JVM reserves additional memory after object creation

Question

Hi everybody,
i have a problem with starting the JVM from inside a C++application. Starting the VM with JNI_CreateJavaVM works as aspected. When creating the first object it seems like the JVM reserves additional memory (outside the heap). Here is a little c++ test case:
/* Compile/Link: cl -MT -Zi -EHsc -I%JAVA_HOME%\include jvmtest0.cxx user32.lib %JAVA_HOME%\lib\jvm.lib */
#include "windows.h"
#include <string>
#include <jni.h>
int availMem () {
     int i,j;
     void* v[2048];
     for (i=0;i<2048;i++) {
          if ((v=malloc(1024*1024))==0) break;
     for (j=0;j<i;j++) free(v[j]);
     return i;
int WINAPI WinMain(HINSTANCE hinst,HINSTANCE hprev,LPSTR cmd,int show)
     int i1=availMem();
     JavaVMInitArgs arg;
     JavaVMOption opt[1];
     arg.version=JNI_VERSION_1_2;
     arg.ignoreUnrecognized=JNI_TRUE;
     opt[0].optionString="-Xmx256m";
     arg.nOptions=1;
     arg.options=opt;
     JavaVM* vm;
     JNIEnv* en;
     if (JNI_CreateJavaVM(&vm,(void**)&en,&arg)<0) {
          MessageBox(NULL,"Error CreateJavaVM","jvm-Test",MB_OK);
          return (1);
     jclass clFrame=en->FindClass("Ljavax/swing/JFrame;");
     if (clFrame==0) {
          MessageBox(NULL,"Error FindClass","jvm-Test",MB_OK);
          return (1);
     jmethodID methodID=en->GetMethodID(clFrame,"<init>","(Ljava/lang/String;)V");
     if (methodID==0) {
          MessageBox(NULL,"Error GetMethodID","jvm-Test",MB_OK);
          return (1);
     int i2=availMem();
     jobject window=en->NewObject(clFrame,methodID,en->NewStringUTF("Test"));
     if (window==0) {
          MessageBox(NULL,"Error NewObject","jvm-Test",MB_OK);
          return (1);
     int i3=availMem();
     char o[256];
     sprintf_s(o,"Available memory:\nStart: %d MB\nAfter CreateJavaVM: %d MB\nAfter JFrame: %d MB",i1,i2,i3);
     MessageBox(NULL,o,"jvm-Test",MB_OK);
     return (0);
I'am running on Windows XP with JDK 1.5.0_02 and get the following result:
Start: 1906 MB
After CreateJavaVM: 1558 MB
After JFrame: 1312 MB
I would expect that the available memory after constructing the Java object is similar to the available memory after creating the JVM. On Linux it is.
Thanks

Answer 1

a=null
MyClass a = new MyClass();
is the same as
MyClass a = new MyClass();
because after MyClass a = new MyClass(), doesn't point to a possible old instance but to a new instance of MyClass.
I guess, the problem is in some other data structure you build. perhaps the vector that contains lots of/big other objects? does any other class point to these objects too? remember: the garbage collector does clean up only unused memory, i.e. objects that are not referenced by any other objects that are valid (i.e. are referenced by the main thread or objects of the main thread).