using SendMessage from user32.dll with jna in java - error

Question

I try to use the:

LRESULT WINAPI SendMessage(_In_  HWND hWnd, _In_  UINT Msg,
                           _In_  WPARAM wParam, _In_  LPARAM lParam);

in Java with jna and i keep getting a error:

Exception in thread "AWT-EventQueue-0" java.lang.UnsatisfiedLinkError: Error looking up function 'SendMessage'

this is my interface:

public interface User32 extends StdCallLibrary {
Pointer GetForegroundWindow();
int SendMessage(Pointer hWnd, int msg, int num1, int num2);

and i call it like that:

Pointer hW = user32.GetForegroundWindow();
user32.SendMessage(hW, 0x0201, 0, 0);
user32.SendMessage(hW, 0x0202, 0, 0);

the hWnd is right. where is my mistake?

Answer 1

JNA can not find the function "SendMessage" in user32.dll because there no function of that name exported.

This is because SendMessage is an old name that is automatically mapped by the other compilers to the matching ANSI or UNICODE version of the function - SendMessageA and SendMessageW.

Using tools that show the exported functions of a DLL like DependencyWalker you can see that user32.dll of Windows 7 for example only knows both functions SendMessageA and SendMessageW but no SendMessage.

The function definition you use looks like the ANSI version, hence you should use SendMessageA instead.

BTW. It doesn't make any difference if you are using 32bit or 64bit Java and user32.dll. What I wrote is true for both versions.