Android系统如何实现每五分钟获取一次IP地址，如果IP地址发生变化则保存到本地文件当中，请写出具体代码

Android系统可以通过使用AlarmManager定时器来实现每五分钟获取一次IP地址，具体代码如下：

public class IPUpdateService extends Service {
    private static final long INTERVAL = 5 * 60 * 1000; // 5分钟
    private AlarmManager alarmManager;
    private PendingIntent pendingIntent;

    @Override
    public void onCreate() {
        super.onCreate();
        alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
        Intent intent = new Intent(this, IPUpdateReceiver.class);
        pendingIntent = PendingIntent.getBroadcast(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        alarmManager.setRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + INTERVAL, INTERVAL, pendingIntent);
        return super.onStartCommand(intent, flags, startId);
    }

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

上面的代码创建了一个IPUpdateService服务，该服务在onCreate方法中获取了AlarmManager实例，并创建了一个PendingIntent，该PendingIntent会在每次定时器触发时启动一个广播接收器IPUpdateReceiver。接下来，我们需要编写IPUpdateReceiver类来处理广播事件：

public class IPUpdateReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        String ipAddress = getIPAddress();
        String savedIP = readSavedIP(context);
        if (!ipAddress.equals(savedIP)) {
            saveIP(context, ipAddress);
        }
    }

    private String getIPAddress() {
        // 获取当前IP地址的逻辑
    }

    private void saveIP(Context context, String ipAddress) {
        try {
            FileOutputStream outputStream = context.openFileOutput("ip_address.txt", Context.MODE_PRIVATE);
            outputStream.write(ipAddress.getBytes());
            outputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    private String readSavedIP(Context context) {
        String savedIP = "";
        try {
            FileInputStream inputStream = context.openFileInput("ip_address.txt");
            byte[] buffer = new byte[1024];
            int length = inputStream.read(buffer);
            savedIP = new String(buffer, 0, length);
            inputStream.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
        return savedIP;
    }
}

在IPUpdateReceiver的onReceive方法中，我们首先获取当前IP地址，并读取之前保存的IP地址。如果当前IP地址和之前保存的IP地址不同，就将当前IP地址保存到本地文件当中。IPUpdateReceiver类中的getIPAddress、saveIP和readSavedIP方法需要根据实际需求进行实现。最后，我们需要在AndroidManifest.xml文件中注册IPUpdateReceiver广播接收器和IPUpdateService服务：

<receiver android:name=".IPUpdateReceiver" />
<service android:name=".IPUpdateService" />

其中，IPUpdateReceiver需要注册在application标签内，而IPUpdateService可以注册在任意位置。