android实现倒计时，需连接SQLite数据库，可自定义倒计时，并使用广播接收器实现倒计时结束提示的完整代码及布局xml文件

布局文件：

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/white">

    <TextView
        android:id="@+id/tv_countdown"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:textSize="50sp"
        android:textColor="@android:color/black"
        android:text="00:00:00" />

    <Button
        android:id="@+id/btn_set"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tv_countdown"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="20dp"
        android:text="设置倒计时" />

</RelativeLayout>

代码文件：

public class MainActivity extends AppCompatActivity {

    private TextView tvCountdown;
    private Button btnSet;

    private CountDownTimer countDownTimer;
    private long timeLeftInMillis;

    private SQLiteDatabase database;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvCountdown = findViewById(R.id.tv_countdown);
        btnSet = findViewById(R.id.btn_set);

        // 创建或打开数据库
        database = SQLiteDatabase.openOrCreateDatabase(getFilesDir().getPath() + "/countdown.db", null);

        // 创建倒计时表
        database.execSQL("CREATE TABLE IF NOT EXISTS countdown (id INTEGER PRIMARY KEY AUTOINCREMENT, time INTEGER)");

        // 设置按钮点击事件
        btnSet.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                showTimePickerDialog();
            }
        });
    }

    // 显示时间选择器对话框
    private void showTimePickerDialog() {
        TimePickerDialog timePickerDialog = new TimePickerDialog(MainActivity.this, new TimePickerDialog.OnTimeSetListener() {
            @Override
            public void onTimeSet(TimePicker view, int hourOfDay, int minute) {
                startCountdown(hourOfDay, minute);
            }
        }, 0, 0, true);
        timePickerDialog.show();
    }

    // 开始倒计时
    private void startCountdown(int hour, int minute) {
        // 计算剩余时间的毫秒数
        timeLeftInMillis = (hour * 60 + minute) * 60 * 1000;

        // 存储倒计时到数据库中
        ContentValues values = new ContentValues();
        values.put("time", timeLeftInMillis);
        database.insert("countdown", null, values);

        // 开始倒计时
        countDownTimer = new CountDownTimer(timeLeftInMillis, 1000) {
            @Override
            public void onTick(long millisUntilFinished) {
                timeLeftInMillis = millisUntilFinished;
                updateCountdownText();
            }

            @Override
            public void onFinish() {
                showCountdownFinishedNotification();
                deleteCountdownFromDatabase();
            }
        }.start();
    }

    // 更新倒计时文本
    private void updateCountdownText() {
        int hours = (int) (timeLeftInMillis / 1000 / 3600);
        int minutes = (int) ((timeLeftInMillis / 1000) % 3600 / 60);
        int seconds = (int) (timeLeftInMillis / 1000 % 60);

        tvCountdown.setText(String.format("%02d:%02d:%02d", hours, minutes, seconds));
    }

    // 显示倒计时结束通知
    private void showCountdownFinishedNotification() {
        Intent intent = new Intent(MainActivity.this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(MainActivity.this, 0, intent, 0);
        NotificationCompat.Builder builder = new NotificationCompat.Builder(MainActivity.this, "default")
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentTitle("倒计时结束")
                .setContentText("您设置的倒计时已经结束")
                .setContentIntent(pendingIntent)
                .setAutoCancel(true);

        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(MainActivity.this);
        notificationManager.notify(1, builder.build());
    }

    // 从数据库中删除倒计时
    private void deleteCountdownFromDatabase() {
        database.delete("countdown", null, null);
    }

    @Override
    protected void onResume() {
        super.onResume();

        // 从数据库中取出倒计时
        Cursor cursor = database.rawQuery("SELECT * FROM countdown", null);
        if (cursor.moveToFirst()) {
            timeLeftInMillis = cursor.getLong(cursor.getColumnIndex("time"));
            countDownTimer = new CountDownTimer(timeLeftInMillis, 1000) {
                @Override
                public void onTick(long millisUntilFinished) {
                    timeLeftInMillis = millisUntilFinished;
                    updateCountdownText();
                }

                @Override
                public void onFinish() {
                    showCountdownFinishedNotification();
                    deleteCountdownFromDatabase();
                }
            }.start();
        }
    }

    @Override
    protected void onPause() {
        super.onPause();

        // 保存当前倒计时到数据库中
        ContentValues values = new ContentValues();
        values.put("time", timeLeftInMillis);
        database.update("countdown", values, null, null);

        // 取消倒计时
        if (countDownTimer != null) {
            countDownTimer.cancel();
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        // 关闭数据库
        database.close();
    }
}