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 $retainedDelayMillis must be at least 0 ms"
}
installCause = RuntimeException("manualInstall() first called here")
this.retainedDelayMillis = retainedDelayMillis
if (application.isDebuggableBuild) {
LogcatSharkLog.install()
}
// Requires AppWatcher.objectWatcher to be set
LeakCanaryDelegate.loadLeakCanary(application)
watchersToInstall.forEach {
it.install()
}
}fun appDefaultWatchers(
application: Application,
reachabilityWatcher: ReachabilityWatcher = objectWatcher
): List<InstallableWatcher> {
return listOf(
ActivityWatcher(application, reachabilityWatcher),
FragmentAndViewModelWatcher(application, reachabilityWatcher),
RootViewWatcher(reachabilityWatcher),
ServiceWatcher(reachabilityWatcher)
)
}在appDefaultWatchers方法中，会默认初始化一些Watcher，在默认情况下，我们只会监控Activity,Fragment,RootView,Service这些对象是否泄漏。
2.2LeakCanary如何触发检测
以ActivityWatcher为例:
class ActivityWatcher(
private val application: Application,
private val reachabilityWatcher: ReachabilityWatcher
) : InstallableWatcher {
private val lifecycleCallbacks =
object : Application.ActivityLifecycleCallbacks by noOpDelegate() {
override fun onActivityDestroyed(activity: Activity) {
reachabilityWatcher.expectWeaklyReachable(
activity, "${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 {
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
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