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
- public class SampleActivity extends Activity {
- /**
- * Instances of static inner classes do not hold an implicit
- * reference to their outer class.
- */
- private static class MyHandler extends Handler {
- private final WeakReference<SampleActivity> mActivity;
- public MyHandler(SampleActivity activity) {
- mActivity = new WeakReference<SampleActivity>(activity);
- }
- public void handleMessage(Message msg) {
- SampleActivity activity = mActivity.get();
- if (activity != null) {
- // ...
- }
- }
- }
- private final MyHandler mHandler = new MyHandler(this);
- /**
- * Instances of anonymous classes do not hold an implicit
- * reference to their outer class when they are "static".
- */
- private static final Runnable sRunnable = new Runnable() {
- public void run() { /* ... */ }
- };
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- // Post a message and delay its execution for 10 minutes.
- mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
- // Go back to the previous Activity.
- finish();
- }
- }
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
- public class SampleActivity extends Activity {
- private ImageView iv;
- /**
- * Instances of static inner classes do not hold an implicit
- * reference to their outer class.
- */
- private static class MyHandler extends Handler {
- private final WeakReference<SampleActivity> mActivity;
- public MyHandler(SampleActivity activity) {
- mActivity = new WeakReference<SampleActivity>(activity);
- }
- public void handleMessage(Message msg) {
- SampleActivity activity = mActivity.get();
- Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了 activity!=null:" + (activity != null));
- if (activity != null) {
- // ...
- }
- }
- }
- private final MyHandler mHandler = new MyHandler(this);
- /**
- * Instances of anonymous classes do not hold an implicit
- * reference to their outer class when they are "static".
- */
- // private static final Runnable sRunnable = new Runnable() {
- // @Override
- // public void run() { /* ... */ }
- // };
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- //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
- iv = new ImageView(this);
- iv.setImageResource(R.drawable.demo);
- // Post a message and delay its execution for 10 minutes.
- //mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
- mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
- // Go back to the previous Activity.
- finish();
- }
- }
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。
- public class SampleTwoActivity extends Activity {
- private ImageView iv;
- /**
- * Instances of static inner classes do not hold an implicit
- * reference to their outer class.
- */
- private static class MyHandler extends Handler {
- // private final WeakReference<SampleTwoActivity> mActivity;
- //
- // public MyHandler(SampleTwoActivity activity) {
- // mActivity = new WeakReference<SampleTwoActivity>(activity);
- // }
- //
- // @Override
- // public void handleMessage(Message msg) {
- // SampleTwoActivity activity = mActivity.get();
- // Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了 activity!=null:" + (activity != null));
- // if (activity != null) {
- // // ...
- // }
- // }
- }
- private final MyHandler mHandler = new MyHandler();
- /**
- * Instances of anonymous classes do not hold an implicit
- * reference to their outer class when they are "static".
- */
- private static class MyRunnable implements Runnable {
- private final WeakReference<SampleTwoActivity> mActivity;
- public MyRunnable(SampleTwoActivity activity) {
- mActivity = new WeakReference<SampleTwoActivity>(activity);
- }
- public void run() {
- SampleTwoActivity activity = mActivity.get();
- Log.e("YAO", "MyRunnable - run ------ 执行延时Runnable activity!=null:" + (activity != null));
- if (activity != null) {
- activity.iv.setVisibility(View.VISIBLE);
- }
- }
- }
- private final MyRunnable mMyRunnable = new MyRunnable(this);
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- //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
- iv = new ImageView(this);
- iv.setImageResource(R.drawable.demo);
- // Post a message and delay its execution for 10 minutes.
- mHandler.postDelayed(mMyRunnable, 1000L * 30);
- //mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
- // Go back to the previous Activity.
- finish();
- }
- }
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
- public class SampleFourActivity extends Activity {
- private ImageView iv;
- private class MyHandler extends Handler {
- public void handleMessage(Message msg) {
- Log.e("YAO", "MyHandler - handleMessage ------ 消息到达了");
- iv.setVisibility(View.VISIBLE);
- }
- }
- private final MyHandler mHandler = new MyHandler();
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- //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
- iv = new ImageView(this);
- iv.setImageResource(R.drawable.demo);
- // Post a message and delay its execution for 10 minutes.
- //mHandler.postDelayed(sRunnable, 1000 * 60 * 10);
- mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
- // Go back to the previous Activity.
- finish();
- }
- protected void onDestroy() {
- super.onDestroy();
- mHandler.removeCallbacksAndMessages(null);
- }
- }
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。
- public class SampleThreeActivity extends Activity {
- private ImageView iv;
- /**
- * Instances of static inner classes do not hold an implicit
- * reference to their outer class.
- */
- private static class MyHandler extends Handler {
- }
- private final MyHandler mHandler = new MyHandler();
- /**
- * Instances of anonymous classes do not hold an implicit
- * reference to their outer class when they are "static".
- */
- private static class MyRunnable implements Runnable {
- private final WeakReference<SampleThreeActivity> mActivity;
- public MyRunnable(SampleThreeActivity activity) {
- mActivity = new WeakReference<SampleThreeActivity>(activity);
- }
- public void run() {
- Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
- SystemClock.sleep(1000L * 30);
- SampleThreeActivity activity = mActivity.get();
- Log.e("YAO", "MyRunnable - run ------ 执行延时Runnable activity!=null:" + (activity != null));
- if (activity != null) {
- activity.mHandler.obtainMessage().sendToTarget();
- }
- }
- }
- private final MyRunnable mMyRunnable = new MyRunnable(this);
- protected void onCreate(Bundle savedInstanceState) {
- super.onCreate(savedInstanceState);
- //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
- iv = new ImageView(this);
- iv.setImageResource(R.drawable.demo);
- // Post a message and delay its execution for 10 minutes.
- //mHandler.postDelayed(mMyRunnable, 1000 * 30);
- //mHandler.sendEmptyMessageDelayed(0, 1000L * 30);
- new Thread(mMyRunnable).start();
- // Go back to the previous Activity.
- finish();
- }
- }
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。
- public void run() {
- SampleThreeActivity activity = mActivity.get();
- if (activity != null) {
- //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。
- String url = activity.url;
- Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
- SystemClock.sleep(1000L * 30);
- activity.mHandler.obtainMessage().sendToTarget();
- }
- }

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。)
- static class MyAsyncTask extends AsyncTask<Void, Void, Void> {
- private final WeakReference<SampleFiveActivity> mActivity;
- public MyAsyncTask(SampleFiveActivity activity) {
- mActivity = new WeakReference<SampleFiveActivity>(activity);
- }
- protected void onPreExecute() {
- SampleFiveActivity activity = mActivity.get();
- Log.e("YAO", "MyAsyncTask - onPreExecute ------ activity!=null:" + (activity != null));
- if (activity != null) {
- activity.iv.setVisibility(View.VISIBLE);
- }
- }
- protected Void doInBackground(Void... params) {
- Log.e("YAO", "TestMemoryActivity.java - run() ---------- 工作线程正在执行耗时任务....");
- SystemClock.sleep(1000L * 30);
- return null;
- }
- protected void onPostExecute(Void aVoid) {
- SampleFiveActivity activity = mActivity.get();
- Log.e("YAO", "MyAsyncTask - onPostExecute ------ activity!=null:" + (activity != null));
- if (activity != null) {
- activity.iv.setVisibility(View.VISIBLE);
- }
- }
- }
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。