如何提高Service的优先级避免被杀死或者杀死后如何再次重启Service?

2014-01-21 16:45:02

我们知道,当进程长期不活动时,如果系统资源吃紧,会杀死一些Service,或不可见的Activity等所在的进程。

如何避免Service被系统杀死,随便在网上搜一下,都能搜到好几种方法,但是每一种方法都有不同的适用环境。

1. 添加android:persistent="true"

添加android:persistent="true"到AndroidManifest.xml,Google文档描述如下:

Whether or not the application should remain running at all times-true" if it should, and "false"if not. The default value is "false". Applications should not normally set this flag; persistence mode is intended only for certain system applications.可见这个属性不能随便用,到目前为止,我所发现使用该属性的应用只有Phone,而且使用是要求权限的,所以这个属性对第三方应用来说意义不是很大;

2. 设置onStartCommand()的返回值

这个思路比较有用,我们着重分析一下,该方法有四种返回值:

START_STICKY

START_NOT_STICKY

START_REDELIVER_INTENT

START_STICKY_COMPATIBILITY

Google官方解释如下,有兴趣的可以展开看看:

    /**
* Constant to return from {@link #onStartCommand}: compatibility
* version of {@link #START_STICKY} that does not guarantee that
* {@link #onStartCommand} will be called again after being killed.
*/
public static final int START_STICKY_COMPATIBILITY = 0; /**
* Constant to return from {@link #onStartCommand}: if this service's
* process is killed while it is started (after returning from
* {@link #onStartCommand}), then leave it in the started state but
* don't retain this delivered intent. Later the system will try to
* re-create the service. Because it is in the started state, it will
* guarantee to call {@link #onStartCommand} after creating the new
* service instance; if there are not any pending start commands to be
* delivered to the service, it will be called with a null intent
* object, so you must take care to check for this.
*
* <p>This mode makes sense for things that will be explicitly started
* and stopped to run for arbitrary periods of time, such as a service
* performing background music playback.
*/
public static final int START_STICKY = 1; /**
* Constant to return from {@link #onStartCommand}: if this service's
* process is killed while it is started (after returning from
* {@link #onStartCommand}), and there are no new start intents to
* deliver to it, then take the service out of the started state and
* don't recreate until a future explicit call to
* {@link Context#startService Context.startService(Intent)}. The
* service will not receive a {@link #onStartCommand(Intent, int, int)}
* call with a null Intent because it will not be re-started if there
* are no pending Intents to deliver.
*
* <p>This mode makes sense for things that want to do some work as a
* result of being started, but can be stopped when under memory pressure
* and will explicit start themselves again later to do more work. An
* example of such a service would be one that polls for data from
* a server: it could schedule an alarm to poll every N minutes by having
* the alarm start its service. When its {@link #onStartCommand} is
* called from the alarm, it schedules a new alarm for N minutes later,
* and spawns a thread to do its networking. If its process is killed
* while doing that check, the service will not be restarted until the
* alarm goes off.
*/
public static final int START_NOT_STICKY = 2; /**
* Constant to return from {@link #onStartCommand}: if this service's
* process is killed while it is started (after returning from
* {@link #onStartCommand}), then it will be scheduled for a restart
* and the last delivered Intent re-delivered to it again via
* {@link #onStartCommand}. This Intent will remain scheduled for
* redelivery until the service calls {@link #stopSelf(int)} with the
* start ID provided to {@link #onStartCommand}. The
* service will not receive a {@link #onStartCommand(Intent, int, int)}
* call with a null Intent because it will will only be re-started if
* it is not finished processing all Intents sent to it (and any such
* pending events will be delivered at the point of restart).
*/
public static final int START_REDELIVER_INTENT = 3;

那么简单的说,四种模式的区别如下:

START_STICKY:kill后会被重启,但是重启后调用onStarfCommand()传进来的Intent参数为null,说明被kill的时候没有保存Intent;

START_STICKY_COMPATIBILITY:START_STICKY的兼容版,但是不能保证onStartCommand()方法被调用,如果应用程序的targetSdkVersion 小于 2.0版本,就会返回该值,否则返回START_STICKY,同时再次启动时只会调用onCreate(),不保证能调用onStartCommand()方法,代码如下:

 public int onStartCommand(Intent intent, int flags, int startId) {
onStart(intent, startId);
return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY;
}
=================================
mStartCompatibility = getApplicationInfo().targetSdkVersion < Build.VERSION_CODES.ECLAIR;
=================================
public static final int ECLAIR = 5; // 对应SDK2.0版本

START_NOT_STICKY:kill之后不会被重启;

START_REDELIVER_INTENT:kill后会被重启,同时重启调用onStartCommand()时再次传入保存的Intent。

启动一个service,然后在recent app里面杀死该进程,使用不同返回值时的log如下:

START_REDELIVER_INTENT

D/PlayerService(16907): onCreate------
D/PlayerService(16907): onStartCommand------and startId = 1
D/PlayerService(16907): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }
W/ActivityManager( 868): Scheduling restart of crashed service com.example.bitmapfun/.ui.PlayerService in 7776ms
I/ActivityManager( 868): Start proc com.example.bitmapfun for service com.example.bitmapfun/.ui.PlayerService: pid=17271 uid=10153 gids={50153, 1028}
D/PlayerService(17271): onCreate------
D/PlayerService(17271): onStartCommand------and startId = 1
D/PlayerService(17271): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }

被杀死的时候没有调用onDestory()方法,ActivityManager负责安排重启该service,此次重启大概需要7776ms,但这个时间不固定,有时很短,几秒,有时很长,可能要几十秒;

START_STICKY

D/PlayerService(17620): onCreate------
D/PlayerService(17620): onStartCommand------and startId = 1
D/PlayerService(17620): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }
W/ActivityManager( 868): Scheduling restart of crashed service com.example.bitmapfun/.ui.PlayerService in 5000ms
I/ActivityManager( 868): Start proc com.example.bitmapfun for service com.example.bitmapfun/.ui.PlayerService: pid=18003 uid=10153 gids={50153, 1028}
D/PlayerService(18003): onCreate------
D/PlayerService(18003): onStartCommand------and startId = 3
D/PlayerService(18003): onStartCommand------and intent = null

同上,不过传入的Intent为null,同时startId发生了变化,startId的官方解释是“A unique integer representing this specific request to start. Use with stopSelfResult(int)”,也就是说重启和第一次启动不是同一个request,也可以认为这是一个全新的request;

START_STICKY_COMPATIBILITY

D/PlayerService(18177): onCreate------
D/PlayerService(18177): onStartCommand------and startId = 1
D/PlayerService(18177): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }
W/ActivityManager( 868): Scheduling restart of crashed service com.example.bitmapfun/.ui.PlayerService in 5000ms
I/ActivityManager( 868): Start proc com.example.bitmapfun for service com.example.bitmapfun/.ui.PlayerService: pid=18578 uid=10153 gids={50153, 1028}
D/PlayerService(18578): onCreate------

这次重启根本就没有调用onStartCommand()方法;

START_NOT_STICKY

D/PlayerService(19436): onCreate------
D/PlayerService(19436): onStartCommand------and startId = 1
D/PlayerService(19436): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }
W/ActivityManager( 868): Scheduling restart of crashed service com.example.bitmapfun/.ui.PlayerService in 29285ms

没有再次启动被杀掉的service。

测试的代码很简单,大家可以自己尝试。现在有一个问题:我们该如何判断启动的service是正常启动还是杀死后被重启的,因为有时候我们需要知道这些信息,代码如下:

     private boolean isApplicationBroughtToBackground() {
ActivityManager am = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
List<RunningTaskInfo> tasks = am.getRunningTasks(1);
if (!tasks.isEmpty()) {
ComponentName topActivity = tasks.get(0).topActivity;
Log.d(TAG, "topActivity.getPackageName() = " + topActivity.getPackageName());
if (!topActivity.getPackageName().equals(getPackageName())) {
return true;
}
}
return false;
}

原理:service所在的activity和running task栈顶的activity做比较,因为一旦service所在的activity被杀死,那么系统会跳转到其他应用,如比桌面,或者SystemUI,或者用户可以打开的task栈中的其他TOP activity,此时的running task栈顶的activity肯定不是被杀死的activity了。

以上测试中所谓的杀死指的是在recent app里面或者Eclipse DDMS 点击Stop杀死,而不是在settings app info中“Force stop”,“Force stop”的log如下:

D/PlayerService(21779): onCreate------
D/PlayerService(21779): onStartCommand------and startId = 1
D/PlayerService(21779): onStartCommand------and intent = Intent { cmp=com.example.bitmapfun/.ui.PlayerService }
W/ActivityManager( 868): Scheduling restart of crashed service com.example.bitmapfun/.ui.PlayerService in 14898ms
I/ActivityManager( 868): Force stopping service ServiceRecord{419a3198 u0 com.example.bitmapfun/.ui.PlayerService}

可以发现,虽然安排了启动,但是很快就被Force Stop了,这样也就失去了被重启的机会,至于在Settings中杀死进程的原理,有机会咱们展开讲。

3. startForeground()提高service的进程等级

我们知道Android进程分为5个等级:foreground process, visible process, Service process, background process, empty process,当系统资源吃紧的时候,会按照进程等级从低到高的顺序,同时根据进程消耗的资源从多到少的原则来kill一些进程,而service正处于第三个等级,如果能够提高service所在进程的等级,那么它被杀死的概率就会小一些。

可以利用Service的startForeground()方法将Service的进程等级从第三级提升到第一级foreground process。源代码如下:

     /**
* Make this service run in the foreground, supplying the ongoing
* notification to be shown to the user while in this state.
* By default services are background, meaning that if the system needs to
* kill them to reclaim more memory (such as to display a large page in a
* web browser), they can be killed without too much harm. You can set this
* flag if killing your service would be disruptive to the user, such as
* if your service is performing background music playback, so the user
* would notice if their music stopped playing.
*
* <p>If you need your application to run on platform versions prior to API
* level 5, you can use the following model to call the the older setForeground()
* or this modern method as appropriate:
*
* {@sample development/samples/ApiDemos/src/com/example/android/apis/app/ForegroundService.java
* foreground_compatibility}
*
* @param id The identifier for this notification as per
* {@link NotificationManager#notify(int, Notification)
* NotificationManager.notify(int, Notification)}.
* @param notification The Notification to be displayed.
*
* @see #stopForeground(boolean)
*/
public final void startForeground(int id, Notification notification) {
try {
mActivityManager.setServiceForeground(
new ComponentName(this, mClassName), mToken, id,
notification, true);
} catch (RemoteException ex) {
}
}

至于使用嘛,可以在在onCreate()或者onStartComman()方法中调用,然后可以在onDestroy()或者其他地方调用stopForeground(boolean removeNotification)方法来stop。

关于进程等级可访问:http://blog.csdn.net/llbupt/article/details/7358360

当然啦,网上还有一些其他的避免Service被杀死或者kill后重启的方法,比如监听android.intent.action.USER_PRESENT,来启动service,或者提高service IntentFilter的priority等,都能算是一些在某些特殊情况下可以其作用的方法,倒也不妨尝试一下。

还有人说用AlarmManager,如下:

 public void onReceive(Context context, Intent mintent) {  

     if (Intent.ACTION_BOOT_COMPLETED.equals(mintent.getAction())) {
// 启动完成
Intent intent = new Intent(context, Alarmreceiver.class);
intent.setAction("arui.alarm.action");
PendingIntent sender = PendingIntent.getBroadcast(context, 0, intent, 0);
long firstime = SystemClock.elapsedRealtime();
AlarmManager am = (AlarmManager) context
.getSystemService(Context.ALARM_SERVICE); // 10秒一个周期,不停的发送广播
am.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, firstime,
10 * 1000, sender);
}
}

监听系统启动的broadcast,然后每10秒一个周期,不停的发广播,这就是说应用一旦启动,就会不断的发广播,个人觉得这种方式不靠谱,原因如下:

1. 这样做无谓的操作,会消耗系统资源;

2. 一旦APP进程被杀死,怎么保证你的receiver不被杀死?

3. 不停的启动service,加入service中启动了其他的线程在做耗时的操作,这样做会产生大量的线程做重复的操作,即便service中没有启动其他线程,不断的调用onStartCommand()方法都不算是一个好办法。

当然了,如果实在没办法,必须得使用这种solution的话,我们可以判断service是否是alive,至于方法百度一下就有了。

至于有人说在onDestroy()中重启service,上面打出来的log大家也看到了,被kill的时候都没机会去调用onDestroy()。

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