android - 调用 getSystemService() 时得到 NullPointerException

Question

我的问题是，在我的 MainActivity 的 onCreate() 方法中，我正在创建新的 Thread 对象，我想将对 this Activity 的引用传递给该对象，然后在该线程中使用它来调用 getSystemService ().但最后，当我启动应用程序时，它崩溃了，我得到了 NullPointerException。

我已经发现问题可能是我在 super.onCreate() 之前传递了对 Activity 的引用，但在我的代码中，super.onCreate() 是在传递引用之前执行的。

这是我的 MainActivity 的 onCreate() 方法

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

    // Instance which contains thread for obtaining wifi info
    final WifiInfoThread wifi_info = new WifiInfoThread(this);
....
}

这是 Thread 类，我试图在其中获取对系统服务的引用

public class WifiInfoThread extends Thread {
// Constructor for passing context to this class to be able to access xml resources
Activity activity;
WifiInfoThread(Activity current) {
    activity = current;
}

// Flag for stopping thread
boolean flag = false;
// Obtain service and WifiManager object
WifiManager current_wifi = (WifiManager) activity.getSystemService(Context.WIFI_SERVICE);

// Runnable object passed to UIThread
Runnable uirunnable = new Runnable() {
    @Override
    public void run() {
        // Get current wifi status
        WifiInfo wifi_info = current_wifi.getConnectionInfo();

        // Things with showing it on screen
        TextView tv_output = (TextView) activity.findViewById(R.id.tv_output);
        String info = "SSID: " + wifi_info.getSSID();
        info += "\nSpeed: " + wifi_info.getLinkSpeed() + " Mbps";
        tv_output.setText(info);
    }
};

public void run() {
    flag = true;

    for(; flag; ) {
        activity.runOnUiThread(uirunnable);
        try {
            this.sleep(500);
        }
        catch(InterruptedException e) {}
    }
}

Answer 1

在初始化 activity 之前，您正在使用 activity.getSystemService。要解决这个问题，请将下面的行移至 Constructor

// Obtain service and WifiManager object
WifiManager current_wifi = (WifiManager) activity.getSystemService(Context.WIFI_SERVICE);

喜欢

WifiManager current_wifi;
WifiInfoThread(Activity current) {
    activity = current;
    current_wifi = (WifiManager) activity.getSystemService(Context.WIFI_SERVICE);
}