CallObjectMethod returns no string
My goal is to instantiate a class Java
in C++
and then call the methods defined in the class Java
.
Here's a summary of my class Java
:
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;
}
Here's a quick summary of the code C++
used to interact with this class:
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;
}
So my problem is that the call CallObjectMethod
doesn't return anything. I tried to change the Java method to static
, but it didn't change.
In general, how would you return a string from Java
to C++
? Maybe my starting idea is bad.
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So, sorry for taking so long, but I solved the problem. Basically, this is a small typo causing havoc (at least in your MWE):
this->IDgetString = this->JNIEnvironment->GetStaticMethodID(this->javaClass
, "getString"
, "(()Ljava/lang/String;");
// ^ This parenthesis must be removed.
Also, I noticed that you did not initialize the object. I ditched your code to use instace methods instead of static methods and MWE works fine for me.
Here is the code for EXICodec.java
I used and here is the code for JavaInterface.cpp
.
I looked at the original code again and found something else.
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;");
//} <- This brace must be further down
if (this->IDgetInputXML == NULL)
{
throw std::runtime_error("JAVA_INIT_METHOD_EXCEPTION");
}
} // <- To here
I'm sure this is just a typo, but if not, it might be causing the problems in your original example.
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