Java 从其他类（扩展广播接收器）调用MainActivity方法

我试图从其他类（PhoneStateReceiver）调用MainActivity的display\u通知方法

但出现以下错误：

java.lang.RuntimeException:无法启动receiver com.weekendproject.roshu.spamblocker.PhoneStateReceiver:java.lang.NullPointerException:尝试在空对象引用上调用虚拟方法“java.lang.String android.content.Context.getPackageName（）”

在MainActivity中，错误特别指向了此行

Intent resultent=新的Intent（getApplicationContext（），MainActivity.class）

但是，我尝试将

getApplicationContext（）

更改为

“this”

和

“getActivity（）”

，但它不起作用

这是完整的代码

MainActivity.java

public class MainActivity extends ActionBarActivity {

    public void display_notification(String incoming_number) {

    NotificationCompat.Builder mBuilder =
            new NotificationCompat.Builder(getApplicationContext())
                    .setSmallIcon(R.drawable.ic_action)
                    .setContentTitle("SpamBlocker alert !!")
                    .setContentText("SpamBlocker blocked number : " + incoming_number);

    // **Error occured in following line**
    Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class);

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext());
    stackBuilder.addParentStack(MainActivity.class);
    stackBuilder.addNextIntent(resultIntent);
    PendingIntent resultPendingIntent =
            stackBuilder.getPendingIntent(
                    0,
                    PendingIntent.FLAG_UPDATE_CURRENT
            );
    mBuilder.setContentIntent(resultPendingIntent);
    NotificationManager mNotificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    Random r = new Random();
    int rand = r.nextInt(1000);
    mNotificationManager.notify(rand, mBuilder.build());
    }
}

public class PhoneStateReceiver extends BroadcastReceiver {

    MainActivity mActivity;

    @Override
    public void onReceive(Context context, Intent intent) {
        mActivity = new MainActivity();
        mActivity.display_notification(incomingNumber);           // call main activity methods

    }
}

PhoneStateReceiver.java

public class MainActivity extends ActionBarActivity {

    public void display_notification(String incoming_number) {

    NotificationCompat.Builder mBuilder =
            new NotificationCompat.Builder(getApplicationContext())
                    .setSmallIcon(R.drawable.ic_action)
                    .setContentTitle("SpamBlocker alert !!")
                    .setContentText("SpamBlocker blocked number : " + incoming_number);

    // **Error occured in following line**
    Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class);

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext());
    stackBuilder.addParentStack(MainActivity.class);
    stackBuilder.addNextIntent(resultIntent);
    PendingIntent resultPendingIntent =
            stackBuilder.getPendingIntent(
                    0,
                    PendingIntent.FLAG_UPDATE_CURRENT
            );
    mBuilder.setContentIntent(resultPendingIntent);
    NotificationManager mNotificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    Random r = new Random();
    int rand = r.nextInt(1000);
    mNotificationManager.notify(rand, mBuilder.build());
    }
}

public class PhoneStateReceiver extends BroadcastReceiver {

    MainActivity mActivity;

    @Override
    public void onReceive(Context context, Intent intent) {
        mActivity = new MainActivity();
        mActivity.display_notification(incomingNumber);           // call main activity methods

    }
}

---

您没有使用onReceive提供的上下文。尝试传递该上下文以显示_通知，而不是使用在接收广播时可能未运行的活动上下文

因此将方法签名更改为：

 public void display_notification(String incoming_number, Context context) 

并且，按如下方式从onReceive传递上下文：

 mActivity.display_notification(incomingNumber, context);

 Intent resultIntent = new Intent(context, MainActivity.class);

并在将其传递给Intent时使用此上下文，如下所示：

 mActivity.display_notification(incomingNumber, context);

 Intent resultIntent = new Intent(context, MainActivity.class);

已编辑

将整个方法放在接收器中并使用onReceive提供的上下文

或者

将该行更改为：

NotificationManager mNotificationManager =
        (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);

重要的部分是，context.getSystemService，其中context来自onReceive。

使用以下命令：

    Intent i = new Intent();
 i.setClassName("com.test", "com.test.MainActivity");
i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
    context.startActivity(i);

---

我认为新建活动对象不是一个好主意，否则生命周期将无法正确维护，其字段可能为空，这可能会导致NPE。
我建议您将通知生成器移动到一个util类中，详细代码可能与@dhun的答案类似。

---

顺便说一下，将这个util类的util方法设置为静态并不是一个好的实践，因为这样会使代码难以进行单元测试。更多详细信息，请参见。

您正在向后执行

活动

和

广播接收器

。接收者应该是您活动的成员，反之亦然。另外，您不应该像这样显式地调用

newmainActivity（）

。Android在内部完成了这项工作，并在通话中包含了一些与系统和应用程序相关的信息（比如软件包名称！）。请尝试以下方法：

MainActivity.java

public class MainActivity extends ActionBarActivity {

    public void display_notification(String incoming_number) {

    NotificationCompat.Builder mBuilder =
            new NotificationCompat.Builder(getApplicationContext())
                    .setSmallIcon(R.drawable.ic_action)
                    .setContentTitle("SpamBlocker alert !!")
                    .setContentText("SpamBlocker blocked number : " + incoming_number);

    // **Error occured in following line**
    Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class);

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext());
    stackBuilder.addParentStack(MainActivity.class);
    stackBuilder.addNextIntent(resultIntent);
    PendingIntent resultPendingIntent =
            stackBuilder.getPendingIntent(
                    0,
                    PendingIntent.FLAG_UPDATE_CURRENT
            );
    mBuilder.setContentIntent(resultPendingIntent);
    NotificationManager mNotificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    Random r = new Random();
    int rand = r.nextInt(1000);
    mNotificationManager.notify(rand, mBuilder.build());
    }
}

public class PhoneStateReceiver extends BroadcastReceiver {

    MainActivity mActivity;

    @Override
    public void onReceive(Context context, Intent intent) {
        mActivity = new MainActivity();
        mActivity.display_notification(incomingNumber);           // call main activity methods

    }
}

PhoneStateReceiver.java

public class MainActivity extends ActionBarActivity {

    public void display_notification(String incoming_number) {

    NotificationCompat.Builder mBuilder =
            new NotificationCompat.Builder(getApplicationContext())
                    .setSmallIcon(R.drawable.ic_action)
                    .setContentTitle("SpamBlocker alert !!")
                    .setContentText("SpamBlocker blocked number : " + incoming_number);

    // **Error occured in following line**
    Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class);

    TaskStackBuilder stackBuilder = TaskStackBuilder.create(getApplicationContext());
    stackBuilder.addParentStack(MainActivity.class);
    stackBuilder.addNextIntent(resultIntent);
    PendingIntent resultPendingIntent =
            stackBuilder.getPendingIntent(
                    0,
                    PendingIntent.FLAG_UPDATE_CURRENT
            );
    mBuilder.setContentIntent(resultPendingIntent);
    NotificationManager mNotificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
    Random r = new Random();
    int rand = r.nextInt(1000);
    mNotificationManager.notify(rand, mBuilder.build());
    }
}

public class PhoneStateReceiver extends BroadcastReceiver {

    MainActivity mActivity;

    @Override
    public void onReceive(Context context, Intent intent) {
        mActivity = new MainActivity();
        mActivity.display_notification(incomingNumber);           // call main activity methods

    }
}

---

此外，我不确定发送广播的应用程序中发生了什么，但我感觉这可能仍然不起作用。如果仍然出现错误，请发布两个应用程序的完整代码。

谢谢您的回答。我修改了它。但是，我得到了错误“java.lang.IllegalStateException:System services不可用于onCreate（）之前的活动”此错误发生在“NotificationManager mNotificationManager=（NotificationManager）getSystemService（Context.NOTIFICATION_SERVICE）；”这一行中，因此最终必须将该方法，在广播接收器中显示_通知，因为尚未创建活动且正在使用系统服务。查看我编辑的答案。谢谢你的回答。我是android应用程序开发的初学者。你能展示一些关于“如何将其放入util类”的链接吗