Calling a java function from C++ via JNI that returns a string

Question

Possible Duplicate:
How to access the Java method in a C++ application

Suppose I have a Java class like this :

class MyClass
{
  String value = "a string value";

  String getValue()
  {
    return value;
  }
}

I've been trying for hours to implement a JNI function that calls a Java function and returns a string. Could someone show me through a snippet how to call the "getValue" function from a C++ using JNI and obtain a jstring variable with the value of String variable from "MyClass.

// C++

jobject result;

jMethodID method_getValue = m_env->GetMethodID(native_object,"getValue","()Ljava/lang/String;");

result = m_env->CallObjectMethod(native_object, method_getValue);

Show us what you've tried and tell us why it isn't acceptable. — President James K. Polk, Dec 05 '10 at 15:22
Did you call your C program from Java. JNI must be initiated by the JVM. The C++ program can call java classes, but, it must have been called from Java to begin with. — James Anderson, Nov 08 '11 at 09:05
@JamesAnderson That's not actually so. The program can initially be a C or C++ program and it can start the JVM itself via the Invocation API. Basically this is all irrelevant to the OP's actual problem. — user207421, Sep 22 '12 at 09:52

score 6 · Answer 1 · edited Nov 08 '11 at 08:10

jMethodID method_getValue = m_env->GetMethodID(native_object,"getValue","()Ljava/lang/String;");

here, native_object is supposed to be the class definition object (jclass) of MyClass

jmethodID GetMethodID(JNIEnv *env, jclass clazz, const char *name, const char *sig);

whereas to here:

result = m_env->CallObjectMethod(native_object, method_getValue);

NativeType CallMethod(JNIEnv *env, jobject obj, jmethodID methodID, ...);

Your CallObjectMethod expects as first parameter an object from MyClass, no jclass. http://download.oracle.com/javase/1.4.2/docs/guide/jni/spec/functions.html

so either one of the calls is wrong here...

probably the getMethodID... you should definitely check for NULL there.

cheers,

Calling a java function from C++ via JNI that returns a string

1 Answers1

Linked