浅析Android内存泄漏检测工具LeakCanary

LeakCanary
A memory leak detection library for Android and Java.

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

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如何使用？

1. build.gradle添加LeakCanary库:

dependencies {
  debugImplementation 'com.squareup.leakcanary:leakcanary-android:1.5.4'
  releaseImplementation 'com.squareup.leakcanary:leakcanary-android-no-op:1.5.4'
}

2.在Application初始化LeakCanary:

public class ExampleApplication extends Application {

  @Override public void onCreate() {
    super.onCreate();
    if (LeakCanary.isInAnalyzerProcess(this)) {
      // This process is dedicated to LeakCanary for heap analysis.
      // You should not init your app in this process.
      return;
    }
    LeakCanary.install(this);
    // Normal app init code...
  }
}

以上两步操作完之后，当activity发生内存泄漏的时候（debug模式下），LeakCanary会自动弹出通知。

如果想在单元测试的时候，禁用LeakCanary，可以在build.gradle添加如下代码

// Ensure the no-op dependency is always used in JVM tests.
configurations.all { config ->
  if (config.name.contains('UnitTest')) {
    config.resolutionStrategy.eachDependency { details ->
      if (details.requested.group == 'com.squareup.leakcanary' && details.requested.name == 'leakcanary-android') {
        details.useTarget(group: details.requested.group, name: 'leakcanary-android-no-op', version: details.requested.version)
      }
    }
  }
}

If you want to also disable leak detection in instrumentation tests(底层的Andr​​oid测试API), add || config.name.contains('AndroidTest') to the if check above.

因为LeakCanary默认只监测Activity，如何使用LeakCanary检测fragment是否存在内存泄漏？
使用RefWatcher来观察fragment的内存使用，如下：

public abstract class BaseFragment extends Fragment {

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

原理分析

1. ActivityRefWatcher中，注册Activity生命周期监听接口，当Activity onDestroy()被调用时，将当前Activity加入内存泄漏监听队列；

private final Application.ActivityLifecycleCallbacks lifecycleCallbacks =
            new Application.ActivityLifecycleCallbacks() {
                @Override
                public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
                }

                @Override
                public void onActivityStarted(Activity activity) {
                }

                @Override
                public void onActivityResumed(Activity activity) {
                }

                @Override
                public void onActivityPaused(Activity activity) {
                }

                @Override
                public void onActivityStopped(Activity activity) {
                }

                @Override
                public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
                }

                @Override
                public void onActivityDestroyed(Activity activity) {
                    ActivityRefWatcher.this.onActivityDestroyed(activity);
                }
            };

    public void watchActivities() {
        // Make sure you don't get installed twice.
        stopWatchingActivities();
        application.registerActivityLifecycleCallbacks(lifecycleCallbacks);
    }

  public void stopWatchingActivities() {
    application.unregisterActivityLifecycleCallbacks(lifecycleCallbacks);
  }

leakcanary只在application onCreate时调用了LeakCanary.install(this)，它是怎么监控activity的呢？主要靠的 application.registerActivityLifecycleCallbacks(listener)，app中的每个activity的每一个生命周期都会回调listener中相关方法，从而leakcanary在生命周期相关方法中能取到Activity，注意注册Activity生命周期监听接口只在Android 4.0以上支持。

2. RefWatcher中，watch方法将对象用WeakReference引起来,监听对象是否内存泄漏。

• leakcanary 的watch流程是在主线程空闲时，delay 5秒后检查对象有没有被回收,其中空闲时执行check是用的IdleHandler。
• 判断一个对象有没有被回收是用的WeakReference与ReferenceQuene, 使用WeakReference(T referent, ReferenceQueue queue) 构造方法构造WeakReference时，如果这个WeakReference连接的对象被回收的话，系统会将WeakReference put进 ReferenceQuene，因此，如果ReferenceQuene中有哪个WeakReference，则表示这个WeakReference弱链接的对象已被回收。

主动GC，采用Runtime.getRuntime().gc()（对比System.gc()的优点是能保证及时触发GC），同时GC后等待100ms，等待Java虚拟机把这个弱引用加入ReferenceQueue