LeakCanary原理解析

上篇文章Leakcanary的使用演示了LeakCanary的使用，这篇文章分析下，LeakCanary(基于1.6.1版本)的原理。在使用LeakCanary时，需要在自定义的Application中，调用LeakCanary.install(this);下面看看LeakCanary类的install方法的具体实现：

#LeakCanary.java
public final class LeakCanary {
    public static RefWatcher install(Application application) {
        return ((AndroidRefWatcherBuilder)refWatcher(application)// 创建AndroidRefWatcherBuilder对象
	.listenerServiceClass(DisplayLeakService.class)//配置监听结果的DisplayLeakService
	.excludedRefs(AndroidExcludedRefs.createAppDefaults().build()))// 配置排除的系统泄露
	.buildAndInstall();//返回一个RefWatcher对象，使用这个对象监听Activity的引用
    }
    ...
}

下面看看AndroidRefWatcherBuilder类的buildAndInstall()方法的具体实现：

#AndroidRefWatcherBuilder.java
public final class AndroidRefWatcherBuilder extends RefWatcherBuilder {
    private static final long DEFAULT_WATCH_DELAY_MILLIS;
    private final Context context;
    private boolean watchActivities = true;
    private boolean watchFragments = true;

    ...

    public RefWatcher buildAndInstall() {
        if (LeakCanaryInternals.installedRefWatcher != null) {
            throw new UnsupportedOperationException("buildAndInstall() should only be called once.");
        } else {
            RefWatcher refWatcher = this.build();
            if (refWatcher != RefWatcher.DISABLED) {
                if (this.watchActivities) {
		    //关键代码,这里是监听Activity的引用
                    ActivityRefWatcher.install(this.context, refWatcher);
                }

                if (this.watchFragments) {
		    //关键代码，这里是监听Fragment的引用
                    Helper.install(this.context, refWatcher);
                }
            }

            LeakCanaryInternals.installedRefWatcher = refWatcher;
            return refWatcher;
        }
    }
   ...
}

下面看看监听Activity的引用的逻辑,ActivityRefWatcher类的install方法的具体实现：

#ActivityRefWatcher.java
public final class ActivityRefWatcher {
    private final ActivityLifecycleCallbacks lifecycleCallbacks = new ActivityLifecycleCallbacksAdapter() {
        public void onActivityDestroyed(Activity activity) {
	    //关键代码，这里通过监听到Activity的onDestroy方法使，调用RefWatcher的watch方法来监听该Activity是否泄漏
            ActivityRefWatcher.this.refWatcher.watch(activity);
        }
    };
    ...
    public static void install(Context context, RefWatcher refWatcher) {
        Application application = (Application)context.getApplicationContext();
        ActivityRefWatcher activityRefWatcher = new ActivityRefWatcher(application, refWatcher);
	//监听Activity的生命周期
        application.registerActivityLifecycleCallbacks(activityRefWatcher.lifecycleCallbacks);
    }
    ...
}

这个方法内部，通过Application的registerActivityLifecycleCallbacks方法来监听Activity的生命周期，当Activity调用onDestroy方法时，调用RefWatcher类的watch方法来检测该Activity是否内存泄漏。下面看看RefWatcher类的watch方法的具体实现：

#RefWatcher.java
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 Listener heapdumpListener;
    private final 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