Sending jstring from jni c-code to a java function that takes a string as an argument

Question

How to send jstring code from JNI C to a Java function that receives a String as an argument?

I read about functions like callVoidMethodA(....) , but they take nothing, like jstring .

Answer 1

You did not specify the name of the target class and the signature of the target method. Therefore, consider, for example, the java.lang.StringBuilder.append (java.lang.String) method.

 // First lets assume you have already received the next variables void foo( JNIEnv* env, jobject obj ) { // Call StringBuilder.append() method } 

Now you need the class name and method signature (according to docs )

 static char const StringBuilder_ClassName = "java/lang/StringBuilder"; static char const StringBuilder_append_MethodName = "append"; static char const StringBuilder_append_MethodSignature = "(Ljava/lang/String;)Ljava/lang/StringBuilder;"; 

To call a java method from JNI code, you should get jmethodID

 static jclass StringBuilder_Class = 0; static jmethodID StringBuilder_append_Method = 0; void Init( JNIEnv* env ) { if( StringBuilder_Class == 0 ) { StringBuilder_Class = (*env)->FindClass( env, StringBuilder_ClassName ); // TODO: Handle error if class not found } if( StringBuilder_append_Method == 0 ) { StringBuilder_append_Method = (*env)->GetMethodID( env, StringBuilder_Class, StringBuilder_append_MethodName, StringBuilder_append_MethodSignature ); // TODO: Handle error if method not found } } void foo( JNIEnv* env, jobject obj ) { Init(); char* str; // str = ...; jstring jString = (*env)->NewStringUTF( env, str ); // Because StringBuild.append() returns object, you should call CallObjectMethod jobject ret = (*env)->CallObjectMethod( env, obj, jString ); // Here you can release local references, ie // (*env)->DeleteLocalRef( env, ret ); // (*env)->DeleteLocalRef( env, jString ); // But it is not necessary. Local references are released automatically when // thread returns from JNI code to Java code. // So you can ignore the returned value and not to release the jString local // reference, ie just call // (*env)->CallObjectMethod( env, obj, jString ); } 

Answer 2

 JNIEXPORT jstring JNICALL Java_Prompt_getLine(JNIEnv *env, jobject obj) { char *buf = "Hi !!!!"; jstring jString = (*env)->NewStringUTF(env, buf); free(buf); return jString; }