最近开发中,使用 AsyncTask + ProgressDialog 显示进度信息,但在AsyncTask停止,Activity finish 后该Activity的实例始终不会被gc,多次运行程序后,会存在多个activity,造成内存泄漏。 下面详细分析一下:
一份显示进度条的测试代码:
- publicclassMainextendsActivity{
-
-
- @Override
- protectedvoidonCreate(BundlesavedInstanceState){
- super.onCreate(savedInstanceState);
-
- TextViewtv=newTextView(this);
- tv.setText("InitState");
- setContentView(tv);
-
- tv.setOnClickListener(newOnClickListener(){
-
- @Override
- publicvoidonClick(Viewv){
- showProgress(Main.this);
- }
- });
- }
-
- publicvoidshowProgress(finalActivityactivity){
- newAsyncTask<Void,Void,Void>(){
- ProgressDialogprogressDial;
-
- protectedvoidonPreExecute(){
- progressDial=newProgressDialog(activity);
- progressDial.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
- progressDial.show();
- };
-
- @Override
- protectedVoiddoInBackground(Void...params){
- doSomeHeavyWork(progressDial);
- returnnull;
- }
-
- protectedvoidonPostExecute(Voidresult){
- progressDial.dismiss();
- };
-
- }.execute();
- }
-
- voiddoSomeHeavyWork(ProgressDialogprogress){
- try{
- for(inti=1;i<=10;++i){
- progress.setProgress(i);
- Thread.sleep(1000);
- }
- }catch(Exceptione){
- }
- }
- }
上述代码发生内存泄漏的地方在 doSomeHeavyWork() 的 progress.setProgress(i);部分;我们看一下setProgress()的实现,最终会调用ProgressBar 类的如下方法:
- privatesynchronizedvoidrefreshProgress(intid,intprogress,booleanfromUser){
- if(mUiThreadId==Thread.currentThread().getId()){
- doRefreshProgress(id,progress,fromUser,true);
- }else{
- RefreshProgressRunnabler;
- if(mRefreshProgressRunnable!=null){
-
- r=mRefreshProgressRunnable;
-
- mRefreshProgressRunnable=null;
- r.setup(id,progress,fromUser);
- }else{
-
- r=newRefreshProgressRunnable(id,progress,fromUser);
- }
- post(r);
- }
- }
- privateclassRefreshProgressRunnableimplementsRunnable{
-
- privateintmId;
- privateintmProgress;
- privatebooleanmFromUser;
-
- RefreshProgressRunnable(intid,intprogress,booleanfromUser){
- mId=id;
- mProgress=progress;
- mFromUser=fromUser;
- }
-
- publicvoidrun(){
- doRefreshProgress(mId,mProgress,mFromUser,true);
-
- mRefreshProgressRunnable=this;
- }
-
- publicvoidsetup(intid,intprogress,booleanfromUser){
- mId=id;
- mProgress=progress;
- mFromUser=fromUser;
- }
- }
if 语句表明当调用的该方法的线程是UI线程时,则直接执行doRefreshProgress() 方法以刷新界面;否则,创建一个RefreshProgressRunnable,并通过调用View.pos()方法将其插入到UI线程的消息队列中。 View.post()实现如下:
- publicbooleanpost(Runnableaction){
- Handlerhandler;
- AttachInfoattachInfo=mAttachInfo;
- if(attachInfo!=null){
- handler=attachInfo.mHandler;
- }else{
-
- ViewRootImpl.getRunQueue().post(action);
- returntrue;
- }
-
- returnhandler.post(action);
- }
在post() 函数注释中,明确写着:This method can be invoked from outside of the UI threadonly when this View is attached to a window.
当ProgressDialog还没有attach到当前window时(ProgressDialog.show() 方法是异步执行的),mAttachInfo 值为 null,故而执行 else语句,再看一下getRunQueue()和其post() 方法:
- staticfinalThreadLocal<RunQueue>sRunQueues=newThreadLocal<RunQueue>();
-
- staticRunQueuegetRunQueue(){
- RunQueuerq=sRunQueues.get();
- if(rq!=null){
- returnrq;
- }
- rq=newRunQueue();
- sRunQueues.set(rq);
- returnrq;
- }
- ……
- staticfinalclassRunQueue{
- privatefinalArrayList<HandlerAction>mActions=newArrayList<HandlerAction>();
-
- voidpost(Runnableaction){
- postDelayed(action,0);
- }
-
- voidpostDelayed(Runnableaction,longdelayMillis){
- HandlerActionhandlerAction=newHandlerAction();
- handlerAction.action=action;
- handlerAction.delay=delayMillis;
-
- synchronized(mActions){
- mActions.add(handlerAction);
- }
- }
-
- voidexecuteActions(Handlerhandler){
- synchronized(mActions){
- finalArrayList<handleraction>actions=mActions;
- finalintcount=actions.size();
-
- for(inti=0;i<count;i++){
- finalHandlerActionhandlerAction=actions.get(i);
- handler.postDelayed(handlerAction.action,handlerAction.delay);
- }
-
- actions.clear();
- }
- }
- ……
- }
- andleraction>
这样会把ProgressBar的RefreshProgressRunnable 插入到一个静态的ThreadLocal的RunQueue队列里,针对本文开头给出的例子,刷新进度的Runnable被插入到了AsyncTask 所在线程的RunQueue里; 那么插入的Runnable什么时候得到执行呢?
调用RunQueue.executeActions()方法只有一处,即在ViewRootImpl类的如下非静态方法中
- privatevoidperformTraversals(){
- ……
- if(mLayoutRequested&&!mStopped){
-
-
- getRunQueue().executeActions(attachInfo.mHandler);
- ……
- }
-
- ……
- }
该方法是在UI线程执行的(见ViewRootImpl.handleMessage()), 故当UI线程执行到该performTraversals() 里的 getRunQueue() 时,得到的是UI线程中的RunQueue,这样AsyncTask 线程中的 RunQueue永远不会被执行到, 并且AsyncTask的是用线程池实现的,AsyncTask启动的线程会长期存在,造成如下引用关系:
AsyncTask线程 => 静态的ThreadLocal的RunQueue => Runnable => ProgressBar => Activity;
如此即使activity finish 了,确始终存在一个静态引用链引用这该activity,而 Activity一般又引用着很多资源,比如图片等,最终造成严重资源泄漏。
另外,上述问题不限与ProgressBar,凡是在非UI线程使用view.post()方法,如果view没有被attach,则均存在潜在的内存泄漏的问题!
针对本文给出的ProgressBar例子,一个简单fix方法实在 AsyncTask的doInbackground() 开始处sleep(500) 即可。 更为精准的方式可使用如下循环测试:
- Viewv=progressBar.getWindow().getDecorView();
- while(v.getWindowToken()==null){
- Thread.sleep(10);
- }
上述ProgressBar例子,并不是总能再现内存泄漏的情况的(因为异步执行的不缺定性),下面再给出一个更容易再现类似问题的例子:
- publicclassMainextendsActivity{
-
- publicstaticinta=0;
-
- @Override
- protectedvoidonCreate(BundlesavedInstanceState){
- super.onCreate(savedInstanceState);
-
- TextViewtv=newTextView(this);
- tv.setText("InitState");
- setContentView(tv);
-
-
- if(a++>3){
- return;
- }
- newAsyncTask<TextView,Void,Void>(){
-
- @Override
- protectedVoiddoInBackground(TextView...params){
- try{
- TextViewtv=params[0];
-
- for(inti=1;i<=3;++i){
- doSomeHeavyWork(tv,"AsyncTask:"+i);
- Thread.sleep(1000);
- }
- }catch(Exceptione){
- }
- returnnull;
- }
-
- protectedvoidonPostExecute(Voidresult){
- recreate();
- };
-
- }.execute(tv);
- }
-
- voiddoSomeHeavyWork(finalTextViewtv,finalStringtext){
- tv.post(newRunnable(){
- @Override
- publicvoidrun(){
- tv.setText(text);
- }
- });
- }
现象是: TextView 很大概率不会显示 "AsyncTask: 1“ 文字; 而把 Thread.sleep(500)注释掉后,一切ok!
Log.e("Token:",tv.getWindowToken()+"");//输出的Token为null,这时候还没有和Activity绑定
Thread.sleep(500);
Log.e("Token:",tv.getWindowToken()+"");//输出的Token不为null,这时候已经和Activity绑定