JobScheduler与JobService的使用

一.JobService是什么?
Entry point for the callback from the {@link android.app.job.JobScheduler}.

This is the base class that handles asynchronous requests that were previously scheduled. You are responsible for overriding {@link JobService#onStartJob(JobParameters)}, which is where you will implement your job logic.

This service executes each incoming job on a {@link android.os.Handler} running on your application's main thread. This means that you must offload your execution logic to another thread/handler/{@link android.os.AsyncTask} of your choosing. Not doing so will result in blocking any future callbacks from the JobManager - specifically {@link #onStopJob(android.app.job.JobParameters)}, which is meant to inform you that the scheduling requirements are no longer being met.

public abstract class JobService extends Service {
    private static final String TAG = "JobService";

    /**
     * Job services must be protected with this permission:
     *
     * 
     *     <service android:name="MyJobService"
     *              android:permission="android.permission.BIND_JOB_SERVICE" >
     *         ...
     *     </service>
     * 
     *
     * 
If a job service is declared in the manifest but not protected with this
     * permission, that service will be ignored by the system.
     */
    public static final String PERMISSION_BIND =
            "android.permission.BIND_JOB_SERVICE";

    private JobServiceEngine mEngine;

    /** @hide */
    public final IBinder onBind(Intent intent) {
        if (mEngine == null) {
            mEngine = new JobServiceEngine(this) {
                @Override
                public boolean onStartJob(JobParameters params) {
                    return JobService.this.onStartJob(params);
                }

                @Override
                public boolean onStopJob(JobParameters params) {
                    return JobService.this.onStopJob(params);
                }
            };
        }
        return mEngine.getBinder();
    }

    /**
     * Call this to inform the JobScheduler that the job has finished its work.  When the
     * system receives this message, it releases the wakelock being held for the job.
     * 

     * You can request that the job be scheduled again by passing {@code true} as
     * the wantsReschedule parameter. This will apply back-off policy
     * for the job; this policy can be adjusted through the
     * {@link android.app.job.JobInfo.Builder#setBackoffCriteria(long, int)} method
     * when the job is originally scheduled.  The job's initial
     * requirements are preserved when jobs are rescheduled, regardless of backed-off
     * policy.
     * 

     * A job running while the device is dozing will not be rescheduled with the normal back-off
     * policy.  Instead, the job will be re-added to the queue and executed again during
     * a future idle maintenance window.
     * 
     *
     * @param params The parameters identifying this job, as supplied to
     *               the job in the {@link #onStartJob(JobParameters)} callback.
     * @param wantsReschedule {@code true} if this job should be rescheduled according
     *     to the back-off criteria specified when it was first scheduled; {@code false}
     *     otherwise.
     */
    public final void jobFinished(JobParameters params, boolean wantsReschedule) {
        mEngine.jobFinished(params, wantsReschedule);
    }

    /**
     * Called to indicate that the job has begun executing.  Override this method with the
     * logic for your job.  Like all other component lifecycle callbacks, this method executes
     * on your application's main thread.
     * 

     * Return {@code true} from this method if your job needs to continue running.  If you
     * do this, the job remains active until you call
     * {@link #jobFinished(JobParameters, boolean)} to tell the system that it has completed
     * its work, or until the job's required constraints are no longer satisfied.  For
     * example, if the job was scheduled using
     * {@link JobInfo.Builder#setRequiresCharging(boolean) setRequiresCharging(true)},
     * it will be immediately halted by the system if the user unplugs the device from power,
     * the job's {@link #onStopJob(JobParameters)} callback will be invoked, and the app
     * will be expected to shut down all ongoing work connected with that job.
     * 

     * The system holds a wakelock on behalf of your app as long as your job is executing.
     * This wakelock is acquired before this method is invoked, and is not released until either
     * you call {@link #jobFinished(JobParameters, boolean)}, or after the system invokes
     * {@link #onStopJob(JobParameters)} to notify your job that it is being shut down
     * prematurely.
     * 

     * Returning {@code false} from this method means your job is already finished.  The
     * system's wakelock for the job will be released, and {@link #onStopJob(JobParameters)}
     * will not be invoked.
     *
     * @param params Parameters specifying info about this job, including the optional
     *     extras configured with {@link JobInfo.Builder#setExtras(android.os.PersistableBundle).
     *     This object serves to identify this specific running job instance when calling
     *     {@link #jobFinished(JobParameters, boolean)}.
     * @return {@code true} if your service will continue running, using a separate thread
     *     when appropriate.  {@code false} means that this job has completed its work.
     */
    public abstract boolean onStartJob(JobParameters params);

    /**
     * This method is called if the system has determined that you must stop execution of your job
     * even before you've had a chance to call {@link #jobFinished(JobParameters, boolean)}.
     *
     * 
This will happen if the requirements specified at schedule time are no longer met. For
     * example you may have requested WiFi with
     * {@link android.app.job.JobInfo.Builder#setRequiredNetworkType(int)}, yet while your
     * job was executing the user toggled WiFi. Another example is if you had specified
     * {@link android.app.job.JobInfo.Builder#setRequiresDeviceIdle(boolean)}, and the phone left its
     * idle maintenance window. You are solely responsible for the behavior of your application
     * upon receipt of this message; your app will likely start to misbehave if you ignore it.
     * 

     * Once this method returns, the system releases the wakelock that it is holding on
     * behalf of the job.
     *
     * @param params The parameters identifying this job, as supplied to
     *               the job in the {@link #onStartJob(JobParameters)} callback.
     * @return {@code true} to indicate to the JobManager whether you'd like to reschedule
     * this job based on the retry criteria provided at job creation-time; or {@code false}
     * to end the job entirely.  Regardless of the value returned, your job must stop executing.
     */
    public abstract boolean onStopJob(JobParameters params);
}

当手机灭屏状态下保持一段时间后，系统会进入休眠，一些后台运行的任务就可能得不到正常执行，比如网络下载中断，后台播放音乐暂停等。WakeLock正是为了解决这类问题，应用只要申请了WakeLock，那么在释放WakeLock之前，系统不会进入休眠，即使在灭屏的状态下，应用要执行的任务依旧不会被系统打断。

二.参考
1.https://developer.android.com/reference/android/app/job/JobService
1+.Android 功耗分析之wakelock
https://www.jianshu.com/p/67ccdac38271
2.理解JobScheduler机制
http://gityuan.com/2017/03/10/job_scheduler_service/

3.深入理解JobScheduler与JobService的使用
https://www.jianshu.com/p/1f2103d3d2a2