memory leak while calling java from c++ with jni

Question

I have a Java-Application, that merges some large files. The Java-Application isn't under my control. The result from the Java-Applicationis returned as a approimatly 90 Mb large string to my C++ Program where I use it in some algorythms. I call the execute-mathod several times. My problem is, every time I call the Java-Application it reserves more memory but doesnt free it. From this Garbage collection and JNI call question I had the idea to call the garbage collector manually but it frees no memory. Any idea to fix that problem?

Here is my C++-Program

void JavaWrapperClass::CreateVM(string Classpath)
{
        Classpath.insert(0,"-Djava.class.path=");
                             // Pointer to native interface
        //================== prepare loading of Java VM ============================
        JavaVMInitArgs vm_args;                        // Initialization arguments
        JavaVMOption* options = new JavaVMOption[2];   // JVM invocation options

        options[0].optionString =const_cast<char*>(Classpath.c_str()); // where to find java .class
        string maxMemOption=string("-Xmx")+to_string(logicalSolverMaxMem)+"m";
        options[1].optionString=const_cast<char*>(maxMemOption.c_str());

        vm_args.version = JNI_VERSION_1_8;             // minimum Java version
        vm_args.nOptions = 2;                          // number of options
        vm_args.options = options;
        vm_args.ignoreUnrecognized = false;     // invalid options make the JVM init fail

        //=============== load and initialize Java VM and JNI interface =============
        jint rc = JNI_CreateJavaVM(&jvm, (void**) &env, &vm_args);  // YES !!
        delete options;    // we then no longer need the initialisation options.
        if (rc != JNI_OK)
        {
            throw bad_exception();
        }
}

---

const string* JavaWrapperClass::Execute(const string& Filename, const string& HV, const string& NV,
        const string& FileId)
{

    mergedFilesStr.erase();
    mergedFilesStr.shrink_to_fit();


    jclass javaClass = env->FindClass("Path_to/My_Class");  // try to find the class
    if (javaClass == nullptr)
    {
        throw JavaWrapper_JNI_runtime_exception("class Path_to/My_Class not initialized!");
    }

    jmethodID ctor = env->GetMethodID(javaClass, "<init>", "()V");  // FIND AN OBJECT CONSTRUCTOR
    if (ctor == nullptr)
    {
        env->DeleteLocalRef(javaClass);
        throw JavaWrapper_JNI_runtime_exception("Constructor not found");
    }

    jobject javaObject;
    javaObject = env->NewObject(javaClass, ctor);
    if (javaObject==nullptr)
    {
        env->DeleteLocalRef(javaClass);
        throw JavaWrapper_JNI_runtime_exception("Could not create Java-Object");
    }

    jmethodID mid =
            env->GetMethodID(javaClass, "execute",
                    "(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;"); // find method
    if (mid == nullptr)
    {
        env->DeleteLocalRef(javaObject);
        env->DeleteLocalRef(javaClass);
        throw JavaWrapper_JNI_runtime_exception("Method string execute(String odx_Filename, String HV, String NV, String FileId) not found !");
    }
    else
    {
        logger->debug("Found JAVA method execute. => Call execute");
        jstring filename = env->NewStringUTF(Filename.c_str());
        jstring hv = env->NewStringUTF(HV.c_str());
        jstring nv = env->NewStringUTF(NV.c_str());
        jstring FileId = env->NewStringUTF(FileId.c_str());
        jstring retString = (jstring) env->CallObjectMethod(javaObject, 
            mid, filename, hv, nv, FileId);   // call the method "execute" with arguments.

        jboolean isCopy=JNI_TRUE;
        const char *mergedFilesPtr;
        mergedFilesPtr = env->GetStringUTFChars(retString, &isCopy);
        mergedFilesStr= new string(mergedFilesPtr);


        if (isCopy == JNI_TRUE) 
        {
            //Release memory from Return-String
            env->ReleaseStringUTFChars(retString, mergedFilesPtr);
        }
        callGarbageCollector();

        env->DeleteLocalRef(filename);
        env->DeleteLocalRef(hv);
        env->DeleteLocalRef(nv);
        env->DeleteLocalRef(FileId);
    }

    env->DeleteLocalRef(javaObject);
    env->DeleteLocalRef(javaClass);
    callGarbageCollector();

    return &mergedFilesStr;
}

---

void JavaWrapperClass::callGarbageCollector()
{
    jclass    systemClass    = nullptr;
    jmethodID systemGCMethod = nullptr;

    systemClass    = env->FindClass("java/lang/System");
    systemGCMethod = env->GetStaticMethodID(systemClass, "gc", "()V");
    env->CallStaticVoidMethod(systemClass, systemGCMethod);

    env->DeleteLocalRef(systemClass);
}

Answer 1

There are a couple issues with your memory handling here:

1. in void JavaWrapperClass::CreateVM(string Classpath) function, you are using new[] operator on options variable but then you are deleting it using delete . This is UB. you must use delete[] to delete this pointer correctly.
2. in const string* JavaWrapperClass::Execute(const string& Filename, const string& HV, const string& NV, const string& FileId) function you are returning a pointer to string named mergedFilesStr . In your function buddy you are allocating this pointer and then returning it, So you need to delete it after you are done using it (do you?). I can see you used erase() and shrink_to_fit() in the beginning of your function but beware that this function has nothing to do with deleting your allocated pointer, So you need to delete it your self like this : delete mergedFilesStr;

Anyway the better approach is using smart pointers . you can avoid this kind of manual memory management problems by using them, They are awesome.

Answer 2

You should always call ReleaseStringUTFChars regardless of whether isCopy is true or not: Should you call ReleaseStringUTFChars if GetStringUTFChars returned a copy? .

I don't know how mergedFilesStr is being decalred/used but it could also be the source of your leak. Your example seems to use it as an object at the beginning of ::Execute but then as a pointer later on? In fact I don't see how the code as posted even compiles.