Native android code with QT

ashajg

Hi guys

I am trying to get name of wifi connected to my mobile using QAndroidJniObject.

My code is

.java

public class QtAndroidToastJava extends QtActivity
{
public String getWifiName(Context context) {
    WifiManager manager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
    if (manager.isWifiEnabled()) {
       WifiInfo wifiInfo = manager.getConnectionInfo();
       if (wifiInfo != null) {
          DetailedState state = WifiInfo.getDetailedStateOf(wifiInfo.getSupplicantState());
          if (state == DetailedState.CONNECTED || state == DetailedState.OBTAINING_IPADDR) {
              return wifiInfo.getSSID();
          }
       }
    }
    return null;
}

}

and my cpp file is

#include "wificlass.h"
#include <QtAndroid>
#include <QDebug>
WIFICLASS::WIFICLASS(QObject *parent) : QObject(parent)
{

}

void WIFICLASS::updateAndroidNotification()
{


    //QAndroidJniObject tstMsg = QAndroidJniObject::fromString("Hello world! :)");
    QAndroidJniObject::callStaticMethod <void>("com/amin/QtAndroidToastJava/QtAndroidToastJava",
                                              "sMakeToast",
                                              "(Ljava/lang/String;)J");

**qDebug()<<"here I want a string which is returned by java file"<<????;**


}

My problem is how I can get a string from java class in My cpp file

My java class is returning return wifiInfo.getSSID(); and I want that to be displayed from cpp file in place of this
qDebug()<<"here I want a string which is returned by java file"<<????;

sierdzio

You call wrong method: sMakeToast instead of getWifiName.

You're not checking reading the return value. Use this syntax:

auto returnString = QAndroidJniObject::callStaticMethod <jstring>("com/amin/QtAndroidToastJava/QtAndroidToastJava",
                                              "getWifiName",
                                              "(Ljava/lang/String;)J");

Lastly, you declare that method has a String argument "(Ljava/lang/String;)J", but you fail to provide it's value (it should be provided as following argument to callStaticMethod).

By the way, that method does not need an argument. public String getWifiName(Context context). You can get the context from QtActivity.

ashajg

Hi @sierdzio

Thanks for help sir!

so I can omit "(Ljava/lang/String;)J" from above code?

ashajg

@sierdzio I am getting this error

auto returnString = QAndroidJniObject::callStaticMethod <jstring>("com/amin/QtAndroidToastJava/QtAndroidToastJava",
                                              "getWifiName");

// QString user = juser.toString();
qDebug()<<returnString;

C:\ASHISH\PROGRAMS\Wifipoc\wificlass.cpp:24: error: undefined reference to '_jstring QAndroidJniObject::callStaticMethod<_jstring>(char const*, char const*)**

sierdzio

You do need to provide type information. If you removed the context from your Java file, then that info would read (V;)Ljava/lang/String (no arguments + return string).

One more nitpick: you're calling callStaticMethod but your Java method is not static.

ashajg

ok sir
@sierdzio

Now I have made following changes

package com.amin.QtAndroidToastJava;

import org.qtproject.qt5.android.bindings.QtApplication;
import org.qtproject.qt5.android.bindings.QtActivity;
import android.net.NetworkInfo.DetailedState;
import android.net.wifi.WifiInfo;
import android.net.wifi.WifiManager;
import android.app.Activity;
import android.content.Context;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;



public class QtAndroidToastJava extends QtActivity
{



public static String getWifiName(Context context) {
    WifiManager manager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
    if (manager.isWifiEnabled()) {
       WifiInfo wifiInfo = manager.getConnectionInfo();
       if (wifiInfo != null) {
          DetailedState state = WifiInfo.getDetailedStateOf(wifiInfo.getSupplicantState());
          if (state == DetailedState.CONNECTED || state == DetailedState.OBTAINING_IPADDR) {
              return wifiInfo.getSSID();
          }
       }
    }
    return null;
}

/////////////////////////////////////

#include "wificlass.h"
#include <QtAndroid>
#include <QDebug>
WIFICLASS::WIFICLASS(QObject *parent) : QObject(parent)
{

}

void WIFICLASS::updateAndroidNotification()
{



   auto returnString = QAndroidJniObject::callStaticMethod <jstring>("com/amin/QtAndroidToastJava/QtAndroidToastJava",
                                                "getWifiName","(V;)Ljava/lang/String");

   //QString user = juser.toString();


   qDebug()<<returnString;

}

After running I am getting this error

..\Wifipoc/wificlass.cpp:23: error: undefined reference to '_jstring QAndroidJniObject::callStaticMethod<_jstring>(char const*, char const*, char const*, ...)'
collect2.exe: error: ld returned 1 exit status
make: *** [libWifipoc.so] Error 1
17:34:55: The process "C:\ASHISH\android-ndk-r15c\prebuilt\windows-x86_64\bin\make.exe" exited with code 2.
Error while building/deploying project Wifipoc (kit: Android for armeabi-v7a (GCC 4.9, Qt 5.11.2 for Android ARMv7))
When executing step "Make"
17:34:55: Elapsed time: 00:06.**

sierdzio

Hm, looks like the static method call needs a jclass instead of string. Weird.

Try this:

jclass myClass = "com/amin/QtAndroidToastJava/QtAndroidToastJava";
auto returnString = QAndroidJniObject::callStaticMethod <jstring>(
    myClass, "getWifiName", "(V)Ljava/lang/String;");

ashajg

Hi sir @sierdzio
What I did is in my cpp file I added this line

QAndroidJniObject returnString = QAndroidJniObject::callStaticObjectMethod("org/qtproject/example/QtAndroidToastJava","getWifiName","(Landroid/content/Context)",QtAndroid::androidContext().object());

As I am using getWifiName(Context context) function it requires context from cpp.
What I did is I added "(Landroid/content/Context)",QtAndroid::androidContext().object()); to check if it works but it is throwing these errors:

W System.err: java.lang.NoSuchMethodError: no static method "Lorg/qtproject/example/QtAndroidToastJava;.getWifiName(Landroid/content/Context)"
W System.err: at org.qtproject.qt5.android.QtNative.startQtApplication(Native Method)
W System.err: at org.qtproject.qt5.android.QtNative$6.run(QtNative.java:359)
W System.err: at org.qtproject.qt5.android.QtThread$1.run(QtThread.java:61)

I think I am making some mistake in this line "(Landroid/content/Context)",QtAndroid::androidContext().object());"

how can I figure out what I should pass or where its getting wrong?

sierdzio

@ashajg said in Native android code with QT:

how can I figure out what I should pass or where its getting wrong?

Only by guessing, unfortunately. JNI is not clever enough to point our errors exactly.

"(Landroid/content/Context)" - no return type (your method returns a string).

I did tell you to drop the context arg, by the way. But if you want to keep it - so be it.

ashajg

ok Sir
I ll also try this

jclass myClass = "com/amin/QtAndroidToastJava/QtAndroidToastJava";
auto returnString = QAndroidJniObject::callStaticMethod <jstring>(
myClass, "getWifiName", "(V)Ljava/lang/String;");

it is saying cannot initialize a variable with jclass..

ashajg

@sierdzio

you asked to drop context so should I just write the code like this

public class QtAndroidToastJava extends QtActivity
{
public String getWifiName(){
    WifiManager manager = (WifiManager) context.getSystemService(Context.WIFI_SERVICE);
    if (manager.isWifiEnabled()) {
       WifiInfo wifiInfo = manager.getConnectionInfo();
       if (wifiInfo != null) {
          DetailedState state = WifiInfo.getDetailedStateOf(wifiInfo.getSupplicantState());
          if (state == DetailedState.CONNECTED || state == DetailedState.OBTAINING_IPADDR) {
              return wifiInfo.getSSID();
          }
       }
    }

its throwing error

symbol: variable context
location: class QtAndroidToastJava
Note: Some input files use or override a deprecated API. if I remove context

ashajg

Hi sir @sierdzio

I got the solution :) by using this (Landroid/content/Context;)Ljava/lang/String; I got wifi name.

JoeCFD

I am trying to do the same thing. But
auto returnString = QAndroidJniObject::callStaticMethod <jstring>(
myClass, "getWifiName", "(V)Ljava/lang/String;");
causes compiling error.
undefined reference to `_jstring* QAndroidJniObject::callStaticMethod<_jstring*>(char const*, char const*, char const*, ...)
QAndroidJniObject::callStaticMethod <jstring> is dropped? I tried int and func call works.

I checked the doc of Qt5
https://doc.qt.io/qt-5/qandroidjniobject.html
// C++ code
QAndroidJniObject stringNumber = QAndroidJniObject::callStaticObjectMethod("org/qtproject/qt5/TestClass",
"fromNumber"
"(I)Ljava/lang/String;",
10);
should be work. But the call in my app returns empty object.
My Qt version: 5.15.2 and OS: Ubuntu 18.04

JoeCFD

@JoeCFD It works now. In my Java code, String has to be created with new.

JoeCFD

wifiInfo.getSSID() returns unknown SSID.

JoeCFD

@JoeCFD set permissions for location and wifi state change in manifestfile. And change location permission to my app in the tablet settings. The problem is solved.