Develop Reference

Android JNI/NDK Application Context

I'm using JNI to get the music library from Android with Qt. I call upon the following Java method (which is already implemented),
public String getArtists(Context context)
...
I need to be able to get the Context of the application in order for it to work.
If it helps, when I was using Java, the following code provided the correct context.
MainActivity.this
Could anybody be of assistance in this problem?
Many thanks!

It will depend where you are making the call. Ideally you will cache the MainActivity pointer in C++.
One way to cache a pointer to use in a later JNI call is to add a native function in java such as native void onCreateNative() to you MainActivity class. In C++ you'll implement the method and cache the "thiz" pointer:
JNIEXPORT void JNICALL com_package_MainActivity_onCreateNative(JNIEnv *env, jobject thiz)
{
gCachedActivity = env->NewGlobalRef(thiz);
}
Now you can use gCachedActivity where you would have used MainActivity.this
env->CallObjectMethod(obj, s_getArtistsGetArtistsMethodID, gCachedActivity);
Of course replacing obj with the object you are calling the method on.
Lastly onCreateNative() should be called in the onCreate() method of MainActivity.

UnsatisfiedLinkError when calling native function

I can succesfully load native library with System.loadLibrary(""), but when I call native method from that library, I receive UnsatisfiedLinkError, no implementation found for that method.
But this code works in another application, somehow in mine not.

Did you change the packageName in the jni method, it should be Java_com_example_yourapp_methodname(JNIEnv * env, jobject thiz) where com_example_yourapp is the reference to your package of the class you call the method.
and after that, did you call ndk-build again?

The method Erik N is suggested fine, but it has some performance impact as the VM need to search a function call with the above signature. Instead you can map the functions with signatures and register them on JNI_OnLoad() function call.
http://docs.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html#wp5833

jni table overflow even after deleteLocalRef

When I run the code, I get an error "failed adding to JNI local ref table has 512 entries"
This is my code:
jstring pJNIData = pJNIEnv->NewStringUTF ( variables[0].GetStringValue() );
pJNIEnv->CallStaticVoidMethod ( pJNIActivityClass, pJNIMethodIDStartTime, pJNIData ) ;
pJNIEnv->DeleteLocalRef(pJNIData);
I have read several suggestions, but none of them work! In spite of the DeleteLocalRef, it fails to works. The function is used in a profiler that literally calls all the functions...

I have seen this when a JNI method called Java code (in my case, the method was not static). As I understand, unused local references are not automatically deleted when a Java method is called from JNI (I mean, until the top-level JNI function returns).
IIRC either there already was information about memory objects in the log, or I could add some logging; from that information I identified garbage items that I did not mention before. They were two arrays and a class, created in subsequent calls but not garbage-collected.
// in a function that calls a Java method from JNI
jbyteArray srcArray = env->NewByteArray(len);
jclass cls = env->FindClass("com/something/MyClass");
jmethodID mid = env->GetMethodID(cls, "mymethod", "([BI)[B");
jbyteArray resArray = (jbyteArray)env->CallObjectMethod(obj, mid, srcArray, XXXX);
...
env->DeleteLocalRef(cls);
env->DeleteLocalRef(resArray);
env->DeleteLocalRef(srcArray);
// no need to do anything with mid
Note that although these three local references were obtained differently, all of them were hanging around.
Useful link:
http://www.netmite.com/android/mydroid/dalvik/docs/jni-tips.html#local_vs_global_references
(or find the Dalvik VM docs dalvik/docs/jni-tips.html and locate the section "Local vs. Global References")
Every object that JNI returns is a "local reference". This means that it's valid for the duration of the current native method in the current thread. Even if the object itself continues to live on after the native method returns, the reference is not valid. This applies to all sub-classes of jobject, including jclass and jarray. [...] Note: method and field IDs are just 32-bit identifiers, not object references, and should not be passed to NewGlobalRef. The raw data pointers returned by functions like GetStringUTFChars and GetByteArrayElements are also not objects.

I thought I would chip in just in case anyone else runs into this issue. This is a weird case that kept me confused for hours!
Ok so I have an NDK app and the Java code being called is inside an apk that is loaded at runtime. I have no idea if the runtime loading effects this in any way but I thought I should mention it.
Now in a c++ method I use find class and getmethodid to get the constuctor to a HashMap and call it to get a new HashMap instance. I then populate the HashMap from the c++ side using jni calls. So far so good.
I then pass the HashMap to java code and, again, all is working as expected. Once the java code has returned I call DeleteLocalRef on the HashMap. No errors are thrown but the reference is not deleted.
This only came up when I finally ran over 512 local references (from multiple calls to this function) and the error dump showed that the last 10 items in the localref store were nearly all HashMaps. I would understand that the GC would not collect these references at the end of the method as I am make a multithreaded ndk app. However the DeleteLocalRef should have worked.
The Fix:
In the end I found that creating the HashMap from a jni call to a java method I wrote was fine, and the reference was then free'able. It seems crazy to have a java function that literally just returns a new HashMap but it worked so for now I am living with it :)

Calling a Java function from the Native code using the NDK

I asked this question the other day, but wasn't too specific, so I want to re-clarify.
I am creating an Android Application which uses an existing library in C using the NDK. The problem I have run into is that the C code uses a lot of things java doesn't ( function pointers as parameters is the big problem ).
Anyway, I was wondering if I could write functions in my Java code that the C code calls. Now from what I can tell, you can do it, so I would appreciate it if no one just answered 'Yes you can, LINK." I have been looking into it but its very over my head as to what actually needs to be done.
Can anyone try to explain the process? I know it involves creating a JVM in the C code; any information that will help a newbie get on his feet will be greatly appreciated.
Thanks
EDIT :
So, I don't know what to do for these three steps.
To call a specific Java function from C, you need to do the following:
Obtain the class reference using the FindClass(,,) method.
Obtain the method IDs of the functions of the class that you want to call using the
GetStaticMethodID and GetMethodID function calls.
Call the functions using CallStaticVoidMethod, CallStaticIntMethod, and CallStaticObjectMethod.
This isn't explained too much and I have literally no experience in C. Is FindClass a C method?

Every C function that is callable from Java via JNI has a first parameter of type JNIEnv*. On the C end, this is a pointer to a pointer to a structure with a bunch of pointers to functions. Those functions are your interface to the Java world. FindClass, GetMethodID and the rest are among them.
So when you want to call FindClass from the C side, here's how you do it:
void Java_com_mypackage_MyClass_MyMethod(JNIEnv *jniEnv, jobject thiz)
{
jclass *clazz = (*(*jniEnv)->FindClass)(jniEnv, "com/mypackage/SomeClass");
jmethodID MethodID = (*(*jniEnv)->GetStaticMethodID)(jniEnv, clazz, "SomeMethod", "(I)I");
int result = (*(*jniEnv)->CallStaticIntMethod)(jniEnv, clazz, MethodID, 18);
And so forth. The line dereferences the jniEnv parameter, gets a function pointer and calls the function through it. Class and method names are completely bogus, naturally. How would I know yours.
Note: I'm talking of function pointers here, but not in the same sense as you do; those are function pointers to functions that JNI provides, not to your functions.
The verbosity of call syntax has to do with the limitations of C; in C++, you can write instead
jclass *cl = jniEnv->FindClass("com/mypackage/SomeClass");
as C++ supports function table pointers of this sort natively via virtual functions.
You can probably take some shortcuts along the way. If you're calling methods in the same class as your C point of entry, and it happens to be static, your second parameter already is a class object pointer. If you have a this pointer to the object you want to invoke a method on, you can use GetObjectClass.

calling java from native c++ JNI DIRECTLY

This question has been asked and answered in many posts like this!
But how can I call from c++ directly ? For this how can I get JNIEnv* and jobject ?
Is this possible ?

To get JNIEnv you can write global JNI_OnLoad function that will get called during loading of shared library. This function will get JavaVM pointer as argument. Using it you can get JNIEnv for current thread (or create new one if there was no JNIEnv previously).
As to where get jobject - if that is new object you are instantiating, then you use JNIEnv::NewObject method. Otherwise you need to pass object on which you want to call method from java side to C/C++.

You need to read the Invocation section of the JNI Specification.