Android启动Activity报错：通知类调用getSystemService空指针解决

能否创建独立的通知处理类？ #

完全可以，这是推荐的解耦实践，能把所有通知相关逻辑集中到一个类里，避免重复代码，方便后续维护。

错误原因分析 #

你错误地让LocalNotifications继承了Application类，还在MainActivity里手动实例化它。Application的实例是Android系统在应用启动时自动创建并初始化的，手动new出来的Application对象没有完成系统层面的Context初始化，内部Context为空，调用getSystemService自然会抛出NullPointerException。

正确实现方案 #

方案一：普通工具类（推荐，灵活可控）

让LocalNotifications作为普通工具类，通过传入Context来获取系统服务：

public class LocalNotifications {
    // 创建通知渠道
    public static void createNotificationChannel(Context context) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            CharSequence channelName = "默认通知渠道";
            String channelDesc = "用于展示应用普通通知";
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            
            NotificationChannel channel = new NotificationChannel("DEFAULT_CHANNEL_ID", channelName, importance);
            channel.setDescription(channelDesc);
            
            NotificationManager notificationManager = context.getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(channel);
        }
    }

    // 发送通知示例
    public static void sendNotification(Context context, String title, String content) {
        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(context);
        Notification notification = new NotificationCompat.Builder(context, "DEFAULT_CHANNEL_ID")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentTitle(title)
                .setContentText(content)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT)
                .build();
        
        notificationManager.notify(1, notification);
    }
}

在Activity中调用：

// 创建渠道（建议在Application或主Activity onCreate中调用一次）
LocalNotifications.createNotificationChannel(this);
// 发送通知
LocalNotifications.sendNotification(this, "新消息", "这是一条测试通知");

方案二：基于全局Context的工具类

如果不想每次调用都传Context，可以通过自定义Application获取全局Context：

1. 自定义Application类：

public class MyApp extends Application {
    private static Context sGlobalContext;

    @Override
    public void onCreate() {
        super.onCreate();
        sGlobalContext = getApplicationContext();
    }

    public static Context getGlobalContext() {
        return sGlobalContext;
    }
}

2. 在AndroidManifest.xml中注册：

<application
    android:name=".MyApp"
    ...>
    <!-- 其他组件配置 -->
</application>

3. 改造LocalNotifications：

public class LocalNotifications {
    private static final Context sContext = MyApp.getGlobalContext();

    public static void createNotificationChannel() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            CharSequence channelName = "默认通知渠道";
            String channelDesc = "用于展示应用普通通知";
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            
            NotificationChannel channel = new NotificationChannel("DEFAULT_CHANNEL_ID", channelName, importance);
            channel.setDescription(channelDesc);
            
            NotificationManager notificationManager = sContext.getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(channel);
        }
    }

    public static void sendNotification(String title, String content) {
        NotificationManagerCompat notificationManager = NotificationManagerCompat.from(sContext);
        Notification notification = new NotificationCompat.Builder(sContext, "DEFAULT_CHANNEL_ID")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentTitle(title)
                .setContentText(content)
                .setPriority(NotificationCompat.PRIORITY_DEFAULT)
                .build();
        
        notificationManager.notify(1, notification);
    }
}

在Activity中调用：

LocalNotifications.createNotificationChannel();
LocalNotifications.sendNotification("新消息", "这是一条测试通知");

内容的提问来源于stack exchange，提问作者Sudo Sur