JNA: Can't create object of structure subclass - Invalid structure field

Question

I am mapping a C typedef to a JNA Struct. Below if the C typedef:

typedef struct _GLYCOUNTERS {
unsigned int ulArraySize;
LPGLYCOUNTER lpCounters;
}GLYCOUNTERS, *LPGLYCOUNTERS;

And this is my JNA Structure subclass:

public class GlyCounters extends Structure{
    public class ByReference extends GlyCounters implements Structure.ByReference {};
    public class ByValue extends GlyCounters implements Structure.ByValue {};
    public int ulArraySize;
    public GlyCounter lpCounters;

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList(new String[] { "ulArraySize", "lpCounters"});
}
    }

I am trying to create and object of GlyCounters class with code:

lppCounters = glyTypes.new GlyCounters();

As you can see I am creating the object in another class and GlyCounters is a nested class of yet another class.

This is the error message I receive:

Exception in thread "main" java.lang.IllegalArgumentException: Invalid Structure field in class btc_dll.GloryTypeDef$GlyCounters, field name 'lpCounters' (class btc_dll.GloryTypeDef$GlyCounter): Can't instantiate class btc_dll.GloryTypeDef$GlyCounter (java.lang.InstantiationException: btc_dll.GloryTypeDef$GlyCounter)
at com.sun.jna.Structure.validateField(Structure.java:1038)
at com.sun.jna.Structure.validateFields(Structure.java:1048)
at com.sun.jna.Structure.<init>(Structure.java:179)
at com.sun.jna.Structure.<init>(Structure.java:172)
at com.sun.jna.Structure.<init>(Structure.java:159)
at com.sun.jna.Structure.<init>(Structure.java:151)
at btc_dll.GloryTypeDef$GlyCounters.<init>(GloryTypeDef.java:298)
at btc_gui.Btc_gui.main(Btc_gui.java:54)
Java Result: 1

The field causing the problem is yet another JNA Structure class for another c typedef struct. After googling the error message, I have tried adding no-args constructors to both of these classes but the error still remains.

Has anyone had similar problems?

Answer 1

When you want a field of type struct * within your structure, you need to use a Structure tagged with the interface Structure.ByReference.

EDITED

The most common way to do this:

public class GlyCounter extends Structure {
    public class ByReference extends GlyCounter implements Structure.ByReference {};
    // More definition here...
}
public class GlyCounters extends Structure{
    public int ulArraySize;
    public GlyCounter.ByReference lpCounters;

    public GlyCounters(Pointer p) {
        super(p);
        read();
    }

    @Override
    protected List<String> getFieldOrder() {
        return Arrays.asList(new String[] { "ulArraySize", "lpCounters"});
    }
}

// To obtain the full array (assuming they're allocated as a block)
GlyCounters gc = new GlyCounters(pointer);
GlyCounter[] counters = gc.lpCounters.toArray(gc.ulArraySize());

The field lpCounters is now of pointer type instead of inlined (i.e. sharing the memory of the enclosing struct).