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  /** * Bits returned by {@link #onStartCommand} describing how to continue
     * the service if it is killed.  May be {@link #START_STICKY},
     * {@link #START_NOT_STICKY}, {@link #START_REDELIVER_INTENT},
     * or {@link #START_STICKY_COMPATIBILITY}.
     */
    public static final int START_CONTINUATION_MASK = 0xf;
    
    /**
     * 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;

START_STICKY：如果service进程被kill掉，保留service的状态为开始状态，但不保留递送的intent对象。随后系统会尝试重新创建service，由于服务状态为开始状态，所以创建服务后一定会调用onStartCommand(Intent,int,int)方法。如果在此期间没有任何启动命令被传递到service，那么参数Intent将为null。

START_NOT_STICKY：“非粘性的”。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统不会自动重启该服务。

START_REDELIVER_INTENT：重传Intent。使用这个返回值时，如果在执行完onStartCommand后，服务被异常kill掉，系统会自动重启该服务，并将Intent的值传入。

START_STICKY_COMPATIBILITY：START_STICKY的兼容版本，但不保证服务被kill后一定能重启。