Android 内存泄露检测工具 LeakCanary 的监控原理

一. java 的几种 reference:
1,强引用(Strong Reference, 没有具体的类来标识强引用,正常的使用的对象引用都是强引用,由vm实现)
强引用是使用最普遍的引用。如果一个对象具有强引用,那垃圾回收器绝不会回收它。

当内存空间不足,Java虚拟机宁愿抛出OutOfMemoryError错误,使程序异常终止,也不会靠随意回收具有强引用的对象来解决内存不足的问题。

2,软引用(SoftReference)

如果一个对象只具有软引用,则内存空间足够,垃圾回收器就不会回收它;如果内存空间不足了,就会回收这些对象的内存。

只要垃圾回收器没有回收它,该对象就可以被程序使用。软引用可用来实现内存敏感的高速缓存。

软引用可以和一个引用队列(ReferenceQueue)联合使用,如果软引用所引用的对象被垃圾回收器回收,Java虚拟机就会把这个软引用加入到与之关联的引用队列中。

3,弱引用(WeakReference)

弱引用与软引用的区别在于:只具有弱引用的对象拥有更短暂的生命周期。

在垃圾回收器线程扫描它所管辖的内存区域的过程中,一旦发现了只具有弱引用的对象,不管当前内存空间足够与否,都会回收它的内存。

不过,由于垃圾回收器是一个优先级很低的线程,因此不一定会很快发现那些只具有弱引用的对象。

弱引用可以和一个引用队列(ReferenceQueue)联合使用,如果弱引用所引用的对象被垃圾回收,Java虚拟机就会把这个弱引用加入到与之关联的引用队列中。

4,虚引用(PhantomReference)

“虚引用”顾名思义,就是形同虚设,与其他几种引用都不同,虚引用并不会决定对象的生命周期。如果一个对象仅持有虚引用,那么它就和没有任何引用一样,在任何时候都可能被垃圾回收器回收。

虚引用主要用来跟踪对象被垃圾回收器回收的活动。虚引用与软引用和弱引用的一个区别在于:虚引用必须和引用队列 (ReferenceQueue)联合使用。

当垃圾回收器准备回收一个对象时,如果发现它还有虚引用,就会在回收对象的内存之前,把这个虚引用加入到与之 关联的引用队列中。

二.leakcanary原理
LeakCanary 中的 RefWatcher 就是通过弱引用及其队列来实现监控的:

有两个很重要的结构: retainedKeys 和 queue ,

retainedKeys 代表没被gc 回收的对象,

而queue中的弱引用代表的是被gc了的对象,通过这两个结构就可以监控对象是不是被回收了;

retainedKeys存放了RefWatcher为每个被监控的对象生成的唯一key;

同时每个被监控对象的弱引用(KeyedWeakReference)关联了 其对应的key 和 queue,这样对象若被回收,则其对应的弱引用会被入队到queue中;

removeWeaklyReachableReferences(..)所做的就是把存在与queue中的弱引用的key 从 retainedKeys 中删除。

private final Set retainedKeys;
private final ReferenceQueue queue;

/**

  • Watches the provided references and checks if it can be GCed. This method is non blocking,
  • the check is done on the {@link Executor} this {@link RefWatcher} has been constructed with.
  • @param referenceName An logical identifier for the watched object.
    */
    public void watch(Object watchedReference, String referenceName) {
    checkNotNull(watchedReference, "watchedReference");
    checkNotNull(referenceName, "referenceName");
    if (debuggerControl.isDebuggerAttached()) {
    return;
    }
    final long watchStartNanoTime = System.nanoTime();
    String key = UUID.randomUUID().toString();
    retainedKeys.add(key);
    final KeyedWeakReference reference =
    new KeyedWeakReference(watchedReference, key, referenceName, queue);
watchExecutor.execute(new Runnable() {
  @Override public void run() {
    ensureGone(reference, watchStartNanoTime);
  }
});

}

void ensureGone(KeyedWeakReference reference, long watchStartNanoTime) {
long gcStartNanoTime = System.nanoTime();

long watchDurationMs = NANOSECONDS.toMillis(gcStartNanoTime - watchStartNanoTime);
removeWeaklyReachableReferences();
if (gone(reference) || debuggerControl.isDebuggerAttached()) {
  return;
}
gcTrigger.runGc();
removeWeaklyReachableReferences();
if (!gone(reference)) {
  long startDumpHeap = System.nanoTime();
  long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);

  File heapDumpFile = heapDumper.dumpHeap();

  if (heapDumpFile == HeapDumper.NO_DUMP) {
    // Could not dump the heap, abort.
    return;
  }
  long heapDumpDurationMs = NANOSECONDS.toMillis(System.nanoTime() - startDumpHeap);
  heapdumpListener.analyze(
      new HeapDump(heapDumpFile, reference.key, reference.name, excludedRefs, watchDurationMs,
          gcDurationMs, heapDumpDurationMs));
}

}

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

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);
}
}
什么时候使用RefWatcher进行监控 ?

对于android, 若要监控Activity, 需要在其执行destroy的 时候进行监控:

通过向Application 注册 ActivityLifecycleCallback, 在onActivityDestroyed(Activity activity) 中 开始监听 activity对象, 因为这时activity应该被回收了,若发生内存泄露,则可以没发现;

RefWatcher 检查对象是否被回收是在一个 Executor 中执行的, Android 的监控 提供了 AndroidWatchExecutor , 它在主线程执行, 但是有一个delay 时间(默认5000 milisecs), 因为对于application 来说,执行destroy activity只是把必要资源回收,activity 对象不一定会马上被 gc回收。

private void executeDelayedAfterIdleUnsafe(final Runnable runnable) {
// This needs to be called from the main thread.
Looper.myQueue().addIdleHandler(new MessageQueue.IdleHandler() {
@Override public boolean queueIdle() {
backgroundHandler.postDelayed(runnable, DELAY_MILLIS);
return false;
}
});
}
ActivityRefWatcher:

package com.squareup.leakcanary;

import android.annotation.TargetApi;
import android.app.Activity;
import android.app.Application;
import android.os.Bundle;

import static android.os.Build.VERSION.SDK_INT;
import static android.os.Build.VERSION_CODES.ICE_CREAM_SANDWICH;
import static com.squareup.leakcanary.Preconditions.checkNotNull;

@TargetApi(ICE_CREAM_SANDWICH) public final class ActivityRefWatcher {

public static void installOnIcsPlus(Application application, RefWatcher refWatcher) {
if (SDK_INT < ICE_CREAM_SANDWICH) {
// If you need to support Android < ICS, override onDestroy() in your base activity.
return;
}
ActivityRefWatcher activityRefWatcher = new ActivityRefWatcher(application, refWatcher);
activityRefWatcher.watchActivities();
}

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);
    }
  };

private final Application application;
private final RefWatcher refWatcher;

/**

  • Constructs an {@link ActivityRefWatcher} that will make sure the activities are not leaking
  • after they have been destroyed.
    */
    public ActivityRefWatcher(Application application, final RefWatcher refWatcher) {
    this.application = checkNotNull(application, "application");
    this.refWatcher = checkNotNull(refWatcher, "refWatcher");
    }

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

public void watchActivities() {
// Make sure you don't get installed twice.
stopWatchingActivities();
application.registerActivityLifecycleCallbacks(lifecycleCallbacks);
}

public void stopWatchingActivities() {
application.unregisterActivityLifecycleCallbacks(lifecycleCallbacks);
}
}
若发生了泄露, refWatcher 会执行dump ,生成dump 文件,然后由mat 或haha 等分析工具找到泄露对象的引用路径。

作者: DocMike
链接:https://www.imooc.com/article/19225
来源:慕课网

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