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:
- public class Main extends Activity {
-
-
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
-
- TextView tv = new TextView(this);
- tv.setText("Init State");
- setContentView(tv);
-
- tv.setOnClickListener(new OnClickListener() {
-
- @Override
- public void onClick(View v) {
- showProgress(Main.this);
- }
- });
- }
-
- public void showProgress(final Activity activity) {
- new AsyncTask<Void, Void, Void>() {
- ProgressDialog progressDial;
-
- protected void onPreExecute() {
- progressDial = new ProgressDialog(activity);
- progressDial.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
- progressDial.show();
- };
-
- @Override
- protected Void doInBackground(Void... params) {
- doSomeHeavyWork(progressDial);
- return null;
- }
-
- protected void onPostExecute(Void result) {
- progressDial.dismiss();
- };
-
- }.execute();
- }
-
- void doSomeHeavyWork(ProgressDialog progress) {
- try {
- for (int i = 1; i <= 10; ++i) {
- progress.setProgress(i);
- Thread.sleep(1000);
- }
- } catch (Exception e) {
- }
- }
- }
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:
- private synchronized void refreshProgress(int id, int progress, boolean fromUser) {
- if (mUiThreadId == Thread.currentThread().getId()) {
- doRefreshProgress(id, progress, fromUser, true);
- } else {
- RefreshProgressRunnable r;
- if (mRefreshProgressRunnable != null) {
-
- r = mRefreshProgressRunnable;
-
- mRefreshProgressRunnable = null;
- r.setup(id, progress, fromUser);
- } else {
-
- r = new RefreshProgressRunnable(id, progress, fromUser);
- }
- post(r);
- }
- }
- private class RefreshProgressRunnable implements Runnable {
-
- private int mId;
- private int mProgress;
- private boolean mFromUser;
-
- RefreshProgressRunnable(int id, int progress, boolean fromUser) {
- mId = id;
- mProgress = progress;
- mFromUser = fromUser;
- }
-
- public void run() {
- doRefreshProgress(mId, mProgress, mFromUser, true);
-
- mRefreshProgressRunnable = this;
- }
-
- public void setup(int id, int progress, boolean fromUser) {
- 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:
- public boolean post(Runnable action) {
- Handler handler;
- AttachInfo attachInfo = mAttachInfo;
- if (attachInfo != null) {
- handler = attachInfo.mHandler;
- } else {
-
- ViewRootImpl.getRunQueue().post(action);
- return true;
- }
-
- return handler.post(action);
- }
在post() 函数注释中,明确写着:This method can be invoked from outside of the UI thread only 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:
- static final ThreadLocal<RunQueue> sRunQueues = new ThreadLocal<RunQueue>();
-
- static RunQueue getRunQueue() {
- RunQueue rq = sRunQueues.get();
- if (rq != null) {
- return rq;
- }
- rq = new RunQueue();
- sRunQueues.set(rq);
- return rq;
- }
- ……
- static final class RunQueue {
- private final ArrayList<HandlerAction> mActions = new ArrayList<HandlerAction>();
-
- void post(Runnable action) {
- postDelayed(action, 0);
- }
-
- void postDelayed(Runnable action, long delayMillis) {
- HandlerAction handlerAction = new HandlerAction();
- handlerAction.action = action;
- handlerAction.delay = delayMillis;
-
- synchronized (mActions) {
- mActions.add(handlerAction);
- }
- }
-
- void executeActions(Handler handler) {
- synchronized (mActions) {
- final ArrayList<handleraction> actions = mActions;
- final int count = actions.size();
-
- for (int i = 0; i < count; i++) {
- final HandlerAction handlerAction = 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
- private void performTraversals() {
- ……
- 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:
- View v = 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:
- public class Main extends Activity {
-
- public static int a=0;
-
- @Override
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
-
- TextView tv = new TextView(this);
- tv.setText("Init State");
- setContentView(tv);
-
-
- if (a++> 3) {
- return ;
- }
- new AsyncTask<TextView, Void, Void>() {
-
- @Override
- protected Void doInBackground(TextView... params) {
- try {
- TextView tv = params[0];
-
- for (int i = 1; i <= 3; ++i) {
- doSomeHeavyWork(tv, "AsyncTask: " + i);
- Thread.sleep(1000);
- }
- } catch (Exception e) {
- }
- return null;
- }
-
- protected void onPostExecute(Void result) {
- recreate();
- };
-
- }.execute(tv);
- }
-
- void doSomeHeavyWork(final TextView tv, final String text) {
- tv.post(new Runnable() {
- @Override
- public void run() {
- 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!
附上MAT内存实例分析图:
![Android 非UI线程使用View.post()方法一处潜在的内存泄漏_第1张图片](http://img.e-com-net.com/image/info5/6072e500232e42c09f06a159a8299866.jpg)