Android工具(LeakCanary—内存泄露检测神器)

【Android工具(LeakCanary—内存泄露检测神器)】丈夫志四海,万里犹比邻。这篇文章主要讲述Android工具:LeakCanary—内存泄露检测神器相关的知识,希望能为你提供帮助。
一、LeakCanary简介
LeakCanary是Square公司开源的一个检测内存的泄露的函数库, 可以方便地和你的项目进行集成, 在Debug版本中监控Activity、Fragment等的内存泄露;
LeakCanary集成到项目中之后, 在检测到内存泄露时, 会发送消息到系统通知栏。点击后打开名称DisplayLeakActivity的页面, 并显示泄露的跟踪信息, Logcat上面也会有对应的日志输出。同时如果跟踪信息不足以定位时, DisplayLeakActivity还为开发者默认保存了最近7个dump文件到App的目录中, 可以使用MAT等工具对dump文件进行进一步的分析;
二、内存泄漏简介
在了解了LeakCanary的接入方式后, 我们肯定着急想见识见识LeakCanary的威力。在跟大家演示LeakCanary检测和处理构成之前, 大家应该明应该对内存泄露有基本的了解和认识;
1.为什么会产生内存泄漏?
当一个对象不需要使用本该回收时, 有另外一个正在使用的对象持有它的引用, 从而导致它不能回收停留在堆内存中, 这就产生了内存泄漏;
2.内存泄露对程序产生的影响?
内存泄漏是造成应用程序OOM的主要原因之一。android系统为每个应用程序分配有限的内存, 当应用中内存泄漏较多时, 就难免会导致应用所需要的内存超出系统分配限额, 从而导致OOM应用Crash;
3.Android常见的内存泄露?
相信内存泄露对大家都早有耳闻, 但是它不像一些java异常情况, 会立即造成程序的Crash, 却有让大家比较“陌生”。下面我们就列举出日常开发中常见的内存泄露类型, 让大家对内存泄露的认识不仅仅停留在“有所耳闻 ”的层面;
单例造成: 由于单例静态特性使得单例的生命周期和应用的生命周期一样长, 如果一个对象( 如Context) 已经不使用了, 而单例对象还持有对象的引用造成这个对象不能正常被回收;
非静态内部类创建静态实例造成: 在Acitivity内存创建一个非静态内部类单例, 避免每次启动资源重新创建。但是因为非静态内部类默认持有外部类( Activity) 的引用, 并且使用该类创建静态实例。造成该实例和应用生命周期一样长, 导致静态实例持有引用的Activity和资源不能正常回收;
Handler造成: 子线程执行网络任务, 使用Handler处理子线程发送消息。由于handler对象是非静态匿名内部类的对象, 持有外部类( Activity) 的引用。在Handler-Message中Looper线程不断轮询处理消息, 当Activity退出还有未处理或者正在处理的消息时, 消息队列中的消息持有handler对象引用, handler又持有Activity, 导致Activity的内存和资源不能及时回收;
线程造成: 匿名内部类Runnalbe和AsyncTask对象执行异步任务, 对当前Activity隐式引用。当Activity销毁之前, 任务还没有执行完, 将导致Activity的内存和资源不能及时回收;
资源未关闭造成的内存泄露: 对于使用了BroadcastReceiver, ContentObserver, File, Cursor, Stream, Bitmap等资源的使用, 应该在Activity销毁时及时关闭或者注销, 否则这些资源将不会被回收, 造成内存泄露;
三、LeakCanary接入
下面我们还是以QProject项目进行演示如何在项目中接入LeakCanary, 项目目录如下:

Android工具(LeakCanary—内存泄露检测神器)

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1.添加LeakCanary依赖
在主项目main模块的build.gradle文件中添加LeakCanary相关依赖;
/main/build.gradle文件
apply plugin: ' com.android.application' android { ...... } dependencies { ... ... //添加leakcanary相关的依赖 //在release和test版本中, 使用的是LeakCanary的no-op版本, 也就是没有实际代码和操作的Wrapper版本, 只包含LeakCanary和RefWatcher类的空实现, 这样不会对生成的APK包体积和应用性能造成影响 debugCompile ' com.squareup.leakcanary:leakcanary-android:1.5' releaseCompile ' com.squareup.leakcanary:leakcanary-android-no-op:1.5' testCompile ' com.squareup.leakcanary:leakcanary-android-no-op:1.5' ... ... compile project(' :test' ) }

2.初始化LeakCanary
在主项目main模块的QApplication的onCreate()方法中初始化LeakCanary;
/main/src/main/java/com/qproject/main/QApplication.java文件
public class QAplication extends Application{ @ Override public void onCreate() { super.onCreate(); ... ... //初始化LeakCanary if (LeakCanary.isInAnalyzerProcess(this)) { return; } LeakCanary.install(this); } }

OK, 到这里我们就完成了一个项目的LeankCanary的简单接入;
提示1: 集成LeakCanary后, 构建和安装apk的时候报错如下:
Error:Error converting bytecode to dex:
Cause: com.android.dex.DexException: Multiple dex files define Lcom/squareup/leakcanary/LeakCanary;
Error:Execution failed for task ' :main:transformClassesWithDexForDebug' .
> com.android.build.api.transform.TransformException: com.android.ide.common.process.ProcessException: java.util.concurrent.ExecutionException: java.lang.UnsupportedOperationException
处理1: 添加依赖debugCompile ' com.squareup.haha:haha:2.0.3' , 修改依赖的版本为1.4-beta2;
四、LeakCanary检测
通过对一些常见的内存泄露的学习, 我们已经对内存泄露有所见闻了。那么下面我们通过LeakCanary工具, 让大家感受下它在你的日常开发中的真实存在。我们以常见内存—单例造成的内存泄露为例进行实践;
1.单例内存泄露模拟
test/src/main/com/qproject/test/TestManager.java
public class TestManager { //单例静态特性使得单例的生命周期和应用的生命周期一样长 private static TestManager instance; private Context context; /** * 传入的Context的生命周期很重要: *如果传入的是Application的Context, 则生命周期和单例生命周期一样长; *如果传入的是Activity的Context, 由于该Context和Activity的生命周期一样长, 当Activity退出的时候它的内存不会被回收, 因为单例对象持有它的引用; */ private TestManager(Context context) { this.context = context; }public static TestManager getInstance(Context context) { if (instance = = null) { instance = new TestManager(context); } return instance; } }

test/src/main/com/qproject/test/leakcanary/LeakCanaryActivity.java
public class LeakCanaryActivity extends AppCompatActivity {@ Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_leakcanary); //获取单例对象, 退出Activity即可模拟出内存泄露 TestManager testManager = TestManager.getInstance(this); } }

2.检测消息通知
运行App到LeakCanaryActivit页面并退出, 在检测到内存泄露的时候, 会发送消息到系统通知栏;
Android工具(LeakCanary—内存泄露检测神器)

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Android工具(LeakCanary—内存泄露检测神器)

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3.查看检测详情
点击通知消息, 打开名为DisplayLeakActivity的页面, 并显示泄漏的跟踪信息;
Android工具(LeakCanary—内存泄露检测神器)

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4.查看LogCat日志
除了以上泄漏信息的显示, Logcat上面也会有对应的日志输出;
//内存泄露对象com.qproject.test.leakcanary.LeakCanaryActivity 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * com.qproject.test.leakcanary.LeakCanaryActivity has leaked: //static com.qproject.test.TestManager.instance的com.qproject.test.TestManager.context引用了回收的内存对象 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * GC ROOT static com.qproject.test.TestManager.instance 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * references com.qproject.test.TestManager.context //内存泄露对象大小, Reference Key, Device和Android Version等信息 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * leaks com.qproject.test.leakcanary.LeakCanaryActivity instance 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * Retaining: 46 KB. 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * Reference Key: 3d74d294-70dc-4447-a9a2-64e656ea86b8 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * Device: Genymotion Android PREVIEW - Google Nexus 5X - 7.0.0 - API 24 - 1080x1920 vbox86p 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * Android Version: 7.0 API: 24 LeakCanary: 1.5 00f37f5 12-25 07:50:51.710 4941-5795/com.qproject.main D/LeakCanary: * Durations: watch= 5038ms, gc= 137ms, heap dump= 2390ms, analysis= 27325ms //内存泄露对象详细信息 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: * Details: 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: * Class com.qproject.test.TestManager 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |static instance = com.qproject.test.TestManager@ 316184816 (0x12d898f0) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |static $classOverhead = byte[308]@ 316175745 (0x12d87581) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: * Instance of com.qproject.test.TestManager 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |static instance = com.qproject.test.TestManager@ 316184816 (0x12d898f0) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |static $classOverhead = byte[308]@ 316175745 (0x12d87581) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |context = com.qproject.test.leakcanary.LeakCanaryActivity@ 315059712 (0x12c76e00) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |shadow$_klass_ = com.qproject.test.TestManager 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |shadow$_monitor_ = 0 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: * Instance of com.qproject.test.leakcanary.LeakCanaryActivity 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |static $classOverhead = byte[2228]@ 316203009 (0x12d8e001) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mDelegate = android.support.v7.app.AppCompatDelegateImplN@ 315842128 (0x12d35e50) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mEatKeyUpEvent = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mResources = null 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mThemeId = 2131230884 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mCreated = true 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mFragments = android.support.v4.app.FragmentController@ 316183584 (0x12d89420) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mHandler = android.support.v4.app.FragmentActivity$1@ 316163360 (0x12d84520) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mNextCandidateRequestIndex = 0 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mOptionsMenuInvalidated = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mPendingFragmentActivityResults = android.support.v4.util.SparseArrayCompat@ 316172368 (0x12d86850) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mReallyStopped = true 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mRequestedPermissionsFromFragment = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mResumed = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mRetaining = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mStopped = true 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mStartedActivityFromFragment = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mStartedIntentSenderFromFragment = false 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mExtraDataMap = android.support.v4.util.SimpleArrayMap@ 316171864 (0x12d86658) 12-25 07:50:51.711 4941-5795/com.qproject.main D/LeakCanary: |mActionBar = null 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mActionModeTypeStarting = 0 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mActivityInfo = android.content.pm.ActivityInfo@ 315841984 (0x12d35dc0) 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mActivityTransitionState = android.app.ActivityTransitionState@ 316207336 (0x12d8f0e8) 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mApplication = com.qproject.main.QAplication@ 314916416 (0x12c53e40) 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mCalled = true 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mChangeCanvasToTranslucent = false 12-25 07:50:51.712 4941-5795/com.qproject.main D/LeakCanary: |mChangingConfigurations = false 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mComponent = android.content.ComponentName@ 315998320 (0x12d5c070) 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mConfigChangeFlags = 0 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mCurrentConfig = android.content.res.Configuration@ 316178888 (0x12d881c8) 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mDecor = null 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mDefaultKeyMode = 0 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mDefaultKeySsb = null 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mDestroyed = true 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mDoReportFullyDrawn = false 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mEatKeyUpEvent = false 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mEmbeddedID = null 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mEnableDefaultActionBarUp = false 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mEnterTransitionListener = android.app.SharedElementCallback$1@ 1887062680 (0x707a4a98) 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mExitTransitionListener = android.app.SharedElementCallback$1@ 1887062680 (0x707a4a98) 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mFinished = true 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mFragments = android.app.FragmentController@ 316183536 (0x12d893f0) 12-25 07:50:51.713 4941-5795/com.qproject.main D/LeakCanary: |mHandler = android.os.Handler@ 316163296 (0x12d844e0) 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mHasCurrentPermissionsRequest = false 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mIdent = 20356640 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mInstanceTracker = android.os.StrictMode$InstanceTracker@ 316183552 (0x12d89400) 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mInstrumentation = android.app.Instrumentation@ 314950632 (0x12c5c3e8) 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mIntent = android.content.Intent@ 316215416 (0x12d91078) 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mLastNonConfigurationInstances = null 12-25 07:50:51.714 4941-5795/com.qproject.main D/LeakCanary: |mMainThread = android.app.ActivityThread@ 314966272 (0x12c60100) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mManagedCursors = java.util.ArrayList@ 316171816 (0x12d86628) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mManagedDialogs = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mMenuInflater = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mParent = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mReferrer = java.lang.String@ 316215864 (0x12d91238) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mResultCode = 0 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mResultData = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mResumed = false 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mSearchEvent = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mSearchManager = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mStartedActivity = false 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mStopped = true 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTaskDescription = android.app.ActivityManager$TaskDescription@ 316163328 (0x12d84500) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTemporaryPause = false 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTitle = java.lang.String@ 315129824 (0x12c87fe0) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTitleColor = 0 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTitleReady = true 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mToken = android.os.BinderProxy@ 315867328 (0x12d3c0c0) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mTranslucentCallback = null 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mUiThread = java.lang.Thread@ 1959751680 (0x74cf7000) 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mVisibleBehind = false 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mVisibleFromClient = true 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mVisibleFromServer = true 12-25 07:50:51.715 4941-5795/com.qproject.main D/LeakCanary: |mVoiceInteractor = null 12-25 07:50:51.716 4941-5795/com.qproject.main D/LeakCanary: |mWindow = com.android.internal.policy.PhoneWindow@ 315116864 (0x12c84d40) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mWindowAdded = true 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mWindowManager = android.view.WindowManagerImpl@ 316172152 (0x12d86778) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mInflater = com.android.internal.policy.PhoneLayoutInflater@ 316010352 (0x12d5ef70) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mOverrideConfiguration = null 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mResources = android.content.res.Resources@ 316235992 (0x12d960d8) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mTheme = android.content.res.Resources$Theme@ 316183728 (0x12d894b0) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mThemeResource = 2131230884 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |mBase = android.app.ContextImpl@ 316155392 (0x12d82600) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |shadow$_klass_ = com.qproject.test.leakcanary.LeakCanaryActivity 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: |shadow$_monitor_ = 1316364430 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: * Excluded Refs: 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Field: android.view.Choreographer$FrameDisplayEventReceiver.mMessageQueue (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Thread:FinalizerWatchdogDaemon (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Thread:main (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Thread:LeakCanary-Heap-Dump (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Class:java.lang.ref.WeakReference (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Class:java.lang.ref.SoftReference (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Class:java.lang.ref.PhantomReference (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Class:java.lang.ref.Finalizer (always) 12-25 07:50:51.717 4941-5795/com.qproject.main D/LeakCanary: | Class:java.lang.ref.FinalizerReference (always)

5.获取dump日志文件
如果觉得跟踪信息不足以定位时, DisplayLeakActivity还为开发者默认保存了最近7个dump文件到APP的目录中, 可以使用MAT等工具对dump文件进行进一步的分析;
vbox86p:/data/data/com.qproject.main/files/leakcanary # ls 2016-12-25_07-50-51_718.hprof 2016-12-25_07-50-51_718.hprof.result D:\\> adb pull ./data/data/com.qproject.main/files/leakcanary/2016-12-25_07-50-51_ 718.hprof [100%] ./data/data/com.qproject.main/f...akcanary/2016-12-25_07-50-51_718.hprof

Android Studio-> View-> Tool Windows-> Captures, 打开Captures窗口, 将pull获取的hprof文件剪切到Capture中文件的目录下, 双击打开即可;
Android工具(LeakCanary—内存泄露检测神器)

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具体的分析过程, 这里就不重点讲述, 大家去查询MAT相关的资料;
五、检测其他对象
如果想监听其他的对象( 例如Fragment ) , 可以通过RefWatcher的实例来实现;
Android工具(LeakCanary—内存泄露检测神器)

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Android工具(LeakCanary—内存泄露检测神器)

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六、LeakCanary原理
1.RefWatcher.watch()函数会为被监控的对象创建一个KeyedWeakReference弱引用对象, 是WeakReference对的子类, 增加了键值对信息, 后面会根据指定的键key找到弱引用对象;
2.在后台线程AndroidWatchExecutor中, 检查KeyedWeakReference弱引用是否已经被清楚。如果还存在, 则触发一次垃圾回收之后。垃圾回收之后, 如果弱引用对象依然存在, 说明发生了内存泄露;

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