Native method with String return type

Question

Hi
i am implementing a Native method with String Return type.
i am able call the respective C++ method and my C++ method is printing the String (jstring in c++ code ) correctly
i but i am getting nullpointerexcepti while loading the string in to my Java String .
i am sure my java code calling the C++ code beacause my C++ code is printing the value and one more wonder is after the NPE my c++ code able to print the value
the code follows
HelloWorld.java
public class HelloWorld {
     private native String print();
     static {
         System.loadLibrary("HelloWorld");
     public static void main(String[] args) throws InterruptedException,NullPointerException{
          HelloWorld hW= new HelloWorld();
          for(int i=0;;i++){
               String str= new HelloWorld().print();
               System.out.println(str);
               Thread.sleep(10000);
}and HelloWorld.cpp
// HelloWorld.cpp : Defines the entry point for the DLL application.
#include "stdafx.h"
#include "jni.h"
#include <stdio.h>
#include "HelloWorld.h"
#include <windows.h>
#include "tchar.h"
#include "string.h"
#ifdef _MANAGED
#pragma managed(push, off)
#endif
CHAR cpuusage(void);
typedef BOOL ( __stdcall * pfnGetSystemTimes)( LPFILETIME lpIdleTime, LPFILETIME lpKernelTime, LPFILETIME lpUserTime );
static pfnGetSystemTimes s_pfnGetSystemTimes = NULL;
static HMODULE s_hKernel = NULL;
void GetSystemTimesAddress()
     if( s_hKernel == NULL )
          s_hKernel = LoadLibrary(_T("Kernel32.dll"));
          if( s_hKernel != NULL )
               s_pfnGetSystemTimes = (pfnGetSystemTimes)GetProcAddress( s_hKernel, "GetSystemTimes" );
               if( s_pfnGetSystemTimes == NULL )
                    FreeLibrary( s_hKernel ); s_hKernel = NULL;
// cpuusage(void)
// ==============
// Return a CHAR value in the range 0 - 100 representing actual CPU usage in percent.
CHAR cpuusage()
     FILETIME               ft_sys_idle;
     FILETIME               ft_sys_kernel;
     FILETIME               ft_sys_user;
     ULARGE_INTEGER         ul_sys_idle;
     ULARGE_INTEGER         ul_sys_kernel;
     ULARGE_INTEGER         ul_sys_user;
     static ULARGE_INTEGER      ul_sys_idle_old;
     static ULARGE_INTEGER  ul_sys_kernel_old;
     static ULARGE_INTEGER  ul_sys_user_old;
     CHAR  usage = 0;
     // we cannot directly use GetSystemTimes on C language
     /* add this line :: pfnGetSystemTimes */
     s_pfnGetSystemTimes(&ft_sys_idle,    /* System idle time */
          &ft_sys_kernel,  /* system kernel time */
          &ft_sys_user);   /* System user time */
     CopyMemory(&ul_sys_idle  , &ft_sys_idle  , sizeof(FILETIME)); // Could been optimized away...
     CopyMemory(&ul_sys_kernel, &ft_sys_kernel, sizeof(FILETIME)); // Could been optimized away...
     CopyMemory(&ul_sys_user  , &ft_sys_user  , sizeof(FILETIME)); // Could been optimized away...
     usage  =
          (ul_sys_kernel.QuadPart - ul_sys_kernel_old.QuadPart)+
          (ul_sys_user.QuadPart   - ul_sys_user_old.QuadPart)
          (ul_sys_idle.QuadPart-ul_sys_idle_old.QuadPart)
          (100)
          (ul_sys_kernel.QuadPart - ul_sys_kernel_old.QuadPart)+
          (ul_sys_user.QuadPart   - ul_sys_user_old.QuadPart)
     ul_sys_idle_old.QuadPart   = ul_sys_idle.QuadPart;
     ul_sys_user_old.QuadPart   = ul_sys_user.QuadPart;
     ul_sys_kernel_old.QuadPart = ul_sys_kernel.QuadPart;
     return usage;
BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
    return TRUE;
#ifdef _MANAGED
#pragma managed(pop)
#endif
JNIEXPORT jstring JNICALL
Java_HelloWorld_print(JNIEnv *env, jobject obj)
       int n;
     GetSystemTimesAddress();
     jstring s=(jstring)cpuusage();
     printf("CPU Usage from C++: %3d%%\r",s);
     return s;
}actually in the above code below part does that all, in the below code the printf statement printing correctly
JNIEXPORT jstring JNICALL
Java_HelloWorld_print(JNIEnv *env, jobject obj)
       int n;
     GetSystemTimesAddress();
     jstring s=(jstring)cpuusage();
     printf("CPU Usage from C++: %3d%%\r",s);
     return s;
}and the NPE i get is
Exception in thread "main" java.lang.NullPointerException
     at HelloWorld.print(Native Method)
     at HelloWorld.main(HelloWorld.java:10)
CPU Usage from C++:   6%any solution?
Thanks
R
Edited by: LoveOpensource on Apr 28, 2008 12:38 AM

Answer 1

See the function you wrote:
JNIEXPORT jstring JNICALL Java_HelloWorld_print(JNIEnv *env, jobject obj)
       int n;
     GetSystemTimesAddress();
     _jstring s=(jstring)cpuusage();_
     printf("CPU Usage from C++: %3d%%\r",s);
     return s;
}Here you try to cast from char to jstring, this is your problem, jstring Object should be created:
char str[20];
sprintf(str, "%3d", (int)cpuusage());
printf("CPU Usage from C++: %s%%\r",str);
jstring s=env->NewStringUTF(str);
return s;