我在我的代码中得到一个警告，声明:
这个AsyncTask类应该是静态的，否则可能会发生泄漏(匿名android.os.AsyncTask)
完整的警告是:
This AsyncTask class should be static or leaks might occur (anonymous android.os.AsyncTask)
A static field will leak contexts. Non-static inner classes have an implicit reference to their outer class. If that outer class is for example a Fragment or Activity, then this reference means that the long-running handler/loader/task will hold a reference to the activity which prevents it from getting garbage collected. Similarly, direct field references to activities and fragments from these longer running instances can cause leaks. ViewModel classes should never point to Views or non-application Contexts.
这是我的代码:
new AsyncTask<Void,Void,Void>(){
@Override
protected Void doInBackground(Void... params) {
runOnUiThread(new Runnable() {
@Override
public void run() {
mAdapter.notifyDataSetChanged();
}
});
return null;
}
}.execute();
我该如何纠正呢?
这个AsyncTask类应该是静态的，否则可能会发生泄漏，因为
当Activity被销毁时，AsyncTask(静态或非静态)仍在运行
如果内部类是非静态的(AsyncTask)类，它将引用外部类(Activity)。
如果一个对象没有引用指向它，垃圾收集将释放它。如果一个对象未使用且垃圾回收不能释放它=>泄漏内存
如果AsyncTask是非静态的，Activity不会释放事件，它被破坏=>泄漏
将AsyncTask作为静态类后更新UI的解决方案
1)使用WeakReference，比如@Suragch answer
2)发送和删除AsyncTask的Activity引用
public class NoLeakAsyncTaskActivity extends AppCompatActivity {
private ExampleAsyncTask asyncTask;
@Override
protected void onCreate(Bundle savedInstanceState) {
...
// START AsyncTask
asyncTask = new ExampleAsyncTask();
asyncTask.setListener(new ExampleAsyncTask.ExampleAsyncTaskListener() {
@Override
public void onExampleAsyncTaskFinished(Integer value) {
// update UI in Activity here
}
});
asyncTask.execute();
}
@Override
protected void onDestroy() {
asyncTask.setListener(null); // PREVENT LEAK AFTER ACTIVITY DESTROYED
super.onDestroy();
}
static class ExampleAsyncTask extends AsyncTask<Void, Void, Integer> {
private ExampleAsyncTaskListener listener;
@Override
protected Integer doInBackground(Void... voids) {
...
return null;
}
@Override
protected void onPostExecute(Integer value) {
super.onPostExecute(value);
if (listener != null) {
listener.onExampleAsyncTaskFinished(value);
}
}
public void setListener(ExampleAsyncTaskListener listener) {
this.listener = listener;
}
public interface ExampleAsyncTaskListener {
void onExampleAsyncTaskFinished(Integer value);
}
}
}
如何使用静态内部AsyncTask类
为了防止泄漏，可以将内部类设置为静态的。但是，这样做的问题是，您不再能够访问Activity的UI视图或成员变量。你可以传递一个Context的引用，但是你也会有内存泄漏的风险。(如果AsyncTask类有强引用，Android不能在Activity关闭后进行垃圾收集。)解决方案是对活动(或任何您需要的上下文)进行弱引用。
public class MyActivity extends AppCompatActivity {
int mSomeMemberVariable = 123;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// start the AsyncTask, passing the Activity context
// in to a custom constructor
new MyTask(this).execute();
}
private static class MyTask extends AsyncTask<Void, Void, String> {
private WeakReference<MyActivity> activityReference;
// only retain a weak reference to the activity
MyTask(MyActivity context) {
activityReference = new WeakReference<>(context);
}
@Override
protected String doInBackground(Void... params) {
// do some long running task...
return "task finished";
}
@Override
protected void onPostExecute(String result) {
// get a reference to the activity if it is still there
MyActivity activity = activityReference.get();
if (activity == null || activity.isFinishing()) return;
// modify the activity's UI
TextView textView = activity.findViewById(R.id.textview);
textView.setText(result);
// access Activity member variables
activity.mSomeMemberVariable = 321;
}
}
}
笔记
As far as I know, this type of memory leak danger has always been true, but I only started seeing the warning in Android Studio 3.0. A lot of the main AsyncTask tutorials out there still don't deal with it (see here, here, here, and here).
You would also follow a similar procedure if your AsyncTask were a top-level class. A static inner class is basically the same as a top-level class in Java.
If you don't need the Activity itself but still want the Context (for example, to display a Toast), you can pass in a reference to the app context. In this case the AsyncTask constructor would look like this:
private WeakReference<Application> appReference;
MyTask(Application context) {
appReference = new WeakReference<>(context);
}
There are some arguments out there for ignoring this warning and just using the non-static class. After all, the AsyncTask is intended to be very short lived (a couple seconds at the longest), and it will release its reference to the Activity when it finishes anyway. See this and this.
Excellent article: How to Leak a Context: Handlers & Inner Classes
科特林
在Kotlin中，不要包含内部类的inner关键字。这使它默认为静态。
class MyActivity : AppCompatActivity() {
internal var mSomeMemberVariable = 123
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
// start the AsyncTask, passing the Activity context
// in to a custom constructor
MyTask(this).execute()
}
private class MyTask
internal constructor(context: MyActivity) : AsyncTask<Void, Void, String>() {
private val activityReference: WeakReference<MyActivity> = WeakReference(context)
override fun doInBackground(vararg params: Void): String {
// do some long running task...
return "task finished"
}
override fun onPostExecute(result: String) {
// get a reference to the activity if it is still there
val activity = activityReference.get()
if (activity == null || activity.isFinishing) return
// modify the activity's UI
val textView = activity.findViewById(R.id.textview)
textView.setText(result)
// access Activity member variables
activity.mSomeMemberVariable = 321
}
}
}
这个AsyncTask类应该是静态的，否则可能会发生泄漏，因为
当Activity被销毁时，AsyncTask(静态或非静态)仍在运行
如果内部类是非静态的(AsyncTask)类，它将引用外部类(Activity)。
如果一个对象没有引用指向它，垃圾收集将释放它。如果一个对象未使用且垃圾回收不能释放它=>泄漏内存
如果AsyncTask是非静态的，Activity不会释放事件，它被破坏=>泄漏
将AsyncTask作为静态类后更新UI的解决方案
1)使用WeakReference，比如@Suragch answer
2)发送和删除AsyncTask的Activity引用
public class NoLeakAsyncTaskActivity extends AppCompatActivity {
private ExampleAsyncTask asyncTask;
@Override
protected void onCreate(Bundle savedInstanceState) {
...
// START AsyncTask
asyncTask = new ExampleAsyncTask();
asyncTask.setListener(new ExampleAsyncTask.ExampleAsyncTaskListener() {
@Override
public void onExampleAsyncTaskFinished(Integer value) {
// update UI in Activity here
}
});
asyncTask.execute();
}
@Override
protected void onDestroy() {
asyncTask.setListener(null); // PREVENT LEAK AFTER ACTIVITY DESTROYED
super.onDestroy();
}
static class ExampleAsyncTask extends AsyncTask<Void, Void, Integer> {
private ExampleAsyncTaskListener listener;
@Override
protected Integer doInBackground(Void... voids) {
...
return null;
}
@Override
protected void onPostExecute(Integer value) {
super.onPostExecute(value);
if (listener != null) {
listener.onExampleAsyncTaskFinished(value);
}
}
public void setListener(ExampleAsyncTaskListener listener) {
this.listener = listener;
}
public interface ExampleAsyncTaskListener {
void onExampleAsyncTaskFinished(Integer value);
}
}
}
