Call Android Activity Mtehod in C++ using JNI qt

[user1618]

I am accessing a Method from Java class which has to show a Toast on Screen. But the method is in other class which is not an Activity.  I have two scenario, lets me explain them one by one..

Common idea
I can access the methods in both Acitivity and normal Java class from C++ through JNI but both have issues

Issue on Calling a method from Activty

I can access method from Activity and also have the activity context to show anything on UI. But in this case I am getting crash with this error
 

     java.lang.RuntimeException: Can't create handler inside thread that has not called Looper.prepare()

I have tried all general stuff like using AsyncTask, Handler, runOnUI etc but same I guess It is issue with some JNI call which also runs mainly in Threads

Issue on Calling a Method from Normal Java Class

I can access method here too, but I do not have the context of Activity in this class to show Toast on UI, so Issue here is to get the Activity context which I can not figured out how to pass it in JNI and all. I tried with passing through constructor but question is how to get it once in JNI to that It could be passed to this class. Error here is

     java.lang.NullPointerException
     E/AndroidRuntime( 4594):     at android.widget.Toast.<init>(Toast.java:92)
     E/AndroidRuntime( 4594):     at android.widget.Toast.makeText(Toast.java:238)

I have searched alot and tried many things but could not able to make it work, and also did not find good help, tutorials on JNI andorid. 
So I do know if there is any workaround availble to achive it

here is my JNI call code
[JNI Code][1]

here is my method in Activity

        public void ShowMessage(String message){   
        messageData = message;
       

        if(MainActivity.this == null){
            Log.d("Acitivtry", "issue");
            }    
       
            MainActivity.this.runOnUiThread(new Runnable() {
           
            public void run() {
                Log.d("in RunUIThread", "hmmm");
                Toast.makeText(MainActivity.this, "messageData", Toast.LENGTH_SHORT).show();
            }
                });
            }


  [1]: http://pastebin.kde.org/pmnrvwb22

[J Decker]

I assume 'MainActivity.this' is a static member; are you sure you set that?

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[inderjeet]

MainActivity is current activity don't think being static would make any difference but I tried both ways still same issue. For me it looks like it is about accessing the android native activity object instead of using findclass for created MainActivity. But I am not sure how to do it. 

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INDERJEET SINGH

+33-751674894

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[J Decker]

I found an example a long time ago to show/hide the input keyboard; I modified that to just call the class in MyNativeActivity extends NativeActivity ... the original was more complex than this.... this way simplifies it kinda

{

    public void setSuspendSleep() {

        this.runOnUiThread( new Runnable() {

    public void run() 

                {

        getWindow().addFlags( WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON );

                }        

        } );

    }

    

    public void setAllowSleep() {

        this.runOnUiThread( new Runnable() {

    public void run() {

       getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

                }

        } );

    }

}

------- and then in C++ ------

// getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

// getWindow().clearFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

extern "C" void SuspendSleep( int bStopSleep )

{

static int init;

static jobject  WindowManager_LayoutParams_FLAG_KEEP_SCREEN_ON;

    // Attaches the current thread to the JVM. 

    jint lResult; 

    jint lFlags = 0; 

    JavaVM* lJavaVM = engine.app->activity->vm; 

    JNIEnv* lJNIEnv = engine.app->activity->env; 

    JavaVMAttachArgs lJavaVMAttachArgs; 

    lJavaVMAttachArgs.version = JNI_VERSION_1_6; 

    lJavaVMAttachArgs.name = "NativeThread"; 

    lJavaVMAttachArgs.group = NULL; 

    lResult=lJavaVM->AttachCurrentThread(&lJNIEnv, &lJavaVMAttachArgs);

    if (lResult == JNI_ERR) { 

        return; 

    } 

    // Retrieves NativeActivity. 

    jobject lNativeActivity = engine.app->activity->clazz; 

jclass ClassNativeActivity = lJNIEnv->GetObjectClass(lNativeActivity);

if( bStopSleep )

{

jmethodID MethodSetFlags = lJNIEnv->GetMethodID(      ClassNativeActivity, "setSuspendSleep", "()V");

if( MethodSetFlags )

lJNIEnv->CallVoidMethod( lNativeActivity,  MethodSetFlags );

}

else

{

jmethodID MethodSetFlags = lJNIEnv->GetMethodID(      ClassNativeActivity, "setAllowSleep", "()V");

if( MethodSetFlags )

lJNIEnv->CallVoidMethod( lNativeActivity,  MethodSetFlags );

}

// Finished with the JVM. 

lJavaVM->DetachCurrentThread();

}

[james Lee]

Hi~

 

What is the engine.app->activity ?

Build error is generated..

 

Yorr code seem to helpful for me..

I can't access an android activity dialog by using Qt-C++.

 

 

 

 

 JavaVM* lJavaVM = engine.app->activity->vm; 

 JNIEnv* lJNIEnv = engine.app->activity->env; 

 

 

2014년 3월 11일 화요일 오후 6시 34분 19초 UTC+9, J Decker 님의 말:

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[VS Hải Long]

I call java method from jni native method. but it can't access any API UI.

now your code solved my problem. Thank J Decker very much

[suresh kumar]

Iam redirected from following post "https://groups.google.com/forum/#!topic/android-ndk/-vN0maMKtOs"

I tried to use mentioned in the post. Iam getting build error.

Please let me know engine object means here ?

[J Decker]

Engine is a structure that's in a sample NDK thing... it contains a 

struct engine {

struct android_app* app;

}

which the app is what it really wants.

it's set from android_main 

/**

 * This is the main entry point of a native application that is using

 * android_native_app_glue.  It runs in its own thread, with its own

 * event loop for receiving input events and doing other things.

 */

void android_main(struct android_app* state) {

const char *data_path = engine.data_path;

   struct event_queue queue = engine.events;

// Make sure glue isn't stripped.

app_dummy();

memset(&engine, 0, sizeof(engine));

engine.data_path = data_path;

engine.events = queue;

if( !engine.events.msgpipe[0] )

if (pipe(engine.events.msgpipe)) {

LOGI("could not create pipe: %s", strerror(errno));

}

state->userData = &engine;

state->onAppCmd = engine_handle_cmd;

state->onInputEvent = engine_handle_input;

engine.app = state;

[suresh kumar]

Thanks for your reply. How come i can get engine object instance in my code when it is in defined in ndk native code glue code?

Please explain it

[J Decker]

On Mon, Jan 5, 2015 at 5:55 AM, suresh kumar <sureshk...@gmail.com> wrote:

Thanks for your reply. How come i can get engine object instance in my code when it is in defined in ndk native code glue code?

Please explain it

because GCC by default makes all non static or hidden symbols public ?

since engine is shared... it ended up having a public name you can link against?

I don't understand your question 

--

[suresh kumar]

iam unable to include engine structure in my native application.Getting compile error.

my implementation will be like this

http://stackoverflow.com/questions/27605301/unable-to-cache-java-vm-while-calling-java-from-nativec-c

[J Decker]

On Tue, Jan 6, 2015 at 2:44 AM, suresh kumar <sureshk...@gmail.com> wrote:

iam unable to include engine structure in my native application.Getting compile error.

my implementation will be like this

http://stackoverflow.com/questions/27605301/unable-to-cache-java-vm-while-calling-java-from-nativec-c

#include <android_native_app_glue.h>

but this is from NativeActivity which extends Activity...

inbetween the java... 

http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.4_r1/android/app/NativeActivity.java#177

 private native int loadNativeCode(String path, String funcname, MessageQueue queue,

             String internalDataPath, String obbPath, String externalDataPath, int sdkVersion,

             AssetManager assetMgr, byte[] savedState);

I haven't found the implementation of this function which creates a 

ANativeActivity* activity,

... defined here....

http://mobilepearls.com/labs/native-android-api/include/android/native_activity.h

which has a JavaVM and JaveEnv  etc in it... which is used in all of my examples that work...

so how NativeActivity_loadNativeCode creates a ANativeActivity?