Memory leak when calling java code from C using JNI

Question

I have a C program that stores some object in java store using JNI. (Before someone ask, using java store is a requirment here and I have to write a client in C which would be able to add and retrieve objects from this store).

I made the program and tried to add 100000 object of size 1KB. But after adding only 50000 objects I am getting 'out of memory' messages (please note that I am printing these 'out of memory' messages whenever I am unable to allocate a new string or byte array using NewStringUTF and NewByteArray functions). At that time my application is using only 80MB of memory. I dont why these methods are returning NULL. Is there something I am missing.

Furthermore, the memory keeps on increasing even though I am releasing byte array and string created for java.

Here is the source code.

    void create_jvm(void)
{
    JavaVMInitArgs vm_args;     
    JavaVMOption vm_options;

    vm_options.optionString = "-Djava.class.path=c:\\Store";
    vm_args.version = JNI_VERSION_1_4;
    vm_args.nOptions = 1;
    vm_args.options = &vm_options;
    vm_args.ignoreUnrecognized = 0;

    JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);

    if(env != null)
    {
        j_store = (*env)->FindClass(env, "com/store");
        if(j_store == null)
        {
            printf("unable to find class. class name: JStore");
        }       
    }   
}

void add(char* key, char* value, int length)
{
    jstring j_key = (*env)->NewStringUTF(env, key);
    jbyteArray j_value = (*env)->NewByteArray(env, length);

    (*env)->SetByteArrayRegion(env, j_value, 0, length, (jbyte *)value);
    ret = (*env)->CallStaticBooleanMethod(env, j_store, method_id, j_key, j_value);

    if(j_value != null)
    {
        (*env)->ReleaseByteArrayElements(env, j_value, (jbyte *)value, 0);
    }
    if(j_key != null)
    {
        (*env)->ReleaseStringUTFChars(env, j_key, key);
    }
}

The java side recieves the data in byte[] and stores it in a hashtable. The issue is that everytime the code runs the memory only adds up and is never released. I tried to add 1 MB object and debugged it.

The process memory increases by 1MB when I call NewByteArray. But when CallStaticBooleanMethod is called the process memory increase by 4MB. And the call to ReleaseByteArrayElements do not release any memory at all.

If I add another 1MB object after this, then process memory remains same when I call NewByteArray and it increase by 1MB when I call CallStaticBooleanMethod but remains the same when I try to release the byte array.

Answer 1

When you call New... functions, you create a "local reference" - reference to this object in local stack frame. This prevents Java VM from GC this object while you still need it. This is fine if you are implementing some native method - its local frame is created only for method call duration. But when you are creating object from a native java-attached thread, it becomes bound to this thread stack frame, which will be destroyed only with this thread.

So, when you done with an object you can call DeleteLocalRef() to tell you no longer need it. Or you can surround the whole add() function with pair of PushLocalFrame()/PopLocalFrame() to make a separate local frame for its duration.

Answer 2

The purpose of the functions ReleaseByteArrayElements and ReleaseStringUTFChars is not to delete the object but to unlock it after a pointer has been obtained with GetByteArrayElements or GetStringUTFChars. The two if statements should be removed.