一. java 的幾種 reference：
1咐刨，強(qiáng)引用（Strong Reference, 沒(méi)有具體的類來(lái)標(biāo)識(shí)強(qiáng)引用码俩，正常的使用的對(duì)象引用都是強(qiáng)引用度帮，由vm實(shí)現(xiàn)）
強(qiáng)引用是使用最普遍的引用。如果一個(gè)對(duì)象具有強(qiáng)引用握玛，那垃圾回收器絕不會(huì)回收它够傍。

當(dāng)內(nèi)存空間不足，Java虛擬機(jī)寧愿拋出OutOfMemoryError錯(cuò)誤挠铲，使程序異常終止冕屯，也不會(huì)靠隨意回收具有強(qiáng)引用的對(duì)象來(lái)解決內(nèi)存不足的問(wèn)題。

2拂苹，軟引用（SoftReference）

如果一個(gè)對(duì)象只具有軟引用安聘，則內(nèi)存空間足夠，垃圾回收器就不會(huì)回收它瓢棒；如果內(nèi)存空間不足了浴韭，就會(huì)回收這些對(duì)象的內(nèi)存。

只要垃圾回收器沒(méi)有回收它脯宿，該對(duì)象就可以被程序使用念颈。軟引用可用來(lái)實(shí)現(xiàn)內(nèi)存敏感的高速緩存。

軟引用可以和一個(gè)引用隊(duì)列（ReferenceQueue）聯(lián)合使用连霉，如果軟引用所引用的對(duì)象被垃圾回收器回收榴芳，Java虛擬機(jī)就會(huì)把這個(gè)軟引用加入到與之關(guān)聯(lián)的引用隊(duì)列中嗡靡。

3,弱引用（WeakReference）

弱引用與軟引用的區(qū)別在于：只具有弱引用的對(duì)象擁有更短暫的生命周期。

在垃圾回收器線程掃描它所管轄的內(nèi)存區(qū)域的過(guò)程中窟感，一旦發(fā)現(xiàn)了只具有弱引用的對(duì)象讨彼，不管當(dāng)前內(nèi)存空間足夠與否，都會(huì)回收它的內(nèi)存柿祈。

不過(guò)哈误，由于垃圾回收器是一個(gè)優(yōu)先級(jí)很低的線程，因此不一定會(huì)很快發(fā)現(xiàn)那些只具有弱引用的對(duì)象躏嚎。

弱引用可以和一個(gè)引用隊(duì)列（ReferenceQueue）聯(lián)合使用蜜自，如果弱引用所引用的對(duì)象被垃圾回收，Java虛擬機(jī)就會(huì)把這個(gè)弱引用加入到與之關(guān)聯(lián)的引用隊(duì)列中卢佣。

4袁辈，虛引用（PhantomReference）

“虛引用”顧名思義，就是形同虛設(shè)珠漂，與其他幾種引用都不同，虛引用并不會(huì)決定對(duì)象的生命周期尾膊。如果一個(gè)對(duì)象僅持有虛引用媳危，那么它就和沒(méi)有任何引用一樣，在任何時(shí)候都可能被垃圾回收器回收冈敛。

虛引用主要用來(lái)跟蹤對(duì)象被垃圾回收器回收的活動(dòng)待笑。虛引用與軟引用和弱引用的一個(gè)區(qū)別在于：虛引用必須和引用隊(duì)列 （ReferenceQueue）聯(lián)合使用。

當(dāng)垃圾回收器準(zhǔn)備回收一個(gè)對(duì)象時(shí)抓谴，如果發(fā)現(xiàn)它還有虛引用暮蹂，就會(huì)在回收對(duì)象的內(nèi)存之前，把這個(gè)虛引用加入到與之 關(guān)聯(lián)的引用隊(duì)列中癌压。

二.leakcanary原理
LeakCanary 中的 RefWatcher 就是通過(guò)弱引用及其隊(duì)列來(lái)實(shí)現(xiàn)監(jiān)控的：

有兩個(gè)很重要的結(jié)構(gòu)： retainedKeys 和 queue 仰泻，

retainedKeys 代表沒(méi)被gc 回收的對(duì)象，

而queue中的弱引用代表的是被gc了的對(duì)象滩届，通過(guò)這兩個(gè)結(jié)構(gòu)就可以監(jiān)控對(duì)象是不是被回收了集侯；

retainedKeys存放了RefWatcher為每個(gè)被監(jiān)控的對(duì)象生成的唯一key；

同時(shí)每個(gè)被監(jiān)控對(duì)象的弱引用（KeyedWeakReference）關(guān)聯(lián)了 其對(duì)應(yīng)的key 和 queue帜消，這樣對(duì)象若被回收棠枉，則其對(duì)應(yīng)的弱引用會(huì)被入隊(duì)到queue中；

removeWeaklyReachableReferences（..）所做的就是把存在與queue中的弱引用的key 從 retainedKeys 中刪除泡挺。

private final Set<String> retainedKeys;
private final ReferenceQueue<Object> 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);
}
}
什么時(shí)候使用RefWatcher進(jìn)行監(jiān)控 辈讶？

對(duì)于android， 若要監(jiān)控Activity娄猫， 需要在其執(zhí)行destroy的 時(shí)候進(jìn)行監(jiān)控：

通過(guò)向Application 注冊(cè) ActivityLifecycleCallback贱除， 在onActivityDestroyed（Activity activity） 中 開(kāi)始監(jiān)聽(tīng) activity對(duì)象生闲， 因?yàn)檫@時(shí)activity應(yīng)該被回收了，若發(fā)生內(nèi)存泄露勘伺，則可以沒(méi)發(fā)現(xiàn)跪腹；

RefWatcher 檢查對(duì)象是否被回收是在一個(gè) Executor 中執(zhí)行的， Android 的監(jiān)控 提供了 AndroidWatchExecutor 飞醉， 它在主線程執(zhí)行冲茸， 但是有一個(gè)delay 時(shí)間（默認(rèn)5000 milisecs）， 因?yàn)閷?duì)于application 來(lái)說(shuō)缅帘，執(zhí)行destroy activity只是把必要資源回收轴术，activity 對(duì)象不一定會(huì)馬上被 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);
}
}
若發(fā)生了泄露钦无， refWatcher 會(huì)執(zhí)行dump 逗栽，生成dump 文件，然后由mat 或haha 等分析工具找到泄露對(duì)象的引用路徑失暂。

作者： DocMike
鏈接：https://www.imooc.com/article/19225
來(lái)源：慕課網(wǎng)