Android Studio使用单例类检测Wi-Fi连接报空对象引用错误

在Android Studio中使用单例类实现Wi-Fi连接检测功能，点击按钮触发对应逻辑时抛出如下空指针错误：
Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

关联代码 #

原有单例类实现 #

public class WifiConnectivity {

    private static WifiConnectivity INSTANCE = null ;
    private ConnectivityManager cnmgr;
    Context context;
    int ipadress;
    String ipAddress;


    public static WifiConnectivity getInstance() {
        if (INSTANCE == null){
            INSTANCE = new WifiConnectivity();
        }
        return INSTANCE;
    }

    public ConnectivityManager getWifiConnectivty(int ipadress , String ipAddress){
        this.ipadress = ipadress;
        this.ipAddress = ipAddress;
        return cnmgr;
    }

    public void Connectivity() {
        cnmgr = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        if (cnmgr.getActiveNetworkInfo() != null && cnmgr.getActiveNetworkInfo().isAvailable()
                && cnmgr.getActiveNetworkInfo().isConnected()) {
            WifiManager wifiMgr = (WifiManager) context.getApplicationContext().getSystemService(Context.WIFI_SERVICE);
            WifiInfo wifiInfo = wifiMgr.getConnectionInfo();
            ipadress = wifiInfo.getIpAddress();
            ipAddress = Formatter.formatIpAddress(ipadress);
            Log.d(TAG, "wifi_connectivity:");
        }else {
            Toast.makeText(context , "Wifi Not Connect" , Toast.LENGTH_SHORT).show();
          
        }

    }
    private WifiConnectivity(){}
}

原有按钮点击调用逻辑 #

WifiConnectivity.getInstance().getWifiConnectivty(ipadress, ipAddress);
WifiConnectivity.getInstance().Connectivity();

• 核心触发点：单例类中的context成员变量从未被赋值，始终为null。执行context.getSystemService()时相当于在空对象上调用方法，直接抛出空指针异常。
• 单例设计缺陷：getInstance()方法没有接收Context入参，私有构造方法也没有初始化Context属性，整个类中无任何给context赋值的逻辑。
• 逻辑顺序错误：先调用getWifiConnectivty()方法时，cnmgr还未初始化，返回值永远为null，方法执行顺序颠倒。
• 潜在隐患：如果直接传入Activity/Fragment的Context，单例生命周期长于页面时会造成内存泄漏。
• 代码缺失：类中使用了TAG常量但未定义，会直接编译失败。

1. 修改单例获取逻辑，传入Application级Context初始化，避免内存泄漏
2. 在构造方法中提前初始化系统服务，减少重复调用开销
3. 调整方法调用顺序，先执行WiFi状态检测再获取结果
4. 补全缺失的常量、权限声明

修复后的单例类代码 #

public class WifiConnectivity {
    private static final String TAG = "WifiConnectivity";
    private static WifiConnectivity INSTANCE = null;
    private final ConnectivityManager cnmgr;
    private final WifiManager wifiMgr;
    private final Context appContext;
    private int ipadress;
    private String ipAddress;

    public static WifiConnectivity getInstance(Context context) {
        if (INSTANCE == null){
            INSTANCE = new WifiConnectivity(context.getApplicationContext());
        }
        return INSTANCE;
    }

    private WifiConnectivity(Context context){
        appContext = context;
        cnmgr = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
        wifiMgr = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
    }

    public String checkWifiConnectivity(){
        NetworkInfo activeNet = cnmgr.getActiveNetworkInfo();
        if (activeNet != null && activeNet.isAvailable() && activeNet.isConnected()) {
            WifiInfo wifiInfo = wifiMgr.getConnectionInfo();
            ipadress = wifiInfo.getIpAddress();
            ipAddress = Formatter.formatIpAddress(ipadress);
            Log.d(TAG, "wifi connected, ip: " + ipAddress);
            return ipAddress;
        }else {
            Toast.makeText(appContext , "Wifi Not Connect" , Toast.LENGTH_SHORT).show();
            return null;
        }
    }

    public int getIpadress() {
        return ipadress;
    }

    public String getIpAddress() {
        return ipAddress;
    }
}

修复后的调用方式 #

在按钮点击事件中按如下逻辑调用：

WifiConnectivity wifiHelper = WifiConnectivity.getInstance(getApplicationContext());
String currentIp = wifiHelper.checkWifiConnectivity();
if (currentIp != null) {
    int currentIpInt = wifiHelper.getIpadress();
    // 后续IP相关业务逻辑
}

必要配置 #

在AndroidManifest.xml中声明所需权限：

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />

注：Android 10及以上版本getActiveNetworkInfo()已被标记废弃，可按需使用NetworkCallbackAPI做高版本适配。

内容的提问来源于stack exchange，提问作者Farzan Siddiqui