要使用FirebaseMessagingService作为Android 14及更高版本中的前台服务类型，您可以按照以下步骤进行操作：

1. 首先，确保您的应用程序已添加Firebase依赖项。在您的项目级build.gradle文件中，添加以下代码：

dependencies {
    // ...
    implementation 'com.google.firebase:firebase-messaging:23.0.0'
    // ...
}

2. 创建一个继承自FirebaseMessagingService的类。在这个类中，您需要实现onMessageReceived方法来处理接收到的推送通知。以下是一个示例：

import android.content.Intent;
import android.util.Log;

import androidx.core.app.NotificationCompat;
import androidx.core.app.NotificationManagerCompat;

import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMessagingService";

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        // 处理接收到的推送通知
        Log.d(TAG, "From: " + remoteMessage.getFrom());
        if (remoteMessage.getNotification() != null) {
            Log.d(TAG, "Notification Message Body: " + remoteMessage.getNotification().getBody());
            // 在这里创建和显示通知
            showNotification(remoteMessage.getNotification().getTitle(), remoteMessage.getNotification().getBody());
        }
    }

    private void showNotification(String title, String message) {
        // 创建通知
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "channel_id")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentTitle(title)
                .setContentText(message)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT);

        // 显示通知
        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);
        notificationManager.notify(0, builder.build());
    }
}

3. 在AndroidManifest.xml文件中注册您的FirebaseMessagingService类。添加以下代码：

4. 确保您的应用程序在前台运行时，将FirebaseMessagingService设置为前台服务类型。在您的Activity或Service中，添加以下代码：

import android.app.Notification;
import android.app.NotificationChannel;
import android.app.NotificationManager;
import android.content.Context;
import android.os.Build;
import android.os.Bundle;

import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.NotificationCompat;

public class MainActivity extends AppCompatActivity {

    private static final int NOTIFICATION_ID = 1;
    private static final String CHANNEL_ID = "channel_id";
    private static final String CHANNEL_NAME = "channel_name";

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

        // 创建通知渠道（仅适用于Android O及更高版本）
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager notificationManager = getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(channel);
        }

        // 将FirebaseMessagingService设置为前台服务类型
        Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID)
                .setContentTitle("Foreground Service")
                .setContentText("Firebase Messaging Service")
                .setSmallIcon(R.drawable.ic_notification)
                .build();

        startForeground(NOTIFICATION_ID, notification);
    }

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

        // 停止前台服务
        stopForeground(true);
    }
}

通过按照上述步骤操作，您可以在Android 14及更高版本中使用FirebaseMessagingService作为前台服务类型来处理推送通知。