最近在使用android 4.1系统的时候，发现在手机休眠一段时间后（1-2小时），后台运行的服务被强行kill掉，有可能是系统回收内存的一种机制，要想避免这种情况可以通过startForeground让服务前台运行，当stopservice()的时候通过stopForeground()去掉。

Running a Service in the Foreground

A foreground service is a service that's considered to be something the user is actively aware of and thus not a candidate for the system to kill when low on memory. A foreground service must provide a notification for the status bar, which is placed under the "Ongoing" heading, which means that the notification cannot be dismissed unless the service is either stopped or removed from the foreground.

For example, a music player that plays music from a service should be set to run in the foreground, because the user is explicitly aware of its operation. The notification in the status bar might indicate the current song and allow the user to launch an activity to interact with the music player.

To request that your service run in the foreground, call startForeground(). This method takes two parameters: an integer that uniquely identifies the notification and the Notification for the status bar. For example:

Notification notification = new Notification(R.drawable.icon, getText(R.string.ticker_text),System.currentTimeMillis());
Intent notificationIntent = new Intent(this, ExampleActivity.class);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);
notification.setLatestEventInfo(this, getText(R.string.notification_title),getText(R.string.notification_message), pendingIntent);
startForeground(ONGOING_NOTIFICATION, notification);

To remove the service from the foreground, call stopForeground(). This method takes a boolean, indicating whether to remove the status bar notification as well. This method does not stop the service. However, if you stop the service while it's still running in the foreground, then the notification is also removed.

Note: The methods startForeground() and stopForeground() were introduced in Android 2.0 (API Level 5). In order to run your service in the foreground on older versions of the platform, you must use the previoussetForeground() method—see the startForeground() documentation for information about how to provide backward compatibility.

For more information about notifications, see Creating Status Bar Notifications.

要想实现需求，我们只需要在onStartCommand里面调用 startForeground，然后再onDestroy里面调用stopForeground即可！

实际情况就譬如手机里面的音乐播放器一样，不管手机如何休眠，只要开始播放音乐了，就不会kill掉这个服务，一旦停止播放音乐，服务就可能被清掉。

    /*** 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 {@link #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) {}}

    /*** Remove this service from foreground state, allowing it to be killed if* more memory is needed.* @param removeNotification If true, the notification previously provided* to {@link #startForeground} will be removed.  Otherwise it will remain* until a later call removes it (or the service is destroyed).* @see #startForeground(int, Notification)*/public final void stopForeground(boolean removeNotification) {try {mActivityManager.setServiceForeground(new ComponentName(this, mClassName), mToken, 0, null,removeNotification);} catch (RemoteException ex) {}}