LeakCanary，在开发阶段，可以用来检测内存泄露，项目地址：https://github.com/square/leakcanary

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一、初始化LeakCanary

public class ExampleApplication extends Application {
  @Override public void onCreate() {
    super.onCreate();
    LeakCanary.install(this);
  }
}

public final class LeakCanary {
    public static RefWatcher install(Application application) {
    return refWatcher(application).listenerServiceClass(DisplayLeakService.class)
        .excludedRefs(AndroidExcludedRefs.createAppDefaults().build())
        .buildAndInstall();
  }
}

refWatcher()生成AndroidRefWatcherBuilder类，.buildAnInstall()，会生成RefWatcher，并把相应的对象如（GcTrigger、AndroidWatchExecutor、AndroidHeapDumper、ServiceHeapDumpListener、AndroidExcludedRefs等）。
且Application.registerActivityLifecycleCallbacks这个方法，用来统一管理所有activity的生命周期，LeakCanary就是在onDestory()方法实现监控的.

public final class ActivityRefWatcher {
       ...
  public static void install(Application application, RefWatcher refWatcher) {
    new ActivityRefWatcher(application, refWatcher).watchActivities();
  }
  private final Application.ActivityLifecycleCallbacks lifecycleCallbacks =
      new Application.ActivityLifecycleCallbacks() {
       ...
        @Override public void onActivityDestroyed(Activity activity) {
          ActivityRefWatcher.this.onActivityDestroyed(activity);
        }
      };
  private final Application application;
  private final RefWatcher refWatcher;
  void onActivityDestroyed(Activity activity) {
    refWatcher.watch(activity);
  }
  public void watchActivities() {
    stopWatchingActivities();
    application.registerActivityLifecycleCallbacks(lifecycleCallbacks);
  }
}

二、泄露判断
在RefWatcher中watch函数中：

public final class RefWatcher {
  private final WatchExecutor watchExecutor;
  private final DebuggerControl debuggerControl;
  private final GcTrigger gcTrigger;
  private final HeapDumper heapDumper;
  private final Set<String> retainedKeys;
  private final ReferenceQueue<Object> queue;
  private final HeapDump.Listener heapdumpListener;
  private final ExcludedRefs excludedRefs;
  public void watch(Object watchedReference, String referenceName) {
    ...
    final long watchStartNanoTime = System.nanoTime();
    String key = UUID.randomUUID().toString();
    retainedKeys.add(key);
    final KeyedWeakReference reference =
        new KeyedWeakReference(watchedReference, key, referenceName, queue);
    ensureGoneAsync(watchStartNanoTime, reference);
  }
  private void ensureGoneAsync(final long watchStartNanoTime, final KeyedWeakReference reference) {
    watchExecutor.execute(new Retryable() {
      @Override public Retryable.Result run() {
        return ensureGone(reference, watchStartNanoTime);
      }
    });
  }

其中retainedKeys是用来保存key的集合，KeyedWeakReference是带有key的弱引用对象，queue是ReferenceQueue，用来GC时候，如果弱引用的对象被回收了， 系统会把这个回收对象放到queue里面。这样就可以判断acitivity是否泄漏了。

 private boolean gone(KeyedWeakReference reference) {
    return !retainedKeys.contains(reference.key);
  }
  private void removeWeaklyReachableReferences() {
    KeyedWeakReference ref;
    while ((ref = (KeyedWeakReference) queue.poll()) != null) {
      retainedKeys.remove(ref.key);
    }

public final class AndroidWatchExecutor implements WatchExecutor {
  ...
  @Override public void execute(Retryable retryable) {
    if (Looper.getMainLooper().getThread() == Thread.currentThread()) {
      waitForIdle(retryable, 0);
    } else {
      postWaitForIdle(retryable, 0);
    }
  }
  void waitForIdle(final Retryable retryable, final int failedAttempts) {
    Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
      @Override public boolean queueIdle() {
        postToBackgroundWithDelay(retryable, failedAttempts);
        return false;
      }
    });
  }
}

watchExecutor是AndroidWatchExecutor，在waitForIdle中有一个Looper.myQueue().addIdleHandler，这个是在当前线程中取出MessageQueue对象，添加一个idleHandler，在Looper,loop()循环取出Message时，如果没有message对象来，即Looper处在空闲时，会实行这个idleHandler，相当于在UI线程中页面的操作如onDestroy、onStop等跟AMS通讯完成后，没有handler.post消息message过来时，就实行了idleHandler。

 public final class Looper {
    public static void loop() {
        final Looper me = myLooper();
        final MessageQueue queue = me.mQueue;
        ...
        for (;;) {
            Message msg = queue.next(); // might block
            ...
        }
    }   
}
public final class MessageQueue {
   Message next() {
       ...
        for (;;) {
            ...
            synchronized (this) {
                // Try to retrieve the next message.  Return if found.
                final long now = SystemClock.uptimeMillis();
                Message prevMsg = null;
                Message msg = mMessages;
                ...(return msg)
            }
            for (int i = 0; i < pendingIdleHandlerCount; i++) {
                final IdleHandler idler = mPendingIdleHandlers[i];
                mPendingIdleHandlers[i] = null; // release the reference to the handler
                boolean keep = false;
                try {
                    keep = idler.queueIdle();
                } catch (Throwable t) {
                    Log.wtf(TAG, "IdleHandler threw exception", t);
                }

                if (!keep) {
                    synchronized (this) {
                        mIdleHandlers.remove(idler);
                    }
                }
            }
        }
    }
}

接着实行核心方法ensureGone：

  @SuppressWarnings("ReferenceEquality") // Explicitly checking for named null.
  Retryable.Result ensureGone(final KeyedWeakReference reference, final long watchStartNanoTime) {
    long gcStartNanoTime = System.nanoTime();
    long watchDurationMs = NANOSECONDS.toMillis(gcStartNanoTime - watchStartNanoTime);
    removeWeaklyReachableReferences();//看下检测的弱引用回收了没
    ...
    if (gone(reference)) {//好，回收了，那么这个activity没有泄漏
      return DONE;
    }
    gcTrigger.runGc();//还是没有回收，手动GC一下
    removeWeaklyReachableReferences();//再看看对象回收没有
    if (!gone(reference)) {//还没回收，怀疑是内存泄漏了，dump内存快照.hprof下来分析
      long startDumpHeap = System.nanoTime();
      long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);
      File heapDumpFile = heapDumper.dumpHeap();//生产快照file
      if (heapDumpFile == RETRY_LATER) {
        // Could not dump the heap.
        return RETRY;
      }
      long heapDumpDurationMs = NANOSECONDS.toMillis(System.nanoTime() - startDumpHeap);
      heapdumpListener.analyze(
          new HeapDump(heapDumpFile, reference.key, reference.name, excludedRefs, watchDurationMs,
              gcDurationMs, heapDumpDurationMs));
    }
    return DONE;
  }
  private boolean gone(KeyedWeakReference reference) {
    return !retainedKeys.contains(reference.key);
  }
  private void removeWeaklyReachableReferences() {
    KeyedWeakReference ref;
    while ((ref = (KeyedWeakReference) queue.poll()) != null) {
      retainedKeys.remove(ref.key);
    }
  }
}

三、hrop内存快照生成
解析hprof文件中，是先把这个文件封装成snapshot,然后根据弱引用和前面定义的key值，确定泄漏的对象，最后找到最短泄漏路径，作为结果反馈出来：
File heapDumpFile = heapDumper.dumpHeap();
heapdumpListener.analyze(
new HeapDump(heapDumpFile, reference.key, reference.name, excludedRefs, watchDurationMs,
gcDurationMs, heapDumpDurationMs));
生产file，进入ServiceHeapDumpListener的analyze函数：

@Override public void analyze(HeapDump heapDump) {
    checkNotNull(heapDump, "heapDump");
    HeapAnalyzerService.runAnalysis(context, heapDump, listenerServiceClass);
  }

紧接着调用HeapAnalyzerService.runAnalysis:

public final class HeapAnalyzerService extends IntentService {
  public static void runAnalysis(Context context, HeapDump heapDump,
      Class<? extends AbstractAnalysisResultService> listenerServiceClass) {
    Intent intent = new Intent(context, HeapAnalyzerService.class);
    intent.putExtra(LISTENER_CLASS_EXTRA, listenerServiceClass.getName());
    intent.putExtra(HEAPDUMP_EXTRA, heapDump);
    context.startService(intent);
  }
}

startService:DisplayLeakService 中：

public class DisplayLeakService extends AbstractAnalysisResultService {

  @Override protected final void onHeapAnalyzed(HeapDump heapDump, AnalysisResult result) {
    String leakInfo = leakInfo(this, heapDump, result, true);
    ...
    if (shouldSaveResult) {
      heapDump = renameHeapdump(heapDump);
      resultSaved = saveResult(heapDump, result);
    }
    PendingIntent pendingIntent;
    ...
   pendingIntent = DisplayLeakActivity.createPendingIntent(this, heapDump.referenceKey);

  
     int notificationId = (int) (SystemClock.uptimeMillis() / 1000);
    showNotification(this, contentTitle, contentText, pendingIntent, notificationId);
    afterDefaultHandling(heapDump, result, leakInfo);
  }