《吊打面试官》：LeakCanary源码解析

LeakCanary.install(this)源码如下所示：

public static RefWatcher install(Application application) {
        return ((AndroidRefWatcherBuilder)refWatcher(application).listenerServiceClass(DisplayLeakService.class).excludedRefs(AndroidExcludedRefs.createAppDefaults().build())).buildAndInstall();
    }

listenerServiceClass(DisplayLeakService.class)：用于分析内存泄漏结果信息，然后发送通知给用户。
excludedRefs(AndroidExcludedRefs.createAppDefaults().build())：设置需要忽略的对象，比如某些系统漏洞不需要统计。
buildAndInstall()：真正检测内存泄漏的方法，下面将展开分析该方法。

public RefWatcher buildAndInstall() {
        RefWatcher refWatcher = this.build();
        if(refWatcher != RefWatcher.DISABLED) {
            LeakCanary.enableDisplayLeakActivity(this.context);
            ActivityRefWatcher.installOnIcsPlus((Application)this.context, refWatcher);
        }

        return refWatcher;
    }

可以看到，上面方法主要做了三件事情：
1.实例化RefWatcher对象，该对象主要作用是检测是否有对象未被回收导致内存泄漏；
2.设置APP图标可见；
3.检测内存


RefWatcher的使用后面讲，这边主要看第二件事情的处理过程，及enableDisplayLeakActivity方法的源码

public static void enableDisplayLeakActivity(Context context) {
        LeakCanaryInternals.setEnabled(context, DisplayLeakActivity.class, true);
    }

public static void setEnabled(Context context, final Class<?> componentClass, final boolean enabled) {
        final Context appContext = context.getApplicationContext();
        executeOnFileIoThread(new Runnable() {
            public void run() {
                LeakCanaryInternals.setEnabledBlocking(appContext, componentClass, enabled);
            }
        });
    }

public static void setEnabledBlocking(Context appContext, Class<?> componentClass, boolean enabled) {
        ComponentName component = new ComponentName(appContext, componentClass);
        PackageManager packageManager = appContext.getPackageManager();
        int newState = enabled?1:2;
        packageManager.setComponentEnabledSetting(component, newState, 1);
    }

可见，最后调用packageManager.setComponentEnabledSetting（）方法，实现应用图标的隐藏和显示。


接下来，进入真正的内存检查的方法installOnIcsPlus（）

public static void installOnIcsPlus(Application application, RefWatcher refWatcher) {
        if(VERSION.SDK_INT >= 14) {
            ActivityRefWatcher activityRefWatcher = new ActivityRefWatcher(application, refWatcher);
            activityRefWatcher.watchActivities();
        }
    }

该方法实例化出ActivityRefWatcher 对象，该对象用来监听activity的生命周期，具体实现如下所示：

public void watchActivities() {
        this.stopWatchingActivities();
        this.application.registerActivityLifecycleCallbacks(this.lifecycleCallbacks);
    }

private final ActivityLifecycleCallbacks lifecycleCallbacks = new ActivityLifecycleCallbacks() {
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        }

        public void onActivityStarted(Activity activity) {
        }

        public void onActivityResumed(Activity activity) {
        }

        public void onActivityPaused(Activity activity) {
        }

        public void onActivityStopped(Activity activity) {
        }

        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
        }

        public void onActivityDestroyed(Activity activity) {
            ActivityRefWatcher.this.onActivityDestroyed(activity);
        }
    };

调用了registerActivityLifecycleCallbacks方法后，当Activity执行onDestroy方法后，会触发ActivityLifecycleCallbacks 的onActivityDestroyed方法，在当前方法中，调用refWatcher的watch方法，前面已经讲过RefWatcher对象主要作用是检测是否有对象未被回收导致内存泄漏。下面继续看refWatcher的watch方法源码：

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

    public void watch(Object watchedReference, String referenceName) {
        if(this != DISABLED) {
            Preconditions.checkNotNull(watchedReference, "watchedReference");
            Preconditions.checkNotNull(referenceName, "referenceName");
            long watchStartNanoTime = System.nanoTime();
            String key = UUID.randomUUID().toString();
            this.retainedKeys.add(key);
            KeyedWeakReference reference = new KeyedWeakReference(watchedReference, key, referenceName, this.queue);
            this.ensureGoneAsync(watchStartNanoTime, reference);
        }
    }

可以看到，上面方法主要做了三件事情：
1.生成一个随机数key存放在retainedKeys集合中，用来判断对象是否被回收；
2.把当前Activity放到KeyedWeakReference（WeakReference的子类）中；
3.通过查找ReferenceQueue，看该Acitivity是否存在，存在则证明可以被正常回收，不存在则证明可能存在内存泄漏。
前两件事很简单，这边主要看第三件事情的处理过程，及ensureGoneAsync方法的源码：

private void ensureGoneAsync(final long watchStartNanoTime, final KeyedWeakReference reference) {
        this.watchExecutor.execute(new Retryable() {
            public Result run() {
                return RefWatcher.this.ensureGone(reference, watchStartNanoTime);
            }
        });
    }
    
  Result ensureGone(KeyedWeakReference reference, long watchStartNanoTime) {
        long gcStartNanoTime = System.nanoTime();
        long watchDurationMs = TimeUnit.NANOSECONDS.toMillis(gcStartNanoTime - watchStartNanoTime);
        this.removeWeaklyReachableReferences();
        if(this.debuggerControl.isDebuggerAttached()) {
            return Result.RETRY;
        } else if(this.gone(reference)) {
            return Result.DONE;
        } else {
            this.gcTrigger.runGc();
            this.removeWeaklyReachableReferences();
            if(!this.gone(reference)) {
                long startDumpHeap = System.nanoTime();
                long gcDurationMs = TimeUnit.NANOSECONDS.toMillis(startDumpHeap - gcStartNanoTime);
                File heapDumpFile = this.heapDumper.dumpHeap();
                if(heapDumpFile == HeapDumper.RETRY_LATER) {
                    return Result.RETRY;
                }

                long heapDumpDurationMs = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - startDumpHeap);
                this.heapdumpListener.analyze(new HeapDump(heapDumpFile, reference.key, reference.name, this.excludedRefs, watchDurationMs, gcDurationMs, heapDumpDurationMs));
            }

            return Result.DONE;
        }
    }

该方法中首先执行removeWeaklyReachableReferences()，从ReferenceQueue队列中查询是否存在该弱引用对象，如果不为空，则说明已经被系统回收了，则将对应的随机数key从retainedKeys集合中删除。

 private void removeWeaklyReachableReferences() {
        KeyedWeakReference ref;
        while((ref = (KeyedWeakReference)this.queue.poll()) != null) {
            this.retainedKeys.remove(ref.key);
        }
    }

然后通过判断retainedKeys集合中是否存在对应的key判断该对象是否被回收。

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

如果没有被系统回收，则手动调用gcTrigger.runGc();后再调用removeWeaklyReachableReferences方法判断该对象是否被回收。

GcTrigger DEFAULT = new GcTrigger() {
        public void runGc() {
            Runtime.getRuntime().gc();
            this.enqueueReferences();
            System.runFinalization();
        }

        private void enqueueReferences() {
            try {
                Thread.sleep(100L);
            } catch (InterruptedException var2) {
                throw new AssertionError();
            }
        }
    };

第三行代码为手动触发GC，紧接着线程睡100毫秒，给系统回收的时间，随后通过System.runFinalization()手动调用已经失去引用对象的finalize方法。
通过手动GC该对象还不能被回收的话，则存在内存泄漏，调用heapDumper.dumpHeap()生成.hprof文件目录，并通过heapdumpListener回调到analyze()方法，后面关于dump文件的分析这边就不介绍了，感兴趣的可以自行去看。

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如有错误欢迎指出来，一起学习。