I have a Java instance method which returns a String and I'm calling this method through JNI in C++. I have written the following code:
const char *DiagLayerContainer_getDESC(JNIEnv *env, jobject diagLayer) {
jclass diagLayerClass = env->FindClass(PARSER_CLASS);
jmethodID getDESCDiagLayerMethodID = env->GetMethodID(diagLayerClass, "getDESCDiagLayer", "(Ljava/lang/Object;)Ljava/lang/String;");
jstring returnString = (jstring) env->CallObjectMethod(diagLayer, getDESCDiagLayerMethodID);
return env->GetStringUTFChars(returnString, JNI_FALSE);
}
How do I get the string and convert it to a const char *?
My program crashes on the last line with access violation to 0x00000000. returnString is not NULL.
According to GetStringUTFChars, the last parameter is a pointer to jboolean.
Change
return env->GetStringUTFChars(returnString, JNI_FALSE);
to
return env->GetStringUTFChars(returnString, NULL);
Or better yet, return a std::string
std::string DiagLayerContainer_getDESC(...) {
...
const char *js = env->GetStringUTFChars(returnString, NULL);
std::string cs(js);
env->ReleaseStringUTFChars(returnString, js);
return cs;
}
I've built a similar simple example and the code as is, seems fine so far.
Although, there are two possible error sources.
The first one is the method signature. Try "()Ljava/lang/String;" instead of "(Ljava/lang/Object;)Ljava/lang/String;".
The second one is in the java source itself. If the java method returns a null string, CallObjectMethod() will return a NULL jstring and GetStringUTFChars() fails.
Add a
if (returnString == NULL)
return NULL;
after CallObjectMethod().
So look into the java source and see, whether the method getDESCDiagLayer() might return a null string.
Related
This is my PseudoStream class which interfaces with C++
class PseudoStream{
companion object{
// This native function receives a String as a Input and returns a Modified output
external fun output(input: String): String
}
}
This is the C++ Inteface code to for the output function
// Creates a jstring from a std::string passed as mystr
jstring Create_Java_String(JNIEnv *environment, const std::string& mystr){
return environment->NewStringUTF(mystr.data());
}
// Converts a jstring to std::string and returns it
std::string jstringtostring(JNIEnv *env, jstring jStr) {
if(!jStr)
return "";
const jclass stringClass = env->GetObjectClass(jStr);
const jmethodID getBytes = env->GetMethodID(stringClass, "getBytes", "(Ljava/lang/String;)[B");
const jbyteArray stringJbytes = (jbyteArray) env->CallObjectMethod(jStr, getBytes, env->NewStringUTF("UTF-8"));
size_t length = (size_t) env->GetArrayLength(stringJbytes);
jbyte* pBytes = env->GetByteArrayElements(stringJbytes, NULL);
std::string ret = std::string((char *)pBytes, length);
env->ReleaseByteArrayElements(stringJbytes, pBytes, JNI_ABORT);
env->DeleteLocalRef(stringJbytes);
env->DeleteLocalRef(stringClass);
return ret;
}
// This is the actual function called by jni to modify the String
extern "C"
JNIEXPORT jstring JNICALL
Java_com_phantom_automath_PseudoStream_00024Companion_output(JNIEnv *env, jobject thiz, jstring input) {
std::string expression = jstringtostring(env, input);
return Create_Java_String(env, expression + " (Modified by C++)");
}
Now, this code is the main UI code
var text_input = remember {mutableStateOf("Initial value ")} // Used by the TextField for Input
var text_output = remember {mutableStateOf("")} // Used by Text() for modified output
TextField(
value = text_input.value,
onValueChange = { text_input.value = it },
)
Button(onClick = {
// The jni method is called here
text_output.value = PseudoStream.output(text_input.value)
}
){
Text("Modify")
}
Text(text_output.value) // Displays the output by the jni call
So here is my problem, Whenever I click the Modify Button without changing the initial value of the TextField. The code works as expected. But, once I modify the TextField and then click Modify Button again this runtime error occurs in kotlin code.
java.lang.NullPointerException: Parameter specified as non-null is null: method kotlin.jvm.internal.Intrinsics.checkNotNullParameter, parameter interactionSource
at androidx.compose.foundation.ClickableKt.clickable(Unknown Source:7)
at androidx.compose.foundation.ClickableKt.clickable$default(Clickable.kt:112)
at androidx.compose.material.ButtonKt.Button(Button.kt:117)
The code works fine as long as I don't modifiy the value inside the TextField
This is not the error caused by JNI , this issue is with how you are passing your TextField value to the jniMethod . The error in the stack trace says :
java.lang.NullPointerException: Parameter specified as non-null is null.
This means that you are passing null value to the function .
The code works fine as long as I don't modifiy the value inside the TextField -> This is because you have set the initial value of text_input , so a non-null value is passed ,thus causing no-error .
You need to do the following to correct your error :
Just change this line :
text_output.value = PseudoStream.output(text_input.value)
To :
text_output.value = PseudoStream.output(text_input.value.text)
Here you pass the text that you given as an input to the TextField instead of the TextField data class which will do the trick .
To know more about the above said , debug text_input.value and know your mistake .
I have finally figured out why my app is not working. It has something to do with nullables inside the code logic of recompostion. Since, I have updated my android project to use the stable release of compose. The code is actually working as expected.
i am using a JNI to fetch signature of apk and i am getting it very well. when i calling this method from java first time i am getting the exact value. calling it again i am getting appended values with exact value (eg 1234456123456). PFB the code which i am using
char* getSignatureMd5(JNIEnv* env, jobject obj)
{
char* sign = loadSignature(env, obj);
MD5_CTX context = { 0 };
MD5Init(&context);
MD5Update(&context, (unsigned char*)sign, strlen(sign));
unsigned char dest[16] = { 0 };
MD5Final(dest, &context);
int i;
static char destination[32]={0};
for (i = 0; i < 16; i++) {
sprintf(destination, "%s%02x", destination, dest[i]);
}
return destination;
}
getToken JNI method
JNIEXPORT jstring JNICALL Java_com_sign_signaturecapturesbi_MyAdapter_getToken(JNIEnv *env, jobject obj)
{
char* signValue = getSignatureMd5(env, obj);
__android_log_print(ANDROID_LOG_VERBOSE, "MyApp", "signValue %s", signValue);
return (*env)->NewStringUTF(env, signValue);
}
These lines cause undefined behavior:
for (i = 0; i < 16; i++) {
sprintf(destination, "%s%02x", destination, dest[i]);
}
man 3 printf:
C99 and POSIX.1-2001 specify that the results are undefined if a call
to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause
copying to take place between objects that overlap (e.g., if the
target string array and one of the supplied input arguments refer to
the same buffer).
Moreover destination is static and because of this it keeps its content between calls. Together these points give you such a weird behavior.
Since dest size is well known, you can simply unroll loop, also don't forget to add one extra cell to destination for terminating \0. And, if possible, you should use snprintf() instead:
static char destination[33];
snprintf(destination, sizeof destination,
"%02x%02x%02x%02x%02x%02x%02x%02x"
"%02x%02x%02x%02x%02x%02x%02x%02x",
dest[0], dest[1], dest[2], dest[3],
dest[4], dest[5], dest[6], dest[7],
dest[8], dest[9], dest[10], dest[11],
dest[12], dest[13], dest[14], dest[15]);
In this case you can leave destination as static one, since your code doesn't relay on its content anymore. But note that getSignatureMd5() returns pointer to the same buffer each time you call it, as result subsequent calls erase result obtained by previous calls.
I have implemented Jansson in Android with C and made a function which calculates values from json and that works in C, I tried to use that code in NDK with JNI it builds with no errors, but as i tried to arrange the code to work with JNI it gives me pointer error warning: return from incompatible pointer type. I have read that i need to use jlong for pointers but i cant figure out how that works, it is my first time working in it.
This is my code from C (gives no errors and compiles)
char *doCalc (char *invoice_str) {
json_error_t error;
json_t *invoice = json_loads (invoice_str, JSON_DISABLE_EOF_CHECK, &error);
...
char *result = json_dumps (json_data, JSON_PRESERVE_ORDER);
return result;
}
C code Arranged to work with JNI (gives me error warning: return from incompatible pointer type, which if im correct is because of jchar)
JNIEXPORT jchar JNICALL *Java_com_example_test_doCalc (JNIEnv* env, jobject obj,char const *invoice_str) {
json_error_t error;
json_t *invoice = json_loads (invoice_str, JSON_DISABLE_EOF_CHECK, &error);
...
char *result = json_dumps (json_data, JSON_PRESERVE_ORDER);
return result;
}
Then in my Activity I like to would run doCalc(charJ);, charJ has Json in it. Which would then give me dump of calculated values.
Also I might be looking at this completely wrong, any help is appreciated.
Try to use jstring instead of char*
JNIEXPORT jchar JNICALL * Java_com_example_test_doCalc(JNIEnv * env, jobject obj, jstring invoice_jstring) {
//convert invoice_jstring to char* link bellow
json_error_t error;
json_t * invoice = json_loads(invoice_str, JSON_DISABLE_EOF_CHECK, & error);
...
char * result = json_dumps(json_data, JSON_PRESERVE_ORDER);
return result;
}
for conversion jstring to char* you can use this answer:
JNI converting jstring to char *
I want to call the Java method from Jni code with the int and int[] arguments. for that i have Go-ogled and found the following sample .
Calling a java method from c++ in Android
And it worked fine with String parameter . But While trying with int i got issues .
Please help me .
JNI CODE:
jstring Java_com_calljavafromjni_MainActivity_getJniString( JNIEnv* env, jobject obj){
jstring jstr = (*env)->NewStringUTF(env, "RAJESH TEST from JNI ");
jint sss=1111;
jclass clazz = (*env)->FindClass(env, "com/calljavafromjni/MainActivity");
jmethodID messageMe = (*env)->GetMethodID(env, clazz, "messageMe", "(Ljava/lang/String;)Ljava/lang/Integer;");
jobject result = (*env)->CallObjectMethod(env, obj, messageMe, sss);
const char* str = (*env)->GetStringUTFChars(env,(jstring) result, NULL); // should be released but what a heck, it's a tutorial :)
printf("%s\n", str);
return (*env)->NewStringUTF(env, str);
}
Java code
public String messageMe(Integer text) {
System.out.println( "aaaaaaaaaaaaaaaa "+text);
return "test";
}
I don't see where int[] come into your problem, but with int it should be easy to solve.
You need to look at your GetMethodId() call, specifically the method signature argument (the last one). The JNI Specification provides a list of all its Type Signatures here. That should also help you when you eventually come to pass your int arrays too.
So we can see at the moment your signature is:
String messageMe(Integer text)
but you told JNI it was (Ljava/lang/String;)Ljava/lang/Integer; which translates to something like:
java.lang.Integer messageMe(String text)
The Type Signatures show us that the signature for an int is simply I so your argument for GetMethodId() should be something like this:
jmethodID messageMe = (*env)->GetMethodID(env, clazz, "messageMe", "(I)Ljava/lang/String;");
I hope that helps. As I said before, JNI isn't the easiest thing to get into but the answers really are all in the Specification, you just have to look quite hard.
EDIT: I corrected my signature.
Basically, you were almost there - you just got the arguments and return value the wrong way around in the signature. It should be (<arguments>)<return type>. You also made the easy mistake of specifying the class for Integer, instead of the primitive type.
This is a follow-up to another question I asked: Android -- get MEID from JNI
I am trying to get the ID of a phone in Android. I have some JNI code and a simple test app to call the JNI code. Here is working Java code from my simple test app:
TelephonyManager tm = (TelephonyManager)this.getSystemService(Context.TELEPHONY_SERVICE);
String id = tm.getDeviceId();
The string id is set to the phone ID value that I want. But I need to get it from JNI, and just using the above code and passing the ID value in is not an acceptable solution. (This is to make some JNI code somewhat tamper-proof and I shouldn't trust the Java layer to send a correct ID value.)
Here is the JNI code that I have written, with error handling removed so it is easier to follow. It works until the indicated line, and then the whole app crashes.
// "env" and "obj" are passed to a JNI function and are used unmodified in this code
// JNIEnv *env, jobject obj
jclass cls_context = NULL;
jclass cls_tm = NULL;
jobject tm = NULL;
jmethodID mid;
jfieldID fid;
jstring jstr;
jsize len_jstr;
cls_context = (*env)->FindClass(env, "android/content/Context");
fid = (*env)->GetStaticFieldID(env, cls_context, "TELEPHONY_SERVICE",
"Ljava/lang/String;");
jstr = (*env)->GetStaticObjectField(env, cls_context, fid);
mid = (*env)->GetMethodID(env, cls_context, "getSystemService",
"(Ljava/lang/String;)Ljava/lang/Object;");
tm = (*env)->CallObjectMethod(env, obj, mid, jstr); // THIS LINE CRASHES
cls_tm = (*env)->FindClass(env, "android/telephony/TelephonyManager");
mid = (*env)->GetMethodID(env, cls_tm, "getDeviceId",
"()Ljava/lang/String;");
jstr = (*env)->CallObjectMethod(env, tm, mid);
len_jstr = (*env)->GetStringUTFLength(env, jstr);
(*env)->GetStringUTFRegion(env, jstr, 0, len_jstr, buf_devid);
I think the problem is that obj isn't the right thing to pass, but if so I have no idea what is the right thing. Isn't obj that gets passed to the JNI function the same thing as this in the Java code?
EDIT: Okay, we have figured out that if we add an extra argument of type jobject to the JNI function, and explicitly pass a copy of this in that argument, and then pass that to CallObjectMethod() (the one that crashes in the above code), everything works. We get our TelephonyManager instance and we can query the Phone ID value.
Using a logging macro, I logged the obj pointer and the passed-in this pointer. They are similar numbers (addresses close to each other) but not identical. So I think obj is some sort of object reference from inside the Java VM... it is not actually the same as this.
In a JNI function, the first two arguments are JNIEnv *env and jobject obj. What is that second one for? What can I do with it? Is there any way I can use it to call getSystemService or will I have to pass an extra argument and pass in this?
The problem seems to be related to Java inheritance: in Java, you can call this.getSystemService() and it works, even though this is not actually an instance of Context. When you make the JNI call, the call simply fails.
So our solution was to have our Android app add a .getApplicationContext() method function actually as part of its own class. This in turn calls the actual getSystemService() and returns the result.
The code didn't change: we are still calling
mid = (*env)->GetMethodID(env, cls_context, "getSystemService", "(Ljava/lang/String;)Ljava/lang/Object;");
but now it works, when there is a .getSystemService() method in the class that is calling our JNI function. So, the env argument in a JNI call does represent this (it's not identical... I printed the value of this as a pointer, and printed env, and they are not the same but they are definitely related).
If you want to call a superclass method on object, you should use CallNonvirtualObjectMethod()
How to call an overriden method in JNI
Try this... It works... "context" came from java :)
jclass ctx = env->FindClass("android/content/Context");
jfieldID fid = env->GetStaticFieldID(ctx,"TELEPHONY_SERVICE","Ljava/lang/String;");
jstring str = (jstring) env->GetStaticObjectField(ctx, fid);
jmethodID mid = env->GetMethodID(ctx, "getSystemService","(Ljava/lang/String;)Ljava/lang/Object;");
jobject tm = env->CallObjectMethod(context, mid, str);
jclass ctx_tm = env->FindClass("android/telephony/TelephonyManager");
jmethodID mid_tm = env->GetMethodID(ctx_tm,"getDeviceId","()Ljava/lang/String;");
jstring str_tm = (jstring) env->CallObjectMethod(tm, mid_tm);
strReturn = env->GetStringUTFChars(str_tm, 0);