LeakCanary使用学习（未完）

Posted 2022-11-25 且听真言

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。

2.2 LeakCanary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。

2.3 LeakCanary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)

2022-02-27 17:43:04.181 16634-16634/com.example.myapplication V/reference_tag: java.lang.Object@c87946a
2022-02-27 17:43:06.182 16634-16634/com.example.myapplication V/reference_tag: null