Java from c++ (JNI) memory leaks

Hi all.. I am beginner in Java. My problem is: I am calling a Java class's method from c++. For this i am using JNI. Everythings works correct, but i have some memory LEAKS in the process of c++ program...

So.. i did simple example..
1) I create a java machine (jint res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);)

2) then i take a pointer on java class (jclass cls = env->FindClass("test_jni"));

3) after that i create a java class object object, by calling the constructor (testJavaObject = env->NewObject(cls, testConstruct);)

AT THIS very moment in the process of c++ program is allocated 10 MB of memory

3) Next i delete the class , the object, and the Java Machine ..

AT THIS very moment the 10 MB of memory are not free

.................
So below i have a few lines of code

C++ prgram:

Code:

---

#include <jni.h>

void main()
{
    {
        //Env
        JNIEnv *env;
        // java virtual machine
        JavaVM *jvm;
        JavaVMOption* options = new JavaVMOption[1];
        //class paths
        options[0].optionString = "-Djava.class.path=C:/Sun/SDK/jdk/lib;D:/jms_test/java_jni_leak;";
        // other options
        JavaVMInitArgs vm_args;
        vm_args.version = JNI_VERSION_1_6;
        vm_args.options = options;
        vm_args.nOptions = 1;
        vm_args.ignoreUnrecognized = false;
        // создаем виртуальную машину
        jint res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
        //Creating java virtual machine
        jclass cls = env->FindClass("test_jni");
        // Id of a class constructor
        jmethodID testConstruct = env->GetMethodID(cls, "<init>", "()V");
        // The Java Object
        // Calling the constructor, is allocated 10 MB of memory in c++ process
        jobject testJavaObject = env->NewObject(cls, testConstruct);
        // function DeleteLocalRef,
        // In this very moment memory not free
        env->DeleteLocalRef(testJavaObject);
        env->DeleteLocalRef(cls);
        // 1!!!!!!!!!!!!!
        res = jvm->DestroyJavaVM();
        // In this very moment memory not free. TO ///
    }
}

---

AND Java class

Code:

---

import java.util.*;

public class test_jni
{
  ArrayList<String> testStringList;
  test_jni()
  {
    System.out.println("start constructor");
    testStringList = new ArrayList<String>();
    for(int i = 0; i < 1000000; ++i)
    {
      testStringList.add("TEEEEEEEEEEEEEEEEST");
    }
  }
}

---

Ok i tested the code...

so.. when i call res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args);
allocating some memory... (res == 0)
then:
i call res = jvm->DestroyJavaVM();
(res == 0)
And memory is not deallocating... so why it is ?

The JVM does release its memory but it can't return it to the OS; the JVM and your C++ program run in the same process space (in a different thread most likely) but both subprocesses (C++ and the JVM) claim memory from the OS. After the JVM has released its memory it'll be available to your C++ part of the process.

kind regards,

Jos

ok ... next simple test:
In cycle i'm creating java object and trying to destroy it.... but i cant... so what is wrong
i am doing so:

Code:

---

for(int i = 0; i < 100; ++i)
{
  // call constructor
  // in this moment i am taking  +1MB
  jobject testJavaObject = env->NewObject(cls, testConstruct);
  env->DeleteLocalRef(testJavaObject);
  // after DeleteLocalRef memory are not deallocating.
}
// so after for() i am having + 100MB

---

Quote:

---

Originally Posted by Gigabitze

ok ... next simple test:
In cycle i'm creating java object and trying to destroy it.... but i cant... so what is wrong
i am doing so:

Code:

---

for(int i = 0; i < 100; ++i)
{
  // call constructor
  // in this moment i am taking  +1MB
  jobject testJavaObject = env->NewObject(cls, testConstruct);
  env->DeleteLocalRef(testJavaObject);
  // after DeleteLocalRef memory are not deallocating.
}
// so after for() i am having + 100MB

---

---

How did you check the amount of allocated memory? Did you ask the OS by using 'top' or similar. As I wrote above the memory isn't returned to the OS, it is just available for your process again. You probably don't want to go into the gory details of the brk() and sbrk() system calls.

kind regards,

Jos

I checked the amount of allocated memory, by using task manager...
So when i am doing env->NewObject(cls, testConstruct);
In task manager, my program process is taking + 1MB.

I understood, that the memory availabel for my process again... But i dont whant it :) .... I need free this memory... how can i do it ?

Quote:

---

Originally Posted by Gigabitze

I checked the amount of allocated memory, by using task manager...
So when i am doing env->NewObject(cls, testConstruct);
In task manager, my program process is taking + 1MB.

I understood, that the memory availabel for my process again... But i dont whant it :) .... I need free this memory... how can i do it ?

---

You have to be extremely careful here; the memory was originally obtained from the OS in a stack manner, i.e. more and more pages were claimed from the OS. The JVM (when it was still alive) did the same (it's the only way a process can obtain memory from the OS). When the JVM terminates a chunk of memory isn't needed anymore. If that chunk of memory is at the top of that 'stack' it can be returned to the OS, otherwise, the chunk obtained by the JVM was somewhere in the middle because your C++ part of the process had claimed more memory, nothing can be returned to the OS.

kind regards,

Jos

So... sorry for me so noobi :)... But i cant understand, What i must do ?
Other words... How i must free the memory from c++ process ?

Quote:

---

Originally Posted by Gigabitze

So... sorry for me so noobi :)... But i cant understand, What i must do ?
Other words... How i must free the memory from c++ process ?

---

The C++ heap (used by malloc/free new/delete) might've 'wrapped around' the chunk of memory previously taken by the JVM. If you're new to all this highly complex crapola, forget about it and let the memory be where it is.

kind regards,

Jos

Ok... create thx, that u are helping me..
But, can u give simple example (c++ code), how is it possible... (The C++ heap (used by malloc/free new/delete) might've 'wrapped around' the chunk of memory previously taken by the JVM.)

Quote:

---

Originally Posted by Gigabitze

Ok... create thx, that u are helping me..
But, can u give simple example (c++ code), how is it possible... (The C++ heap (used by malloc/free new/delete) might've 'wrapped around' the chunk of memory previously taken by the JVM.)

---

I can't do that; it all depends on the size of the initial C/C++ heap; let the following depict the initial C/C++ heap:

Code:

---

CCCCCCCCCC

---

after the JVM has been installed by your C/C++ code the memory looks like this:

Code:

---

CCCCCCCCCCJJJJJJJJJJJJJJJJJJJJ

---

where the Js denote the space taken by the JVM; now if C/C++ allocates a bunch of memory, it'll end up here:

Code:

---

AAAAAAAAAAJJJJJJJJJJJJJJJJJJJJ

---

where the As denote the allocated C/C++ objects; if your C/C++ code wants to allocate more memory, it'll end up here:

Code:

---

AAAAAAAAAAJJJJJJJJJJJJJJJJJJJJAAAAAACCCC

---

where the As on the right are the newly allocated memory and the Cs at the end are just some more memory obtained from the OS but (not yet) needed. Note that the part of memory taken by the JVM is 'embedded' in the memory taken by your C++/C part of the process and can't be returned to the OS. Only memory at the end of the process data space can be returned to the OS; there's nothing you can do about it without quite some sophisticated memory fiddling.

kind regards,

Jos

Ok.. thank you very much... I have understood this situation.. and will be think... how can i kill this leak

Quote:

---

Originally Posted by Gigabitze

Ok.. thank you very much... I have understood this situation.. and will be think... how can i kill this leak

---

You might give the following scenario a try:

1) start the C++ part of your process;
2) allocate quite a large chunk of memory;
3) start the JVM;
4) free the chunk allocated in step 2)

This way (if that chunk is large enough) no object will be allocated beyond the position where the JVM is allocated. When the JVM is freed you can safely get rid (i.e. return to the OS) the memory previously taken by the JVM. It requires a bit of experimenting.

kind regards,

Jos