Android 性能优化之 LeakCanary使用方法

背景

在Android应用中，一个好的产品，除了功能强大，好的性能也是必不可少的。有调查显示，近90%的受访者会因为App卡顿，内存大等问题而卸载该应用，因此手机的性能问题会影响用户的体验，如果用户觉得该应用的体验度不好，会直接卸载或切换其他平台。

对于性能优化，很多大公司会专门招聘性能优化的人员。也有些初级工程师会接触到这部分的工作，但是无从下手，对专业工具和专业代码使用以及分析比较吃力，排查起来也比较费劲。如果有专业的工具能够只管的把这些记录并标记好。这样初级工程师也可以通过详情的问题去排查，那么LeaksCanary就是这款工具了。

LeaksCanary 介绍

LeakCanary是Square公司为Android开发者提供的一个自动检测内存泄漏的工具。

LeakCanary本质上是一个基于MAT进行Android应用程序内存泄漏自动化检测的的开源工具，我们可以通过集成LeakCanary提供的jar包到自己的工程中，一旦检测到内存泄漏，LeakCanary就会dump Memory信息，并通过另一个进程分析内存泄漏的信息并展示出来，随时发现和定位内存泄漏问题，而不用每次在开发流程中都抽出专人来进行内存泄漏问题检测，极大地方便了Android应用程序的开发。

使用方法

1.LeakCanary 如何自动初始化

LeakCanary只需添加依赖就可以实现自动初始化。LeakCanary是通过ContentProvider实现初始化的，在ContentProvider 的 onCreate方法中初始化LeakCanary。并且MainProcessAppWatcherInstaller是在主线程中初始化的。注意：ContentProvider的初始化是在Application的onCreate之前完成的，所以LeakCanary的初始化方法AppWatcher.manualInstall(application)也是在Application的onCreate之前完成的。

internal class MainProcessAppWatcherInstaller : ContentProvider() {
   override fun onCreate(): Boolean {
      val application = context!!.applicationContext as Application
      AppWatcher.manualInstall(application)
      return true
    }
     ... ...
}

2.LeakCanary如何检测内存泄漏

2.1LeakCanary初始化时做了什么

AppWatcher.kt

@JvmOverloads
fun manualInstall(
  application: Application,
  retainedDelayMillis: Long = TimeUnit.SECONDS.toMillis(5),
  watchersToInstall: List<InstallableWatcher> = appDefaultWatchers(application)
) {
  checkMainThread()
  if (isInstalled) {
    throw IllegalStateException(
      "AppWatcher already installed, see exception cause for prior install call", installCause
    )
  }
  check(retainedDelayMillis >= 0) {
    "retainedDelayMillis $retainedDelayMillis must be at least 0 ms"
  }
  installCause = RuntimeException("manualInstall() first called here")
  this.retainedDelayMillis = retainedDelayMillis
  if (application.isDebuggableBuild) {
    LogcatSharkLog.install()
  }
  // Requires AppWatcher.objectWatcher to be set
  LeakCanaryDelegate.loadLeakCanary(application)
  watchersToInstall.forEach {
    it.install()
  }
}

fun appDefaultWatchers(
  application: Application,
  reachabilityWatcher: ReachabilityWatcher = objectWatcher
): List<InstallableWatcher> {
  return listOf(
    ActivityWatcher(application, reachabilityWatcher),
    FragmentAndViewModelWatcher(application, reachabilityWatcher),
    RootViewWatcher(reachabilityWatcher),
    ServiceWatcher(reachabilityWatcher)
  )
}

在appDefaultWatchers方法中，会默认初始化一些Watcher，在默认情况下，我们只会监控Activity,Fragment,RootView,Service这些对象是否泄漏。

2.2LeakCanary如何触发检测

以ActivityWatcher为例:

/**
 * Expects activities to become weakly reachable soon after they receive the [Activity.onDestroy]
 * callback.
 */
class ActivityWatcher(
  private val application: Application,
  private val reachabilityWatcher: ReachabilityWatcher
) : InstallableWatcher {
  private val lifecycleCallbacks =
    object : Application.ActivityLifecycleCallbacks by noOpDelegate() {
      override fun onActivityDestroyed(activity: Activity) {
        reachabilityWatcher.expectWeaklyReachable(
          activity, "${activity::class.java.name} received Activity#onDestroy() callback"
        )
      }
    }
  override fun install() {
    application.registerActivityLifecycleCallbacks(lifecycleCallbacks)
  }
  override fun uninstall() {
    application.unregisterActivityLifecycleCallbacks(lifecycleCallbacks)
  }
}

在Activity.onDestory时，就会触发检测内存泄漏。通过ActivityLifecycleCallbacks监听生命周期变化，在onActivityDestroyed方法中调用ReachabilityWatcher的expectWeaklyReachable方法。

2.3LeakCanary如何检测泄漏的对象

以Activity为例，通过ReachabilityWatcher的expectWeaklyReachable方法检测。

fun interface ReachabilityWatcher {
  /**
   * Expects the provided [watchedObject] to become weakly reachable soon. If not,
   * [watchedObject] will be considered retained.
   */
  fun expectWeaklyReachable(
    watchedObject: Any,
    description: String
  )
}
ObjectWatcher.kt
ObjectWatcher实现ReachabilityWatcher接口。
private val watchedObjects = mutableMapOf()
private val queue = ReferenceQueue()
@Synchronized override fun expectWeaklyReachable(
  watchedObject: Any,
  description: String
) {
  if (!isEnabled()) {
    return
  }
  removeWeaklyReachableObjects()
  val key = UUID.randomUUID()
    .toString()
  val watchUptimeMillis = clock.uptimeMillis()
  val reference =
    KeyedWeakReference(watchedObject, key, description, watchUptimeMillis, queue)
  SharkLog.d {
    "Watching " +
      (if (watchedObject is Class<*>) watchedObject.toString() else "instance of ${watchedObject.javaClass.name}") +
      (if (description.isNotEmpty()) " ($description)" else "") +
      " with key $key"
  }
  watchedObjects[key] = reference
  checkRetainedExecutor.execute {
    moveToRetained(key)
  }
}

1.通过观察的实例watchedObject构建弱引用KeyedWeakReference实例，watchedObject与ReferenceQueue关联，当对象被回收时，该弱引用对象将被存入ReferenceQueue当中。

2.弱引用KeyedWeakReference实例会被被存储在watchedObjects中（Map）。

3.检测过程中，会调用removeWeaklyReachableObjects,将已回收对象从watchedObjects中移除。

4.如果watchedObjects中没有移除对象，证明它没有被回收，那么就会调用moveToRetained。

private fun removeWeaklyReachableObjects() {
  // 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.
  var ref: KeyedWeakReference?
  do {
    ref = queue.poll() as KeyedWeakReference?
    if (ref != null) {
      watchedObjects.remove(ref.key)
    }
  } while (ref != null)
}

@Synchronized private fun moveToRetained(key: String) {
  removeWeaklyReachableObjects()
  val retainedRef = watchedObjects[key]
  if (retainedRef != null) {
    retainedRef.retainedUptimeMillis = clock.uptimeMillis()
    onObjectRetainedListeners.forEach { it.onObjectRetained() }
  }
}

2.4弱引用 WeakReference

只要 GC 发现一个对象只有弱引用，则就会回收此弱引用对象。

public class WeakReference<T> extends Reference<T> {
    public WeakReference(T referent) {
        super(referent);
    }
    public WeakReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }
}

var str: Any? = Any()
val quque = ReferenceQueue<Any>()
val weakReference = WeakReference<Any>(str, quque)
val weakReference_before_gc = weakReference.get()
Log.v("reference_tag", weakReference_before_gc.toString())
str = null
System.gc()
Handler().postDelayed( {
    val weakReference_after_gc = weakReference.get()
    Log.v("reference_tag", weakReference_after_gc.toString())
}, 2000)

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Android性能优化修炼手册：http: //​docs.qq.com/doc/DWGRIR1hVWkFoZWVK