Develop Reference

access xml part from c++ android

i have a textview in xml file with "tv" id
if i declare
TextView tv ;
tv= findViewByID(R.id.tv);
then i can call setText Method with this part
extern "C"
JNIEXPORT void JNICALL
Java_org_lotka_brilliance_activity_MainActivity_textViewInit(JNIEnv *env, jobject
thiz) {
jclass MainCLass = env->FindClass("org/lotka/brilliance/activity/MainActivity");
jclass TextViewClass = env->FindClass("android/widget/TextView");
jfieldID tvfieild = env->GetFieldID(MainCLass , "tv",
"Landroid/widget/TextView;");
jmethodID tvsettext = env->GetMethodID(TextViewClass , "setText", "
(Ljava/lang/CharSequence;)V");
jobject jobject1 = env->GetObjectField(thiz , tvfieild);
env->CallVoidMethod(jobject1 ,tvsettext , env->NewStringUTF("hello from jni"));
}
im using the TextView which is declared in java part , now the question is that , if i want to find TextView id in jni what should i do ??

There will be a generated class com.your.package.R.id which contains a bunch of static ints with the view IDs. So it's just a matter of reading the particular field you're interested in, and then calling findViewById to get the TextView.
I'm not sure why anyone would want to manipulate their app's views in this cumbersome way, but it can be done:
jclass idClass = env->FindClass("com/your/package/R$id");
jfieldID tvField = env->GetStaticFieldID(idClass, "tv", "I");
jint tvId = env->GetStaticIntField(idClass, tvField);
// thiz is a jobject corresponding to the calling Activity instance
jmethodID findViewById = env->GetMethodID(env->GetObjectClass(thiz), "findViewById", "(I)Landroid/view/View;");
jobject tv = env->CallObjectMethod(thiz, findViewById, tvId);
jmethodID setText = env->GetMethodID(env->GetObjectClass(tv), "setText", "(Ljava/lang/CharSequence;)V");
env->CallVoidMethod(tv, setText, env->NewStringUTF("Hello from JNI!"));

How to set IntField from native jni to Java Android 10

After update my project to support Android 10, a crash produced in the JNI level related to non-SDK interface restrictions in Android 10 :
JNI DETECTED ERROR IN APPLICATION: JNI SetIntField called with pending exception java.lang.NoSuchFieldError: no "I" field "value" in class "Ljava/lang/Integer;" or its superclasses
which come from this part of code :
jclass clazz = (*env)->GetObjectClass(env, outputObj);
jfieldID mi = (*env)->GetFieldID(env, clazz, "value", "I");
(*env)->SetIntField(env, outputObj, mi, pListLen);
To fix that i replace it by :
jclass clazz = (*env)->GetObjectClass(env, outputObj);
jmethodID intValueMethod = (*env)->GetMethodID(env, clazz, "intValue", "()I");
jint result = (*env)->CallIntMethod(env, outputObj,intValueMethod,pListLen);
After that my application doesn't crash and find the integer value correctly, but i want to set the result integer on Java code using the same method SetIntField.
Could you please give me a way or method to set the result on the Java part.

As changing the intger field is not a good practice on JNI part, so i changed the way instead changing it by reflection method
jclass clazz = (*env)->GetObjectClass(env, outputBufLen);
jmethodID value_of = (*env)->GetStaticMethodID(env,clazz, "valueOf", "(I)Ljava/lang/Integer;");
jobject result = (*env)->CallStaticObjectMethod(env,clazz, value_of, pOutBufLen);
I set the result under an int Array and i return it :
int result = (*env)->CallStaticObjectMethod(env,clazz, value_of, pOutBufLen);
jintArray resultArray = (*env)->NewIntArray(env, 2);
jint fill[2];
fill[0] = result;
fill[1] = (jint) pOutBufLen;
(*env)->SetIntArrayRegion(env, result, 0, 2, fill);
return (resultArray);

java lang class exception -Unable to cast the java vector<user defined java class> return from c++ jni object callback

I am not able to cast back the user self defined class return from C++ jni callback
code snippet as follow:
//Kotlin class
data class class_record(var id:String?,var class_name:String?,var class_type)
// com.example.MainActivity
public native Object Cls(String id);
Vector vec_ classrecord=new Vector();
vec_classrecord=(Vector)Cls("1234");
// c++ jni code
extern "C"
JNIEXPORT jobject JNICALL
Java_com_example_MainActivity_Cls(JNIEnv *env, jobject instance, jstring id,
) {
jclass java_vector_class;
jmethodID java_vector_method;
jobject java_vector_object ;
auto vec_record=// call c++ method that return vector for class record pointer
jstring jni_str;
jclass javaClassRef;
// jni for java.util.Vector
java_vector_class = env->FindClass("java/util/Vector");
java_vector_method_constructor = env->GetMethodID(java_vector_class, "<init>", "()V");
java_vector_object = env->NewObject(java_vector_class, java_vector_method_constructor, "");
for (auto record_it = vec_record.begin(); record_it < vec_record.end(); ++record_it) {
// jni for class_record
jclass java_class = env->FindClass("com/example/class_record");
javaClassRef = (jclass) env->NewGlobalRef(java_class);
jmethodID cls_constructor = env->GetMethodID(javaClassRef, "<init>", "()V");
jobject cls_object = env->NewObject(javaClassRef, cls_constructor, "");
// set id
javaMethodRef = env->GetMethodID(javaClassRef, "setId", "(Ljava/lang/String;)V");
std::string strval = record_it.id;
jni_str = env->NewStringUTF(strval.c_str());
env->CallVoidMethod(cls_object, javaMethodRef, jni_str);
// set class_name
javaMethodRef = env->GetMethodID(javaClassRef, "setClass_name","(Ljava/lang/String;)V");
std::string strval = record_it.class_name;
jni_str = env->NewStringUTF(strval.c_str());
env->CallVoidMethod(cls_object, javaMethodRef, jni_str);
//set class_type
javaMethodRef = env->GetMethodID(javaClassRef, "setClass_type","(Ljava/lang/String;)V");
std::string strval = record_it.class_type;
jni_str = env->NewStringUTF(strval.c_str());
env->CallVoidMethod(cls_object, javaMethodRef, jni_str);
jmethodID java_vector_add = env->GetMethodID(java_vector_class, "addElement","(Ljava/lang/Object;)V");
**env->CallVoidMethod(java_vector_object, java_vector_add, javaClassRef);**
}
return java_vector_object;
}

env->CallVoidMethod(java_vector_object, java_vector_add, cls_object);
Under Kotlin environment , it it much better to express jni c++ callback in ArrayList instead of vector which grow almost double the size with snippet as follow in situation where the returned arraylist is fixed size immuatable.
java_util_class = env->FindClass("java/util/ArrayList");
jmethodID java_add= env->GetMethodID(java_util_class, "add","(Ljava/lang/Object;)Z");
env->CallBooleanMethod(java_object, java_add, cls_object);
sample tutorial

How to call JNI Function from a method in C library

I'm currently developping an android app with android studio, for this project i need to use a custom library written in c/c++ .
So i in order to do that i needed to use NDK.
The library contain methods that i need to implements in order to have access to specifics fonctions in android
My question is : how can I call my jni method inside the method existing in c
which will call a java method to store session key in the android system
So for a pratical exemple since it's maybe not clear:
In a file called lib_exemple.c i have three methods
the first one is the initialisation call to the library (jni to library c) -->working
JNIEXPORT void JNICALL
Java_com_example_libExemple_Libs_ExempleLib_lib_1Exemple_1init
(JNIEnv *env, jobject instance) {
lib_Example_init('0'); // c func of the library
}
then the second one is the jni who call a java method (jni to java) -> working
JNIEXPORT jint
JNICALL Java_com_example_libExemple_Libs_ExempleLib_lib_1Exemple_1store_1session_1key
(JNIEnv * env, jobject jobject1, jbyte jbyte1, jchar jchar1, jbyte jbyte2){
jclass clazz = (*env)->FindClass(env, "com/example/libExemple/Libs/ExempleLib");
jmethodID mCurrentActivityId = (*env)->GetMethodID(env, clazz, "KeyStoreSessionKey", "(BCB)I");
jint result = (*env)->CallIntMethod(env, jobject1, mCurrentActivityId, jbyte1, jchar1, jbyte2);
return result;
}
and the third method is the one the library c have in it (library C to jni)
int lib_Exemple_store_session_key(uint8_t Session, P_KEY_ST_T pKey, uint8_t keyType) {
//i want to call jni func here , so the librairy can access the native android function
return 0;
}
then to configure the ndk in a file called ExempleLib.java i have defined
static {
System.loadLibrary("LibExemple");
}
then the prototype of the initialisation of the library
public native void libExemple_init();
the prototype of the first method in lib_exemple.c
public native int lib_Exemple_store_session_key(byte pKey, char key_length, byte keyIndex);
and the java fonction called by it
protected int KeyStoreSessionKey(byte pKey, char key_length, byte keyIndex) {
...
}
my mainActivity contain
ExempleLib LibFunc = new ExempleLib();
Log.d(TAG, "init lib" );
LibFunc.lib_Exemple_init();
My goal is that after initialising my library and call an init function , it can call the first method in the second method of lib_exemple.c (the one used in the library )
thanks
EDIT :
The solution of my problem is when i initialize the lib , i need to save jvm
so i can access the JNIenv in the library method directly
jobject Savedinstance = NULL;
static JavaVM *cachedJVM;
JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM *jvm, void *reserved) {
cachedJVM = jvm;
return JNI_VERSION_1_6;
}
JNIEnv *AttachJava() {
JavaVMAttachArgs args = {JNI_VERSION_1_6, 0, 0};
JNIEnv *java;
(*cachedJVM)->AttachCurrentThread(cachedJVM, &java, &args);
return java;
}
JNIEXPORT jint JNICALL
Java_com_example_vgosselin_libExemple_Libs_ExempleLib_lib_1init(JNIEnv *env,
jobject instance) {
Savedinstance = instance;
void *element = 0;
jint result;
result = lib_Exemple_init_();
return result;
}
so i can call the java method in
int lib_Exemple_store_session_key(uint8_t Session, P_KEY_ST_T pKey, uint8_t keyType) {
JNIEnv *NewEnv = AttachJava();
jclass clazz = (*NewEnv)->FindClass(NewEnv,"com/example/vgosselin/libExemple/Libs/ExempleLib");
jmethodID mCurrentActivityId = (*NewEnv)->GetMethodID(NewEnv,clazz,"KeystoreStoreKey","([BCB)I");
jint result;
jbyteArray Data = 0;
jchar Key = 0;
jbyte Type = 0;
result = (*NewEnv)->CallIntMethod(NewEnv, Savedinstance, mCurrentActivityId, Data, Key, Type);
return result;
}
and now i don't need the method in jni syntax anymore
JNIEXPORT jint JNICALL Java_com_example_libExemple_Libs_ExempleLib_lib_1Exemple_1store_1session_1key

Invalid indirect reference on NewObject call

OK, so I have the native code below.
I'm trying to return an array of FilePermissionInfo from it, populated with some data returned by stat().
The problem is that I get the following error when NewObject is called the first time:
06-15 20:25:17.621: W/dalvikvm(2287): Invalid indirect reference
0x40005820 in decodeIndirectRef 06-15 20:25:17.621: E/dalvikvm(2287):
VM aborting
It's odd, because the only reference object I have is the jclass (for FilePermissionInfo) and I turn it to a global reference.
The code is:
JNIEXPORT jobjectArray JNICALL
Java_com_mn_rootscape_utils_NativeMethods_getFilesPermissions( JNIEnv* env, jobject thizz, jobjectArray filePathsArray )
{
jobjectArray result;
int size = (*env)->GetArrayLength(env, filePathsArray);
jboolean isCopy;
jclass filePermInfoCls = (*env)->FindClass(env, kFilePermissionInfoPath);
if(!filePermInfoCls)
{
LOGE("getFilesPermissions: failed to get class reference.");
return NULL;
}
gFilePermInfoClass = (jclass)(*env)->NewGlobalRef(env, filePermInfoCls);
LOGI("got gFilePermInfoClass");
jmethodID filePermInfoClsConstructor = (*env)->GetMethodID(env, gFilePermInfoClass, "<init>", kFilePermInfoConstructorSig);
if(!filePermInfoClsConstructor)
{
LOGE("getFilesPermissions: failed to get method reference.");
return NULL;
}
struct stat sb;
LOGI("starting...");
result = (jobjectArray)(*env)->NewObjectArray(env, size, gFilePermInfoClass, NULL);
for(int i = 0; i != size; ++i)
{
jstring string = (jstring) (*env)->GetObjectArrayElement(env, filePathsArray, i);
const char *rawString = (*env)->GetStringUTFChars(env, string, &isCopy);
if(stat(rawString, &sb) == -1)
{
LOGE("stat error for: %s", rawString);
}
LOGI("%ld %ld %ld %ld %ld %ld %ld %ld", sb.st_dev, sb.st_mode, sb.st_nlink, sb.st_uid, sb.st_gid, sb.st_atime, sb.st_mtime, sb.st_ctime);
jobject permInfo = (*env)->NewObject(env,
gFilePermInfoClass,
filePermInfoClsConstructor,
(long)sb.st_dev,
(long)sb.st_mode,
(long)sb.st_nlink,
(long)sb.st_uid,
(long)sb.st_gid,
(long)sb.st_atime,
(long)sb.st_mtime,
(long)sb.st_ctime,
"",
"",
1,
"");
LOGI("xxx1");
(*env)->SetObjectArrayElement(env, result, i, permInfo);
LOGI("xxx2");
(*env)->ReleaseStringUTFChars(env, string, rawString);
LOGI("xxx3");
}
(*env)->DeleteLocalRef(env, filePermInfoCls);
return result;
}
The Java class constructor signature and path are:
const char* kFilePermissionInfoPath = "com/mn/rootscape/utils/FilePermissionInfo";
const char* kFilePermInfoConstructorSig = "(JJJJJJJJLjava/lang/String;Ljava/lang/String;ZLjava/lang/String;)V";
Please note that if I call NewObject on the default constructor then it works fine.

OK, found it.
It was a problem with the jstring parameters. It turns out you cannot pass empty strings (or even NULL for that matter) as a jstring.
Instead I used (*env)->NewStringUTF(env, NULL) to create a NULL jstring.
Seems to work OK now.
Since this question generated somewhat a high activity, I'm posting the final solution below. Note that the nullString variable is being deallocated at the end of its scope (or when you're done using it):
jstring nullString = (*env)->NewStringUTF(env, NULL);
...
jobject permInfo = (*env)->NewObject(env,
gFilePermInfoClass,
filePermInfoClsConstructor,
(jbyte)permsOwner,
(jbyte)permsGroup,
(jbyte)permsOthers,
(jlong)sb.st_uid,
(jlong)sb.st_gid,
(jlong)sb.st_atime,
(jlong)sb.st_mtime,
(jlong)sb.st_ctime,
nullString,
nullString,
(jboolean)1,
nullString);
...
(*env)->DeleteLocalRef(env, nullString);