check if it happens on the latest version of Android, and otherwise use blame to find when it was fixed.
4. If it's still happening, build a simple repro case
5. File an issue on b.android.com with the leak trace and the repro case
6. Create a PR in LeakCanary to update AndroidExcludedRefs.java. Optional: if you find a hack to clear that leak on previous versions of Android, feel free to document it.

This is especially important for new releases of Android. You have the opportunity to help detect new memory leaks early on, which benefits the entire Android community.

Beyond the leak trace

Sometimes the leak trace isn't enough and you need to dig into the heap dump with MAT or YourKit. Here's how you can find the leaking instance in the head dump:

1. Look for all instances of com.squareup.leakcanary.KeyedWeakReference
2. For each of these, look at the key field.
3. Find the KeyedWeakReference that has a key field equal to the reference key reported by LeakCanary.
4. The referent field of that KeyedWeakReference is your leaking object.
5. From then on, the matter is in your hands. A good start is to look at the shortest path to GC Roots (excluding weak references).

Customizing

Icon and label

DisplayLeakActivity comes with a default icon and label, which you can change by providing R.drawable.__leak_canary_icon and R.string.__leak_canary_display_activity_label in your app:

res/
  drawable-hdpi/
    __leak_canary_icon.png
  drawable-mdpi/
    __leak_canary_icon.png
  drawable-xhdpi/
    __leak_canary_icon.png
  drawable-xxhdpi/
    __leak_canary_icon.png
  drawable-xxxhdpi/
    __leak_canary_icon.png

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <string name="__leak_canary_display_activity_label">MyLeaks</string>
</resources>

Stored leak traces

DisplayLeakActivity saves up to 7 heap dumps & leak traces in the app directory. You can change that number by providing R.integer.__leak_canary_max_stored_leaks in your app:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <integer name="__leak_canary_max_stored_leaks">20</integer>
</resources>

Uploading to a server

You can change the default behavior to upload the leak trace and heap dump to a server of your choosing.

Create your own AbstractAnalysisResultService. The easiest way is to extend DisplayLeakService in your debug sources:

public class LeakUploadService extends DisplayLeakService {
  @Override protected void afterDefaultHandling(HeapDump heapDump, AnalysisResult result, String leakInfo) {
    if (!result.leakFound || result.excludedLeak) {
      return;
    }
	myServer.uploadLeakBlocking(heapDump.heapDumpFile, leakInfo);
  }
}

Make sure the release Application class uses the disabled RefWatcher:

public class ExampleApplication extends Application {

  public static RefWatcher getRefWatcher(Context context) {
    ExampleApplication application = (ExampleApplication) context.getApplicationContext();
    return application.refWatcher;
  }

  private RefWatcher refWatcher;

  @Override public void onCreate() {
    super.onCreate();
    refWatcher = installLeakCanary();
  }
  
  protected RefWatcher installLeakCanary() {
    return RefWatcher.DISABLED;
  }
}

Build a custom RefWatcher in your debug Application class:

public class DebugExampleApplication extends ExampleApplication {
  protected RefWatcher installLeakCanary() {
    return LeakCanary.install(app, LeakUploadService.class, AndroidExcludedRefs.createAppDefaults().build());
  }
}

Don't forget to register the service in your debug manifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    >
  <application android:name="com.example.DebugExampleApplication">
    <service android:name="com.example.LeakUploadService" />
  </application>
</manifest>

You can also upload the leak traces to Slack or HipChat, here's an example.

Ignoring specific references

You can create your own version of ExcludedRefs to ignore specific references that you know are causing leaks but you still want to ignore:

public class DebugExampleApplication extends ExampleApplication {
  protected RefWatcher installLeakCanary() {
    ExcludedRefs excludedRefs = AndroidExcludedRefs.createAndroidDefaults()
        .instanceField("com.example.ExampleClass", "exampleField")
        .build();
    return LeakCanary.install(this, DisplayLeakService.class, excludedRefs);
  }
}

Snapshots of the development version

See the CHANGELOG.

Troubleshooting

• if leakcanary-android is not in the list of external libraries in Android Studio, but leakcanary-analyzer and leakcanary-watcher are there: try doing a Clean Build. If it's still a problem, try building from the command line.
• error: package com.squareup.leakcanary does not exist: if you have other build types than debug and release, you need to add a specific dependency for those too (xxxCompile).

Resources

• LeakCanary: Detect all memory leaks! squ.re/leakcanary.
• Wrangling Dalvik series: Memory management in Android.
• Uploading leak traces to a Slack or HipChat channel.
• Dump the heap on OutOfMemoryError crashes.
• StackOverflow: leakcanary tag.

Translations

• squ.re/leakcanary in Chinese.
• squ.re/leakcanary in Russian.
• README.md in Chinese: 中文版说明.

The name LeakCanary is a reference to the expression canary in a coal mine, because LeakCanary is a sentinel used to detect risks by providing advance warning of a danger. Props to @edenman for suggesting it!

License

Copyright 2015 Square, Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.