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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 ApplicationAppWatcher.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 = retainedDelayMillisif (application.isDebuggableBuild) {LogcatSharkLog.install()}// Requires AppWatcher.objectWatcher to be setLeakCanaryDelegate.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] = referencecheckRetainedExecutor.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)

到此这篇关于Android LeakCanary的使用方法介绍的文章就介绍到这了,更多相关Android 性能优化相关只是可参考小编分享在下面的文档，访问下方链接获取。

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