CallObjectMethod不返回字符串

我的目标是在C++中创建一个Java类的实例，然后调用Java类中定义的方法。

这是我的Java类的总结:

EXICodec.java

public class EXICodec {
    ...
    private static String inputXML;
    ...
    public EXICodec()
    {
        System.out.println("Constructor");  
        this.inputXML = "string";
        ...
    }
    ...
    public static void setInputXML(String inXML)
    {
        inputXML = inXML;
    }
    ...
    public static  String getInputXML()
    {
        return inputXML;
    }

下面是C++代码的摘要，用来与这个类交互:

JavaInterface.hh

class JavaInterface
{
    private:
        JNIEnv*     JNIEnvironment;
        JavaVM*     javaVM;
        jclass      javaClass;
        jobject     javaClassInstance;
        jmethodID   IDconstructor;
        jmethodID   IDsetInputXML;
        jmethodID   IDgetInputXML;
    public:
        JavaInterface();                    
        void init_context( void);               
        void init_class( void);
        void init_methods( void);                   
        void setInputXML( std::string);
        std::string getInputXML( void);
}

JavaInterface.cc

void JavaInterface::init_context(){..}    //initiate the JNIEnvironment & javaVM attributes
void JavaInterface::init_class(){..}      //initiate the javaClass attribute
void JavaInterface::init_methods()
{
     this->IDconstructor = this->JNIEnvironment->GetMethodID(this->javaClass, "<init>", "()V");
     if (this->IDconstructor == NULL) 
     {
         throw std::runtime_error("JAVA_INIT_METHOD_EXCEPTION");
     }
     this->IDsetInputXML = this->JNIEnvironment->GetStaticMethodID(this->javaClass, "setInputXML", "(Ljava/lang/String;)V");
     if (this->IDsetInputXML == NULL) 
     {
         throw std::runtime_error("JAVA_INIT_METHOD_EXCEPTION");
     }
     this->IDgetInputXML = this->JNIEnvironment->GetStaticMethodID(this->javaClass, "getInputXML", "()Ljava/lang/String;");
     if (this->IDgetInputXML == NULL) 
     {
         throw std::runtime_error("JAVA_INIT_METHOD_EXCEPTION");
     }
}      
void JavaInterface::j_constructor()
{
    this->javaClassInstance = this->JNIEnvironment->NewObject(this->javaClass, this->IDconstructor);      
    if (this->javaClassInstance == NULL) 
    {
        throw std::runtime_error("JAVA_CONSTRUCTOR_CALL_EXCEPTION");
    }
}
void JavaInterface::j_setInputXML( std::string str)
{   
    jstring argument = this->JNIEnvironment->NewStringUTF(str.c_str());
    this->JNIEnvironment->CallVoidMethod(this->javaClassInstance, this->IDsetInputXML, argument);
}
std::string JavaInterface::j_getInputXML()
{
    jstring javaString = NULL;
    // ---> PROBLEM HERE : javaString is still NULL after the line below : <---
    javaString = (jstring)this->JNIEnvironment->CallObjectMethod(this->javaClassInstance, this->IDgetInputXML, 0);  
    // jString to char*
    const char *nativeString = this->JNIEnvironment->GetStringUTFChars(javaString, JNI_FALSE);
    // char* to std::string
    std::string str(nativeString);
    this->JNIEnvironment->ReleaseStringUTFChars(javaString, nativeString);
    return str;
}   

所以我的问题是CallObjectMethod的调用不返回任何东西。我试图将Java方法更改为static，但它没有改变。

更一般地说，如何从Java返回C++的字符串?