我如何处理JNA中的不透明指针？

Question

My Java application uses JNA to communicate with device drivers through the manufacturer's supplied API. The API communicates using Windows Messages. At a point during execution, the device has a set of data passed as an LPARAM. The LPARAM is a pointer to a vector of structures that contains the data I need.

In C++, I can access this data like this:

ImagesStruct *Images = { 0 };
...
Images = (ImagesStruct *)lParam;

The native ImagesStruct looks like this:

typedef struct _CompressedImage
{
  BYTE *pBuffer;
  int BufferLen;
}
CompressedImage;

typedef struct _ImagesStruct
{
  DWORD DocNumber;
  CompressedImage *Images;
  BOOL SnippetFront;
}
ImagesStruct;

In Java, I have the Structures mapped like this:

// ImagesStruct
public class IMAGES extends Structure {
    public int DocNumber;
    public COMPRESSED_IMAGE.ByReference Images; // pointer
    public boolean SnippetFront;

    public IMAGES() { super(); }
    public IMAGES(Pointer p) { super(p); }

    @Override
    protected List getFieldOrder() {
        return Arrays.asList("DocNumber", "Images", "SnippetFront");
    }
}

// CompressedImage
public class COMPRESSED_IMAGE extends Structure {
    public Pointer pBuffer; // pointer
    public int BufferLen;

    public COMPRESSED_IMAGE() { super(); }
    public COMPRESSED_IMAGE(Pointer p) { super(p); }

    @Override
    protected List getFieldOrder() {
        return Arrays.asList("pBuffer", "BufferLen");
    }

    public static class ByReference extends COMPRESSED_IMAGE implements Structure.ByReference {
        public ByReference() { super(); }
        public ByReference(Pointer p) { super(p); read(); }
    }
}

Here is one of the many ways I've tried to access that data in Java

Pointer p = lparam.toPointer(); // this works
IMAGES pImages = new IMAGES();
IMAGES[] pa = (IMAGES[]) pImages.toArray(pImages.size()); // error here

Everything I try, including the above, seems to result in the same error message:

JNA: Callback desktop.IDeal@1175f37 threw the following exception: java.lang.UnsupportedOperationException: This pointer is opaque : const@0x155f0000 at com.sun.jna.Pointer$Opaque.read(Pointer.java:1320) at com.sun.jna.Pointer.getByteArray(Pointer.java:726) at desktop.IDeal.callback(IDeal.java:199) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:497) at com.sun.jna.CallbackReference$DefaultCallbackProxy.invokeCallback(CallbackReference.java:470) at com.sun.jna.CallbackReference$DefaultCallbackProxy.callback(CallbackReference.java:500) at com.sun.jna.Native.invokeInt(Native Method) at com.sun.jna.Function.invoke(Function.java:384) at com.sun.jna.Function.invoke(Function.java:316) at com.sun.jna.Library$Handler.invoke(Library.java:232) at com.sun.proxy.$Proxy2 .GetMessage(Unknown Source) at win32.Window.run(Window.java:59) at java.lang.Thread.run(Thread.java:745)

Do I have things mapped correctly? If there's nothing I can do to map that data at the other end of the pointer to a set of Java objects, how do I use the Pointer object to access that data?

Answer 1

Change the signature of your callback to accept IMAGES instead of LPARAM . JNA will take care of the conversions for you.

You should also consider naming your JNA Structure exactly like your native struct in order to avoid confusion (use "ImagesStruct" instead of "IMAGES").

It's also good practice to call Structure.read() when initializing a JNA Structure using the Pointer -based constructor. That ensures the Java fields are in synch with native memory before the constructor returns.

Answer 2

I found the answer in the JNA Win32WindowDemo: Just use the LPARAM callback and create your Structure using the YourStructureHere(new Pointer(lParam.longValue())) constructor:

public LRESULT callback(HWND hwnd, int uMsg, WPARAM wParam, LPARAM lParam) {
    if (commandCompletionMessages.contains(uMsg) || eventMessages.contains(uMsg)){
            WFSRESULT wfsresult = new WFSRESULT(new Pointer(lParam.longValue()));
    (...)
    }       
    return User32.INSTANCE.DefWindowProc(hwnd, uMsg, wParam, lParam);