Myujth

I am up with implementing my Java Native Interface functions in Golang using the golang C lib.

Now I want to convert a jstring to an UTF-8 string using the JNI function GetStringUTFChars but I get an error when doing it. These are the steps I have done:

In my Java class (called MyClass) where I have defined the JNI method, I have:

public static native void print(String msg);

Using javah, I have generated the .h-file with the function defined in C language:

JNIEXPORT void JNICALL Java_com_mypackage_MyClass_print
 (JNIEnv *, jclass, jstring);

Then, in my Go code I have the following code:

package main

// #cgo CFLAGS: -I/Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk/Contents/Home/include
// #cgo CFLAGS: -I/Library/Java/JavaVirtualMachines/jdk1.8.0_131.jdk/Contents/Home/include/darwin
/*
#include <jni.h>
*/
import "C"

//export Java_com_mypackage_MyClass_print
func Java_com_mypackage_MyClass_print(env *C.JNIEnv, clazz C.jclass, str C.jstring) 

 _ = C.GetStringUTFChars(env, str, 0)

When I build the go file using:
go build -buildmode=c-shared -o libmyclass.dylib libmyclass.go
then I get the following error:

could not determine kind of name for C.GetStringUTFChars

How should I call the GetStringUTFChars defined in the JNI spec so I then can print the string with fmt.println?

EDIT 2

Removed "edit 1" since the procedure above was correct, it was just the LD_LIBRARY_PATH variable that was not set.

JNI functions like GetStringUTFChars are function pointers and cannot be called directly from Go. You have to wrap the functions you need in a separate C file. e.g.

jx.c

#include <jni.h>

const char* jx_GetStringUTFChars(JNIEnv *env, jstring str, jboolean *isCopy) 
 return (*env)->GetStringUTFChars(env, str, isCopy);

After creating a library from the C file, your Go file will look something like this:

package main

/*
#cgo CFLAGS: -I/usr/java/jdk1.8.0_162/include/ -I/usr/java/jdk1.8.0_162/include/linux/
#cgo LDFLAGS: -L$SRCDIR/ -ljx

#include "jx.h"
*/
import "C"
import (
 "fmt"
)

//export Java_com_mypackage_MyClass_print
func Java_com_mypackage_MyClass_print(env *C.JNIEnv, clazz C.jclass, str C.jstring) 
 s := C.jx_GetStringUTFChars(env, str, (*C.jboolean)(nil))
 fmt.Println(C.GoString(s))


func main() 

The reason why there is a separate C file just for the wrapper function is because of this clause in the documentation:

Using //export in a file places a restriction on the preamble: since it is copied into two different C output files, it must not contain any definitions, only declarations.