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

千里之堤， 毁于蚁穴。 -- 《韩非子·喻老》

LeakCanary是什么？可以从LeakCanary的github很容易的得到定义：
Android|Java的内存检测库

更多使用方法：https://www.liaohuqiu.net/cn/posts/leak-canary-read-me/

简单集成

在的build.gradle:

dependencies {
   debugCompile 'com.squareup.leakcanary:leakcanary-android:1.5.1'
   releaseCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.5.1'
   testCompile 'com.squareup.leakcanary:leakcanary-android-no-op:1.5.1'
 }

在你的Applicationclass:

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...
  }
}

然后什么都不用做，当出现内存泄漏的时候LeakCanary就会在你的通知栏发送一条，点进去就是泄漏日志。

这里写图片描述

Dump泄漏的hprof文件

从初始化入口LeakCanary.install(this)可以很清楚的看到得到了一个AndroidRefWatcherBuilder对象

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

  /** Builder to create a customized {@link RefWatcher} with appropriate Android defaults. */
  public static AndroidRefWatcherBuilder refWatcher(Context context) {
    return new AndroidRefWatcherBuilder(context);
  }

而这个对象extends RefWatcherBuilder进入RefWatcherBuilder可以看出这是一个采用构造者设计模式所以直接进入build方法进行查看

  /** Creates a {@link RefWatcher}. */
  public final RefWatcher build() {
    if (isDisabled()) {
      return RefWatcher.DISABLED;
    }

    ExcludedRefs excludedRefs = this.excludedRefs;
    if (excludedRefs == null) {
      excludedRefs = defaultExcludedRefs();
    }

    HeapDump.Listener heapDumpListener = this.heapDumpListener;
    if (heapDumpListener == null) {
      heapDumpListener = defaultHeapDumpListener();
    }

    DebuggerControl debuggerControl = this.debuggerControl;
    if (debuggerControl == null) {
      debuggerControl = defaultDebuggerControl();
    }

    HeapDumper heapDumper = this.heapDumper;
    if (heapDumper == null) {
      heapDumper = defaultHeapDumper();
    }

    WatchExecutor watchExecutor = this.watchExecutor;
    if (watchExecutor == null) {
      watchExecutor = defaultWatchExecutor();
    }

    GcTrigger gcTrigger = this.gcTrigger;
    if (gcTrigger == null) {
      gcTrigger = defaultGcTrigger();
    }

    return new RefWatcher(watchExecutor, debuggerControl, gcTrigger, heapDumper, heapDumpListener,
        excludedRefs);
  }

HeapDumper可以从名字我们可以猜测用于dump hprof文件的、一样的我们点进去这个类发现是一个接口，所以找到他的实现类AndroidHeapDumper

  @Override public File dumpHeap() {
    File heapDumpFile = leakDirectoryProvider.newHeapDumpFile();

    if (heapDumpFile == RETRY_LATER) {
      return RETRY_LATER;
    }

    FutureResult<Toast> waitingForToast = new FutureResult<>();
    showToast(waitingForToast);

    if (!waitingForToast.wait(5, SECONDS)) {
      CanaryLog.d("Did not dump heap, too much time waiting for Toast.");
      return RETRY_LATER;
    }

    Toast toast = waitingForToast.get();
    try {
      Debug.dumpHprofData(heapDumpFile.getAbsolutePath());
      cancelToast(toast);
      return heapDumpFile;
    } catch (Exception e) {
      CanaryLog.d(e, "Could not dump heap");
      // Abort heap dump
      return RETRY_LATER;
    }
  }

稍微提及一下LeakDirectoryProvider这个类主要用于初始化SD卡存放LeakCanary生成的文件目录，大概位置于/sdcard/download/leakcanary-你的包名 回到我们的AndroidHeapDumper类、我们只需要关心一行代码

Debug.dumpHprofData(heapDumpFile.getAbsolutePath());

采用Android自带的Debug来来doump hprof文件然后保存到LeakDirectoryProvider初始化的文件目录下

什么时候Dump

回到LeakCanary类中的buildAndInstall方法

  public RefWatcher buildAndInstall() {
    RefWatcher refWatcher = build();
    if (refWatcher != DISABLED) {
      LeakCanary.enableDisplayLeakActivity(context);
      ActivityRefWatcher.install((Application) context, refWatcher);
    }
    return refWatcher;
  }

主要做两步
第一步:enableDisplayLeakActivity进入这个方法把DisplayLeakActivity这个Activity组件恢复正常使用

  public static void setEnabledBlocking(Context appContext, Class<?> componentClass,
      boolean enabled) {
    ComponentName component = new ComponentName(appContext, componentClass);
    PackageManager packageManager = appContext.getPackageManager();
    int newState = enabled ? COMPONENT_ENABLED_STATE_ENABLED : COMPONENT_ENABLED_STATE_DISABLED;
    // Blocks on IPC.
    packageManager.setComponentEnabledSetting(component, newState, DONT_KILL_APP);
  }

第二步也是主要步骤进入ActivityRefWatcher类初始化Application.ActivityLifecycleCallbacks监听

  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);
        }
      };

对的，就是监听当一个Activity执行onDestory的时候进行监听它

  void onActivityDestroyed(Activity activity) {
    refWatcher.watch(activity);
  }

自然而然我们需要进入RefWatcher.watch这个方法去查看具体是怎么分析

  public void watch(Object watchedReference, String referenceName) {
    if (this == DISABLED) {
      return;
    }
    checkNotNull(watchedReference, "watchedReference");
    checkNotNull(referenceName, "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);
  }

很巧妙的使用KeyedWeakReference弱引用，为了确保弱引用能被回收“偷偷”的跑了Runtime.getRuntime().gc()来实现对弱引用的回收

  Retryable.Result ensureGone(final KeyedWeakReference reference, final long watchStartNanoTime) {
    long gcStartNanoTime = System.nanoTime();
    long watchDurationMs = NANOSECONDS.toMillis(gcStartNanoTime - watchStartNanoTime);

    removeWeaklyReachableReferences();

    if (debuggerControl.isDebuggerAttached()) {
      // The debugger can create false leaks.
      return RETRY;
    }
    if (gone(reference)) {
      return DONE;
    }
    gcTrigger.runGc();
    removeWeaklyReachableReferences();
    if (!gone(reference)) {
      long startDumpHeap = System.nanoTime();
      long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);

      File heapDumpFile = heapDumper.dumpHeap();
      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;
  }

我们再watch方法中每次都会把当前需要检测的对象或者说是Activity组件加入Set<String> retainedKeys这个Set容器中，然后系统就会调用gcTrigger.runGc()来回收KeyedWeakReference弱引用

  GcTrigger DEFAULT = new GcTrigger() {
    @Override public void runGc() {
      // Code taken from AOSP FinalizationTest:
      // https://android.googlesource.com/platform/libcore/+/master/support/src/test/java/libcore/
      // java/lang/ref/FinalizationTester.java
      // System.gc() does not garbage collect every time. Runtime.gc() is
      // more likely to perfom a gc.
      Runtime.getRuntime().gc();
      enqueueReferences();
      System.runFinalization();
    }

    private void enqueueReferences() {
      // Hack. We don't have a programmatic way to wait for the reference queue daemon to move
      // references to the appropriate queues.
      try {
        Thread.sleep(100);
      } catch (InterruptedException e) {
        throw new AssertionError();
      }
    }
  };

那怎么确定这个Activity发生泄漏了呢？
首先我们需要明白一个道理，每次初始化传入了一个ReferenceQueue这个队列是用来存放每当当前的弱引用被GC回收了，那么当前这个弱引用对象就会被存入到这个Queue中去，所以每次只要能从retainedKeys把当前的KeyedWeakReference弱引用对应的key移除那么就证明没有发生泄漏，而当泄漏的话queue就没有供retainedKeys移除的key值

  private void removeWeaklyReachableReferences() {
    // WeakReferences are enqueued as soon as the object to which they point to becomes weakly
    // reachable. This is before finalization or garbage collection has actually happened.
    KeyedWeakReference ref;
    while ((ref = (KeyedWeakReference) queue.poll()) != null) {
      retainedKeys.remove(ref.key);
    }
  }


  private boolean gone(KeyedWeakReference reference) {
    return !retainedKeys.contains(reference.key);
  }

可以从这两个方法很好的看出来就是这么确认是否发生了泄漏。
一旦发生了泄漏就开始Dump hprof文件了

    if (!gone(reference)) {
      long startDumpHeap = System.nanoTime();
      long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);

      File heapDumpFile = heapDumper.dumpHeap();
      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));
    }

如何解析hprof

当发生了泄漏就会生成HeapDump对象然后就会进入下面这个方法去启动HeapAnalyzerServiceService来进行分析

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

然后这边想一提带过就好，因为这边使用的是Square haha库来进行解析dump下来的hprof文件

  public AnalysisResult checkForLeak(File heapDumpFile, String referenceKey) {
    long analysisStartNanoTime = System.nanoTime();

    if (!heapDumpFile.exists()) {
      Exception exception = new IllegalArgumentException("File does not exist: " + heapDumpFile);
      return failure(exception, since(analysisStartNanoTime));
    }

    try {
      HprofBuffer buffer = new MemoryMappedFileBuffer(heapDumpFile);
      HprofParser parser = new HprofParser(buffer);
      Snapshot snapshot = parser.parse();
      deduplicateGcRoots(snapshot);

      Instance leakingRef = findLeakingReference(referenceKey, snapshot);

      // False alarm, weak reference was cleared in between key check and heap dump.
      if (leakingRef == null) {
        return noLeak(since(analysisStartNanoTime));
      }

      return findLeakTrace(analysisStartNanoTime, snapshot, leakingRef);
    } catch (Throwable e) {
      return failure(e, since(analysisStartNanoTime));
    }
  }

想要了解更多可以去查看haha更多的使用方法。
经过解析之后机会把数据传递到DisplayLeakService这是个Service会根据传入进来的数据发送通知栏通知，然后并存入数据，当你点击对应的通知进入DisplayLeakActivity界面就能显示泄漏日志了。

差不多这就是LeakCanary如何监听泄漏、Dump泄漏、分析泄漏、显示泄漏的主要流程了，当然更重要的是发生了泄漏要懂得修复才是硬道理。