使用 ConnectivityManager 和 NetworkInfo 避免 NullPointerException

Question

在我的应用程序中，我通过使用 PHP 脚本查询数据库。目前，应用程序一出现在屏幕上，它就会立即获取信息。但是，如果您没有互联网连接，它会返回 NullPointerException，因为它没有获取任何数据。此后，我使用 ConnectivityManager 和 NetworkInfo 更新了我的代码，如果用户未连接到其网络或 WiFi，则会显示 toast。问题是它跳过了这一点，只是像平常一样执行其余的代码。

public class Shc_BalloonSat_Activity extends Activity
{
int historyCountFromUser;
httpAPI api;
mapAPI map;
DecimalFormat df = new DecimalFormat("##.#####");
DecimalFormat decimalf = new DecimalFormat("##.######");
SharedPreferences pref;
Editor prefEditor;
String lastpacketsPHP;

// User to determine how many packet the user would like to see.
int userDefinedCount = 5;

/*
 *  Called when the activity is first created. 
*/

@Override
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    String returned = "";
    lastpacketsPHP = "";
    pref = getSharedPreferences("shared_prefs", 0);
    prefEditor = pref.edit();
    prefEditor.putString(lastpacketsPHP, "/* PHP file url goes here */");
    prefEditor.commit();       

    if(!isNetworkConnected(this))
    {
        Toast.makeText(this, "Please connect to the internet to access the full functionality of this app.", Toast.LENGTH_LONG).show();
    }

    else
    {
         api = new httpAPI(this);
         map = new mapAPI(this);

        try
        {
            returned = api.getData();
        }

        catch (Exception e)
        {
            e.printStackTrace();
        }

        TextView infoTV = (TextView)this.findViewById(R.id.info);
        infoTV.setText(returned);
        assignInfoToInfoTextView();
        assignInfoToHistoryTextView();
    }

}

public boolean isNetworkConnected(Context context)
{
    ConnectivityManager connectionManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    if (connectionManager.getActiveNetworkInfo() != null && connectionManager.getActiveNetworkInfo().isAvailable() && connectionManager.getActiveNetworkInfo().isConnected())
    {
        return true;
    }

    else 
    {
        return false;
    }
}

有什么想法我做错了吗？

更新（LogCat错误消息）：

02-19 13:55:19.450: E/log_tag(443): Error in HTTP connection: java.net.UnknownHostException: Host is unresolved: space.uah.edu:80
02-19 13:55:19.450: E/log_tag(443): Error converting result:     java.lang.NullPointerException
02-19 13:55:19.450: E/log_tag(443): Error in HTTP connection: java.net.UnknownHostException: Host is unresolved: space.uah.edu:80
02-19 13:55:19.450: E/log_tag(443): Error converting result: java.lang.NullPointerException
02-19 13:55:19.450: E/AndroidRuntime(443): Uncaught handler: thread main exiting due to uncaught exception
02-19 13:55:19.473: E/AndroidRuntime(443): java.lang.RuntimeException: Unable to start activity    ComponentInfo{shc_BalloonSat.namespace/shc_BalloonSat.namespace.Shc_BalloonSat_Activity}: java.lang.NullPointerException: println needs a message
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread.access$2200(ActivityThread.java:119)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.os.Looper.loop(Looper.java:123)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread.main(ActivityThread.java:4363)
02-19 13:55:19.473: E/AndroidRuntime(443):  at java.lang.reflect.Method.invokeNative(Native Method)
02-19 13:55:19.473: E/AndroidRuntime(443):  at java.lang.reflect.Method.invoke(Method.java:521)
02-19 13:55:19.473: E/AndroidRuntime(443):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
02-19 13:55:19.473: E/AndroidRuntime(443):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
02-19 13:55:19.473: E/AndroidRuntime(443):  at dalvik.system.NativeStart.main(Native Method)
02-19 13:55:19.473: E/AndroidRuntime(443): Caused by: java.lang.NullPointerException: println needs a message
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.util.Log.println(Native Method)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.util.Log.e(Log.java:208)
02-19 13:55:19.473: E/AndroidRuntime(443):  at shc_BalloonSat.namespace.Shc_BalloonSat_Activity.assignInfoToInfoTextView(Shc_BalloonSat_Activity.java:173)
02-19 13:55:19.473: E/AndroidRuntime(443):  at shc_BalloonSat.namespace.Shc_BalloonSat_Activity.onCreate(Shc_BalloonSat_Activity.java:75)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-19 13:55:19.473: E/AndroidRuntime(443):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)
02-19 13:55 :19.473: E/AndroidRuntime(443):     ... 11 more

Answer 1

假设您的连接检查逻辑工作正常，请将其设置为 if/else。如果未连接则显示 toast，否则继续逻辑。

     if(!isNetworkConnected(this))         {     
            Toast.makeText(this, "Please connect to the internet to access the full functionality of this app.", Toast.LENGTH_LONG).show();      
       }
else
    {          
    TextView infoTV = (TextView)this.findViewById(R.id.info);     
    infoTV.setText(returned);      
    assignInfoToInfoTextView();     
    assignInfoToHistoryTextView(); 
    }