public class ForegroundService extends Service {
    public ForegroundService() {
    }
    @Override
    public void onCreate() {
        super.onCreate();
        // 将该服务转为前台服务
        // 需要设置 ID 和 通知
        // 设置 ID 为 0 , 就不显示已通知了 , 但是 oom_adj 值会变成后台进程 11
        // 设置 ID 为 1 , 会在通知栏显示该前台服务
        startForeground(1, new Notification());
    }
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

2021-04-08 19:36:34.736 23830-23830/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: kim.hsl.keep_progress_alive, PID: 23830
    android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=null pri=0 contentView=null vibrate=null sound=null defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1745)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:193)
        at android.app.ActivityThread.main(ActivityThread.java:6718)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)

package kim.hsl.keep_progress_alive.foreground_service;
import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.graphics.Color;
import android.os.Build;
import android.os.IBinder;
import androidx.annotation.RequiresApi;
import androidx.core.app.NotificationCompat;
import kim.hsl.keep_progress_alive.R;
import static androidx.core.app.NotificationCompat.PRIORITY_MIN;
public class ForegroundService extends Service {
    public ForegroundService() {
    }
    @Override
    public void onCreate() {
        super.onCreate();
        // 将该服务转为前台服务
        // 需要设置 ID 和 通知
        // 设置 ID 为 0 , 就不显示已通知了 , 但是 oom_adj 值会变成后台进程 11
        // 设置 ID 为 1 , 会在通知栏显示该前台服务
        //startForeground(1, new Notification());
        startForeground();
    }
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
    /**
     * 启动前台服务
     */
    private void startForeground() {
        String channelId = null;
        // 8.0 以上需要特殊处理
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            channelId = createNotificationChannel("kim.hsl", "ForegroundService");
        } else {
            channelId = "";
        }
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, channelId);
        Notification notification = builder.setOngoing(true)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setPriority(PRIORITY_MIN)
                .setCategory(Notification.CATEGORY_SERVICE)
                .build();
        startForeground(1, notification);
    }
    /**
     * 创建通知通道
     * @param channelId
     * @param channelName
     * @return
     */
    @RequiresApi(Build.VERSION_CODES.O)
    private String createNotificationChannel(String channelId, String channelName){
        NotificationChannel chan = new NotificationChannel(channelId,
                channelName, NotificationManager.IMPORTANCE_NONE);
        chan.setLightColor(Color.BLUE);
        chan.setLockscreenVisibility(Notification.VISIBILITY_PRIVATE);
        NotificationManager service = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        service.createNotificationChannel(chan);
        return channelId;
    }
}