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LeakCanary原理分析

2023-08-18 270 推广

概述

LeakCanary是一个开源的内存泄漏检测库，极大简化了内存泄漏的检测流程。了解其工作原理，有助于我们更好的理解Android的内存管理机制。

使用示例

在 build.gradle中添加配置：

dependencies {
  debugImplementation 'com.squareup.leakcanary:leakcanary-android:1.6.3'
  releaseImplementation 'com.squareup.leakcanary:leakcanary-android-no-op:1.6.3'
  // Optional, if you use support library fragments:
  debugImplementation 'com.squareup.leakcanary:leakcanary-support-fragment:1.6.3'
}

在 Application类中添加代码：

public class ExampleApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();

        if (LeakCanary.isInAnalyzerProcess(this)) {
            // This process is dedicated to LeakCanary for heap analysis.
            // You should not init your app in this process.
            return;
        }
        LeakCanary.install(this);
        // Normal app init code...
    }
}

使用RefWatcher观察那些本该被GC回收掉的对象：

RefWatcher refWatcher = LeakCanary.installedRefWatcher();

// We expect schrodingerCat to be gone soon (or not), let's watch it.
refWatcher.watch(schrodingerCat);

工作机制

  1. RefWatcher.watch()创建一个 KeyedWeakReference到要被监控的对象。
  2. 然后在后台线程检查引用是否被清除，如果没有，调用GC。
  3. 如果引用还是未被清除，把heap内存dump到APP对应的文件系统中的一个 .hprof文件中。
  4. 在另一个进程中的 HeapAnalyzerService有一个 HeapAnalyzer使用HAHA解析这个文件。
  5. 在Heap Dump中， HeapAnalyzer根据唯一的reference key找到了 KeyedWeakReference，并定位了泄漏的引用。
  6. HeapAnalyzer计算到 GCRoots的最短强引用路径，并确定是否泄漏，如果是的话，建立导致泄漏的引用链。
  7. 引用链传递到APP进程中的 DisplayLeakService，并以通知的形式展示出来。

源码分析

创建RefWatcher

public final class LeakCanary {

    public static @NonNull RefWatcher install(@NonNull Application application) {
        return refWatcher(application) // 创建AndroidRefWatcherBuilder对象
            .listenerServiceClass(DisplayLeakService.class) // 配置监听分析结果的服务
            .excludedRefs(AndroidExcludedRefs.createAppDefaults().build()) // 配置排除的系统泄露
            .buildAndInstall(); // 创建一个Refwatcher并监听Activity的引用
    }
    // ...
}

AndroidRefWatcherBuilder#buildAndInstall

public final class AndroidRefWatcherBuilder extends RefWatcherBuilder {

    public @NonNull RefWatcher buildAndInstall() {
        if (LeakCanaryInternals.installedRefWatcher != null) {
            throw new UnsupportedOperationException("buildAndInstall() should only be called once.");
        }
        // 创建RefWatcher对象
        RefWatcher refWatcher = build();
        if (refWatcher != DISABLED) {
            if (enableDisplayLeakActivity) {
                LeakCanaryInternals.setEnabledAsync(context, DisplayLeakActivity.class, true);
            }
            if (watchActivities) {
                // 监听Activity的引用
                ActivityRefWatcher.install(context, refWatcher);
            }
            if (watchFragments) {
                   // 监听Fragment的引用
                FragmentRefWatcher.Helper.install(context, refWatcher);
            }
        }
        LeakCanaryInternals.installedRefWatcher = refWatcher;
        return refWatcher;
    }

  // ...
}

监听Activity的引用

ActivityRefWatcher

public final class ActivityRefWatcher {

    public static void install(@NonNull Context context, @NonNull RefWatcher refWatcher) {
        Application application = (Application) context.getApplicationContext();
        ActivityRefWatcher activityRefWatcher = new ActivityRefWatcher(application, refWatcher);

        application.registerActivityLifecycleCallbacks(activityRefWatcher.lifecycleCallbacks);
    }

    private final Application.ActivityLifecycleCallbacks lifecycleCallbacks = new ActivityLifecycleCallbacksAdapter() {

        @Override
        public void onActivityDestroyed(Activity activity) {
            // 在Activity执行完onDestroyed方法时，调用RefWatcher的watch来监控该Activity是否泄露
            refWatcher.watch(activity);
        }
    };
    // ...
}

检查引用

public final class RefWatcher {

    public static final RefWatcher DISABLED = new RefWatcherBuilder<>().build();

    // 线程控制器，在 onDestroy() 之后并且主线程空闲时执行内存泄漏检测
    private final WatchExecutor watchExecutor;
    // 判断是否处于调试模式，调试模式中不会进行内存泄漏检测，因为在调试过程中可能会保留上一个引用从而导致错误信息上报。
    private final DebuggerControl debuggerControl;
    // 用于主动触发GC操作
    private final GcTrigger gcTrigger;
    // 堆信息转储者，dump 内存泄漏处的 heap 信息到 hprof 文件
    private final HeapDumper heapDumper;
    private final HeapDump.Listener heapdumpListener;
    private final HeapDump.Builder heapDumpBuilder;
    // 保存每个被检测对象所对应的唯一key
    private final Set retainedKeys;
    // 引用队列，和WeakReference配合使用，当弱引用所引用的对象被GC回收，该弱引用就会被加入到这个队列
    private final ReferenceQueue queue;

    public void watch(Object watchedReference) {
        watch(watchedReference, "");
    }

    public void watch(Object watchedReference, String referenceName) {
        if (this == DISABLED) {
            return;
        }
        checkNotNull(watchedReference, "watchedReference");
        checkNotNull(referenceName, "referenceName");
        final long watchStartNanoTime = System.nanoTime();
        // 为被检测对象生成唯一的key值，并保存到retainedKeys
        String key = UUID.randomUUID().toString();
        retainedKeys.add(key);
        // 创建被检测对象的弱引用，并传入该对象的key
        final KeyedWeakReference reference = new KeyedWeakReference(watchedReference, key, referenceName, queue);

        // 异步检测这个对象是否被回收
        ensureGoneAsync(watchStartNanoTime, reference);
    }


    private void ensureGoneAsync(final long watchStartNanoTime, final KeyedWeakReference reference) {
        watchExecutor.execute(new Retryable() {
            @Override public Retryable.Result run() {
                return ensureGone(reference, watchStartNanoTime);
            }
        });
    }

    @SuppressWarnings("ReferenceEquality") // Explicitly checking for named null.
    Retryable.Result ensureGone(final KeyedWeakReference reference, final long watchStartNanoTime) {
        long gcStartNanoTime = System.nanoTime();
        long watchDurationMs = NANOSECONDS.toMillis(gcStartNanoTime - watchStartNanoTime);

        // 移除对象已经被回收的弱引用
        removeWeaklyReachableReferences();

        // 调试模式检测不准确
        if (debuggerControl.isDebuggerAttached()) {
            // The debugger can create false leaks.
            return RETRY;
        }
        // 判断引用是否存在，不存在，表示被对象被回收
        if (gone(reference)) {
            return DONE;
        }
        // 触发GC
        gcTrigger.runGc();
        // GC后再移除对象已经被回收的弱引用
        removeWeaklyReachableReferences();
        // 如果该引用还存在，就表示对象已经泄露
        if (!gone(reference)) {
            long startDumpHeap = System.nanoTime();
            long gcDurationMs = NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);

            // dump出heap的内存快照
            File heapDumpFile = heapDumper.dumpHeap();
            if (heapDumpFile == RETRY_LATER) {
                // Could not dump the heap.
                return RETRY;
            }
            long heapDumpDurationMs = NANOSECONDS.toMillis(System.nanoTime() - startDumpHeap);
            // 构建HeapDump对象
            HeapDump heapDump = heapDumpBuilder.heapDumpFile(heapDumpFile).referenceKey(reference.key)
                                               .referenceName(reference.name)
                                               .watchDurationMs(watchDurationMs)
                                               .gcDurationMs(gcDurationMs)
                                               .heapDumpDurationMs(heapDumpDurationMs)
                                               .build();
            // 分析HeapDump对象
            heapdumpListener.analyze(heapDump);
        }
        return DONE;
    }

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

    private void removeWeaklyReachableReferences() {
        KeyedWeakReference ref;
        // 当弱引用所引用的对象被回收，就会把该引用放到queue中，所以可以通过queue来判断对象是否被回收
        while ((ref = (KeyedWeakReference) queue.poll()) != null) {
            retainedKeys.remove(ref.key);
        }
    }
    // ...
}

Dump Heap

AndroidHeapDumper是HeapDumper的实现类。

public final class AndroidHeapDumper implements HeapDumper {

    @Override @Nullable
    public File dumpHeap() {
        File heapDumpFile = leakDirectoryProvider.newHeapDumpFile();

        if (heapDumpFile == RETRY_LATER) {
            return RETRY_LATER;
        }

        // ...

        try {
            // 生成.hprof文件
            Debug.dumpHprofData(heapDumpFile.getAbsolutePath());
            cancelToast(toast);
            notificationManager.cancel(notificationId);
            return heapDumpFile;
        } catch (Exception e) {
            CanaryLog.d(e, "Could not dump heap");
            // Abort heap dump
            return RETRY_LATER;
        }
    }

    // ...
}

解析hprof

public final class ServiceHeapDumpListener implements HeapDump.Listener {
     // ...

    @Override
    public void analyze(@NonNull HeapDump heapDump) {
        checkNotNull(heapDump, "heapDump");
        // 启动HeapAnalyzerServiceService来分析heapDump
        HeapAnalyzerService.runAnalysis(context, heapDump, listenerServiceClass);
    }
}

public final class HeapAnalyzerService extends ForegroundService implements AnalyzerProgressListener {
    // ...

    public static void runAnalysis(Context context, HeapDump heapDump,  Class extends AbstractAnalysisResultService> listenerServiceClass) {
        setEnabledBlocking(context, HeapAnalyzerService.class, true);
        setEnabledBlocking(context, listenerServiceClass, true);
        Intent intent = new Intent(context, HeapAnalyzerService.class);
        intent.putExtra(LISTENER_CLASS_EXTRA, listenerServiceClass.getName());
        intent.putExtra(HEAPDUMP_EXTRA, heapDump);
        ContextCompat.startForegroundService(context, intent);
    }

    @Override
    protected void onHandleIntentInForeground(@Nullable Intent intent) {
        if (intent == null) {
            CanaryLog.d("HeapAnalyzerService received a null intent, ignoring.");
            return;
        }
        String listenerClassName = intent.getStringExtra(LISTENER_CLASS_EXTRA);
        HeapDump heapDump = (HeapDump) intent.getSerializableExtra(HEAPDUMP_EXTRA);

        HeapAnalyzer heapAnalyzer = new HeapAnalyzer(heapDump.excludedRefs, this, heapDump.reachabilityInspectorClasses);

        // 分析内存泄露的地方
        AnalysisResult result = heapAnalyzer.checkForLeak(heapDump.heapDumpFile, heapDump.referenceKey, heapDump.computeRetainedHeapSize);
        // 发送内存泄露检测结果的通知
        AbstractAnalysisResultService.sendResultToListener(this, listenerClassName, heapDump, result);
    }
}

public final class HeapAnalyzer {
    // ...

    public @NonNull AnalysisResult checkForLeak(@NonNull File heapDumpFile, @NonNull String referenceKey, boolean computeRetainedSize) {
        long analysisStartNanoTime = System.nanoTime();

        if (!heapDumpFile.exists()) {
            Exception exception = new IllegalArgumentException("File does not exist: " + heapDumpFile);
            return failure(exception, since(analysisStartNanoTime));
        }

        try {
            listener.onProgressUpdate(READING_HEAP_DUMP_FILE);
            // 使用haha库解析.hprof文件
            HprofBuffer buffer = new MemoryMappedFileBuffer(heapDumpFile);
            HprofParser parser = new HprofParser(buffer);
            listener.onProgressUpdate(PARSING_HEAP_DUMP);
               // 解析.hprof文件生成对应的快照对象
            Snapshot snapshot = parser.parse();
            listener.onProgressUpdate(DEDUPLICATING_GC_ROOTS);
            // 删除gcRoots中重复的根对象RootObj
            deduplicateGcRoots(snapshot);
            listener.onProgressUpdate(FINDING_LEAKING_REF);
            // 检查对象是否泄露
            Instance leakingRef = findLeakingReference(referenceKey, snapshot);

            // leakingRef为空表示对象没有泄露
            if (leakingRef == null) {
                String className = leakingRef.getClassObj().getClassName();
                return noLeak(className, since(analysisStartNanoTime));
            }
            // 查找引用链
            return findLeakTrace(analysisStartNanoTime, snapshot, leakingRef, computeRetainedSize);
        } catch (Throwable e) {
            return failure(e, since(analysisStartNanoTime));
        }
    }
}

定位泄露的引用

public final class HeapAnalyzer {
    // ...

    private Instance findLeakingReference(String key, Snapshot snapshot) {
        ClassObj refClass = snapshot.findClass(KeyedWeakReference.class.getName());
        if (refClass == null) {
            throw new IllegalStateException(
                    "Could not find the " + KeyedWeakReference.class.getName() + " class in the heap dump.");
        }
        List keysFound = new ArrayList<>();
        for (Instance instance : refClass.getInstancesList()) {
            List values = classInstanceValues(instance);
            Object keyFieldValue = fieldValue(values, "key");
            if (keyFieldValue == null) {
                keysFound.add(null);
                continue;
            }
            String keyCandidate = asString(keyFieldValue);
            if (keyCandidate.equals(key)) {
                return fieldValue(values, "referent");
            }
            keysFound.add(keyCandidate);
        }
        throw new IllegalStateException("Could not find weak reference with key " + key + " in " + keysFound);
    }
}

建立引用链

public final class HeapAnalyzer {
    // ...

    private AnalysisResult findLeakTrace(long analysisStartNanoTime, Snapshot snapshot, Instance leakingRef, boolean computeRetainedSize) {

        listener.onProgressUpdate(FINDING_SHORTEST_PATH);
        // 查找到GC Roots的最短引用路径
        ShortestPathFinder pathFinder = new ShortestPathFinder(excludedRefs);
        ShortestPathFinder.Result result = pathFinder.findPath(snapshot, leakingRef);

        String className = leakingRef.getClassObj().getClassName();

        // False alarm, no strong reference path to GC Roots.
        if (result.leakingNode == null) {
            return noLeak(className, since(analysisStartNanoTime));
        }

        listener.onProgressUpdate(BUILDING_LEAK_TRACE);
        // 构建泄露的引用链
        LeakTrace leakTrace = buildLeakTrace(result.leakingNode);

        long retainedSize;
        if (computeRetainedSize) {

            listener.onProgressUpdate(COMPUTING_DOMINATORS);
            // 计算内存泄露的大小
            snapshot.computeDominators();

            Instance leakingInstance = result.leakingNode.instance;

            retainedSize = leakingInstance.getTotalRetainedSize();

            // TODO: check O sources and see what happened to android.graphics.Bitmap.mBuffer
            if (SDK_INT <= N_MR1) {
                listener.onProgressUpdate(COMPUTING_BITMAP_SIZE);
                retainedSize += computeIgnoredBitmapRetainedSize(snapshot, leakingInstance);
            }
        } else {
            retainedSize = AnalysisResult.RETAINED_HEAP_SKIPPED;
        }

        return leakDetected(result.excludingKnownLeaks, className, leakTrace, retainedSize, since(analysisStartNanoTime));
    }

}

展示分析结果

public class DisplayLeakService extends AbstractAnalysisResultService {
    // ...

    @Override
    protected final void onHeapAnalyzed(@NonNull AnalyzedHeap analyzedHeap) {
        HeapDump heapDump = analyzedHeap.heapDump;
        AnalysisResult result = analyzedHeap.result;

        String leakInfo = leakInfo(this, heapDump, result, true);
        CanaryLog.d("%s", leakInfo);

        heapDump = renameHeapdump(heapDump);
        boolean resultSaved = saveResult(heapDump, result);

        String contentTitle;
        if (resultSaved) {
            PendingIntent pendingIntent = DisplayLeakActivity.createPendingIntent(this, heapDump.referenceKey);
            if (result.failure != null) {
                contentTitle = getString(R.string.leak_canary_analysis_failed);
            } else {
                String className = classSimpleName(result.className);
                // ...
            }
            String contentText = getString(R.string.leak_canary_notification_message);
            showNotification(pendingIntent, contentTitle, contentText);
        } else {
            onAnalysisResultFailure(getString(R.string.leak_canary_could_not_save_text));
        }

        afterDefaultHandling(heapDump, result, leakInfo);
    }

    @Override
    protected final void onAnalysisResultFailure(String failureMessage) {
        super.onAnalysisResultFailure(failureMessage);
        String failureTitle = getString(R.string.leak_canary_result_failure_title);
        showNotification(null, failureTitle, failureMessage);
    }
}

总结

参考链接

  1. [带你读懂 Reference 和 ReferenceQueue]https://blog.csdn.net/gdutxiaoxu/article/details/80738581
  2. [一步步拆解 LeakCanary]https://blog.csdn.net/gdutxiaoxu/article/details/80752876
  3. [深入理解Leakcanary源码]https://jsonchao.github.io/2019/01/06/Android%E4%B8%BB%E6%B5%81%E4%B8%89%E6%96%B9%E5%BA%93%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90%EF%BC%88%E5%85%AD%E3%80%81%E6%B7%B1%E5%85%A5%E7%90%86%E8%A7%A3Leakcanary%E6%BA%90%E7%A0%81%EF%BC%89/
  4. [LeakCanary中文使用说明]https://www.liaohuqiu.net/cn/posts/leak-canary-read-me/
  5. [LeakCanary:让内存泄漏无所遁形]https://www.liaohuqiu.net/cn/posts/leak-canary/
  6. [深入理解 Android 之 LeakCanary 源码解析]https://allenwu.itscoder.com/leakcanary-source
  7. [Customizing LeakCanary]https://github.com/square/leakcanary/wiki/Customizing-LeakCanary

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