Handler还需要用到弱引用(WeakReference)吗?

本文探讨了Android中Handler类的正确使用方法以避免内存泄漏,包括如何使用静态内部类和弱引用,以及推荐使用removeCallbacksAndMessages方法。

转自:https://blog.youkuaiyun.com/alcoholdi/article/details/54948058  写的很好



网上很多文章都说写Hanlder,需要用static声明为静态的,还需要用弱引用包裹构造函数传来的Activity实例。

比如这篇英文博客

http://www.androiddesignpatterns.com/2013/01/inner-class-handler-memory-leak.html


里面的Sample是这样写的

  1. public class SampleActivity extends Activity {
  2. /**
  3. * Instances of static inner classes do not hold an implicit
  4. * reference to their outer class.
  5. */
  6. private static class MyHandler extends Handler {
  7. private final WeakReference<SampleActivity> mActivity;
  8. public MyHandler(SampleActivity activity) {
  9. mActivity = new WeakReference<SampleActivity>(activity);
  10. }
  11. @Override
  12. public void handleMessage(Message msg) {
  13. SampleActivity activity = mActivity.get();
  14. if (activity != null) {
  15. // ...
  16. }
  17. }
  18. }
  19. private final MyHandler mHandler = new MyHandler(this);
  20. /**
  21. * Instances of anonymous classes do not hold an implicit
  22. * reference to their outer class when they are "static".
  23. */
  24. private static final Runnable sRunnable = new Runnable() {
  25. @Override
  26. public void run() { /* ... */ }
  27. };
  28. @Override
  29. protected void onCreate(Bundle savedInstanceState) {
  30. super.onCreate(savedInstanceState);
  31. // Post a message and delay its execution for 10 minutes.
  32. mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
  33. // Go back to the previous Activity.
  34. finish();
  35. }
  36. }

post,postDelayed这种传Runnable的方法是不会触发handleMessage方法的。

所以用一下sendEmptyMessageDelayed测试一下

  1. public class SampleActivity extends Activity {
  2. private ImageView iv;
  3. /**
  4. * Instances of static inner classes do not hold an implicit
  5. * reference to their outer class.
  6. */
  7. private static class MyHandler extends Handler {
  8. private final WeakReference<SampleActivity> mActivity;
  9. public MyHandler(SampleActivity activity) {
  10. mActivity = new WeakReference<SampleActivity>(activity);
  11. }
  12. @Override
  13. public void handleMessage(Message msg) {
  14. SampleActivity activity = mActivity.get();
  15. Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了 activity!=null:" + (activity != null));
  16. if (activity != null) {
  17. // ...
  18. }
  19. }
  20. }
  21. private final MyHandler mHandler = new MyHandler(this);
  22. /**
  23. * Instances of anonymous classes do not hold an implicit
  24. * reference to their outer class when they are "static".
  25. */
  26. // private static final Runnable sRunnable = new Runnable() {
  27. // @Override
  28. // public void run() { /* ... */ }
  29. // };
  30. @Override
  31. protected void onCreate(Bundle savedInstanceState) {
  32. super.onCreate(savedInstanceState);
  33. //塞一张大一点的图片,用来增大Activity的所需内存,可以更好的查看Memory趋势图
  34. iv = new ImageView(this);
  35. iv.setImageResource(R.drawable.demo);
  36. // Post a message and delay its execution for 10 minutes.
  37. //mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
  38. mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
  39. // Go back to the previous Activity.
  40. finish();
  41. }
  42. }

打开这个Activity后会自动finish,然后点一下Android Monitor里面的Initiate GC按钮,触发GC操作。


此时Dump Java Heap看看内存情况


可以看到SampleActivity已经被回收了,什么都没有。


而里面的MyHandler还存在,这里还可以看到Handler只持有一个Message对象。

等了30秒消息到达后,日志打印出

02-11 15:22:15.033 7153-7153/com.yao.memorytest E/YAO: MyHandler - handleMessage ------ 消息到达了  activity!=null:false

说明SampleActivity已经被回收了。



重来一次,这次不点击Initiate GC按钮,Dump Java Heap后



对比前面两张图,可以看到SampleActivity还存在。从MyHandler看,这个实例除了有一个Message对象,还有一个SampleActivity对象(注意这个对象是红色的)。

等30秒结果打印出

02-11 15:24:15.033 7153-7153/com.yao.memorytest E/YAO: MyHandler - handleMessage ------ 消息到达了  activity!=null:true

说明关闭页面30秒后,弱引用Activity还在。



如果把static关键字去掉


由于非静态内部类会持有外部类的一个隐式引用。所以MyHandler持有一个当前SampleActivity对象实例(此时这个对象是白色的)。


可以看到,对于static内部类,用弱引用都会把SampleActivity标成红色。而不用static关键字,则是白色。

猜测一下,红色代表的是待回收内存,下次GC后会被回收。白色代表还在使用,GC后不会被回收的内存。



改写一下代码,现在改成使用postDelayed方法,此时按照之前得出的结论Runnable也必须是静态的。

如果Runnable里面有跟当前Activity相关的代码,也得加个弱引用Activity。

  1. public class SampleTwoActivity extends Activity {
  2. private ImageView iv;
  3. /**
  4. * Instances of static inner classes do not hold an implicit
  5. * reference to their outer class.
  6. */
  7. private static class MyHandler extends Handler {
  8. // private final WeakReference<SampleTwoActivity> mActivity;
  9. //
  10. // public MyHandler(SampleTwoActivity activity) {
  11. // mActivity = new WeakReference<SampleTwoActivity>(activity);
  12. // }
  13. //
  14. // @Override
  15. // public void handleMessage(Message msg) {
  16. // SampleTwoActivity activity = mActivity.get();
  17. // Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了 activity!=null:" + (activity != null));
  18. // if (activity != null) {
  19. // // ...
  20. // }
  21. // }
  22. }
  23. private final MyHandler mHandler = new MyHandler();
  24. /**
  25. * Instances of anonymous classes do not hold an implicit
  26. * reference to their outer class when they are "static".
  27. */
  28. private static class MyRunnable implements Runnable {
  29. private final WeakReference<SampleTwoActivity> mActivity;
  30. public MyRunnable(SampleTwoActivity activity) {
  31. mActivity = new WeakReference<SampleTwoActivity>(activity);
  32. }
  33. @Override
  34. public void run() {
  35. SampleTwoActivity activity = mActivity.get();
  36. Log.e("YAO", "MyRunnable - run ------ 执行延时Runnable activity!=null:" + (activity != null));
  37. if (activity != null) {
  38. activity.iv.setVisibility(View.VISIBLE);
  39. }
  40. }
  41. }
  42. private final MyRunnable mMyRunnable = new MyRunnable(this);
  43. @Override
  44. protected void onCreate(Bundle savedInstanceState) {
  45. super.onCreate(savedInstanceState);
  46. //塞一张大一点的图片,用来增大Activity的所需内存,可以更好的查看Memory趋势图
  47. iv = new ImageView(this);
  48. iv.setImageResource(R.drawable.demo);
  49. // Post a message and delay its execution for 10 minutes.
  50. mHandler.postDelayed(mMyRunnable, 1000L * 30);
  51. //mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
  52. // Go back to the previous Activity.
  53. finish();
  54. }
  55. }

Dump Java Heap看看


MyRunnable也持有一个Activity的弱引用,红色的。执行一下GC,Activity也意料之中的被回收了。

等30秒结果打印出

02-12 03:31:18.215 7501-7501/com.yao.memorytest E/YAO: MyRunnable - run ------ 执行延时Runnable  activity!=null:false



以上是发送用Handler发送延时消息,延时任务的情况。

可以看出用上静态内部类+弱引用Handler的确能解决内存泄露的问题,同时得注意Runnable也需要用上静态弱引用才行。



然而有个更好的方法removeCallbacksAndMessages

  1. public class SampleFourActivity extends Activity {
  2. private ImageView iv;
  3. private class MyHandler extends Handler {
  4. @Override
  5. public void handleMessage(Message msg) {
  6. Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了");
  7. iv.setVisibility(View.VISIBLE);
  8. }
  9. }
  10. private final MyHandler mHandler = new MyHandler();
  11. @Override
  12. protected void onCreate(Bundle savedInstanceState) {
  13. super.onCreate(savedInstanceState);
  14. //塞一张大一点的图片,用来增大Activity的所需内存,可以更好的查看Memory趋势图
  15. iv = new ImageView(this);
  16. iv.setImageResource(R.drawable.demo);
  17. // Post a message and delay its execution for 10 minutes.
  18. //mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
  19. mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
  20. // Go back to the previous Activity.
  21. finish();
  22. }
  23. @Override
  24. protected void onDestroy() {
  25. super.onDestroy();
  26. mHandler.removeCallbacksAndMessages(null);
  27. }
  28. }
只需要在onDestroy里面Handler.removeCallbacksAndMessages(null);,无论runnbale还是message消息全清空,自然也不会关联上Activity。下次GC就能顺利回收了。


发送长时间的延时消息/任务其实是少见,更多的是比如我们经常开线程联网访问或者开线程做一些耗时计算,结束后才通过Handler发送消息更新UI,应该是这样的。

  1. public class SampleThreeActivity extends Activity {
  2. private ImageView iv;
  3. /**
  4. * Instances of static inner classes do not hold an implicit
  5. * reference to their outer class.
  6. */
  7. private static class MyHandler extends Handler {
  8. }
  9. private final MyHandler mHandler = new MyHandler();
  10. /**
  11. * Instances of anonymous classes do not hold an implicit
  12. * reference to their outer class when they are "static".
  13. */
  14. private static class MyRunnable implements Runnable {
  15. private final WeakReference<SampleThreeActivity> mActivity;
  16. public MyRunnable(SampleThreeActivity activity) {
  17. mActivity = new WeakReference<SampleThreeActivity>(activity);
  18. }
  19. @Override
  20. public void run() {
  21. Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
  22. SystemClock.sleep(1000L * 30);
  23. SampleThreeActivity activity = mActivity.get();
  24. Log.e("YAO", "MyRunnable - run ------ 执行延时Runnable activity!=null:" + (activity != null));
  25. if (activity != null) {
  26. activity.mHandler.obtainMessage().sendToTarget();
  27. }
  28. }
  29. }
  30. private final MyRunnable mMyRunnable = new MyRunnable(this);
  31. @Override
  32. protected void onCreate(Bundle savedInstanceState) {
  33. super.onCreate(savedInstanceState);
  34. //塞一张大一点的图片,用来增大Activity的所需内存,可以更好的查看Memory趋势图
  35. iv = new ImageView(this);
  36. iv.setImageResource(R.drawable.demo);
  37. // Post a message and delay its execution for 10 minutes.
  38. //mHandler.postDelayed(mMyRunnable, 1000 * 30);
  39. //mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
  40. new Thread(mMyRunnable).start();
  41. // Go back to the previous Activity.
  42. finish();
  43. }
  44. }

Dump Java Heap


发现Activity是红色的可回收内存,没啥问题。

但是改一下run方法里面的代码。

  1. @Override
  2. public void run() {
  3. SampleThreeActivity activity = mActivity.get();
  4. if (activity != null) {
  5. //通过弱引用去获取Activity的成员变量参数(比如网址url),再跑耗时任务。
  6. String url = activity.url;
  7. Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
  8. SystemClock.sleep(1000L * 30);
  9. activity.mHandler.obtainMessage().sendToTarget();
  10. }
  11. }


所以用了弱引用的get方法后,相当于会把内存中的弱引用转为强引用。


所以如果是这种方式的话,建议退出这个Activity时取消这个任务。Thread是没有提供取消任务的方法的。可以用AsyncTask的cancel方法,ExecutorService的shutdown方法,当然一般网络框架volley、okhttp这些也会提供相应的取消请求方法。

(较真一点如果在使用AsyncTask时也改成静态内部类+弱引用当然也可以,但是非常麻烦,AsyncTask运行在UI线程的onPreExecute、onProgressUpdate、onPostExecute使用到Activity中的成员变量的话,都需要进行弱引用Activity的判空方法,相当麻烦。)

  1. static class MyAsyncTask extends AsyncTask<Void, Void, Void> {
  2. private final WeakReference<SampleFiveActivity> mActivity;
  3. public MyAsyncTask(SampleFiveActivity activity) {
  4. mActivity = new WeakReference<SampleFiveActivity>(activity);
  5. }
  6. @Override
  7. protected void onPreExecute() {
  8. SampleFiveActivity activity = mActivity.get();
  9. Log.e("YAO", "MyAsyncTask - onPreExecute ------ activity!=null:" + (activity != null));
  10. if (activity != null) {
  11. activity.iv.setVisibility(View.VISIBLE);
  12. }
  13. }
  14. @Override
  15. protected Void doInBackground(Void... params) {
  16. Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
  17. SystemClock.sleep(1000L * 30);
  18. return null;
  19. }
  20. @Override
  21. protected void onPostExecute(Void aVoid) {
  22. SampleFiveActivity activity = mActivity.get();
  23. Log.e("YAO", "MyAsyncTask - onPostExecute ------ activity!=null:" + (activity != null));
  24. if (activity != null) {
  25. activity.iv.setVisibility(View.VISIBLE);
  26. }
  27. }
  28. }



总结就是,以后我使用handler估计只会用Handler.removeCallbacksAndMessages(null);这种方法了,方便快捷。


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