通过 JNI 从 C 调用 Java 可变参数 function

Question

I am currently working on creating some Java bindings for a C library I work on. One of our C-structs has a char buffer that is a file system path. After calling the C function, the buffer is correctly populated. I want to take the buffer and convert it to a java.nio.file.Path member on the Java object.

I am having some trouble however. I for some reason am generating a NullPointerException within C, and I can't really see the problem.

The way to create a java.nio.file.Path object is going through java.nio.file.Paths::get() .

Here is the relevant C code:

const jclass paths_class = (*env)->FindClass(env, "java/nio/file/Paths");
if ((*env)->ExceptionCheck(env))
    return;
const jmethodID get_method = (*env)->GetStaticMethodID(
    env, paths_class, "get", "(Ljava/lang/String;[Ljava/lang/String;)Ljava/nio/file/Path;");
if ((*env)->ExceptionCheck(env))
    return;
const jstring path_str = (*env)->NewStringUTF(env, info.mi_path);
if ((*env)->ExceptionCheck(env))
    return;
const jobject path_obj =
   (*env)->CallStaticObjectMethod(env, paths_class, get_method, path_str); // exception generated here
if ((*env)->ExceptionCheck(env))
    return;

And the Java class for good measure:

public final class MclassInfo {
    private native void _get(final Kvdb kvdb, Mclass mclass) throws HseException;

    private long allocatedBytes;
    private long usedBytes;
    private Path path;

    MclassInfo(final Kvdb kvdb, final Mclass mclass) throws HseException {
        _get(kvdb, mclass);
    }

    public long getAllocatedBytes() {
        return allocatedBytes;
    }

    public long getUsedBytes() {
        return usedBytes;
    }

    public Path getPath() {
        return path;
    }
}

All I can think is that somehow I am not calling the Java variadic function properly. I have also tried passing NULL as an extra argument to the method call, but ended up with the same issue.

Answer 1

The method you are trying to invoke is declared as get(String first, String... more) . The variadic syntax in Java is just sugar for an array of the specified type, ie the two arguments of this method are really String and String[] -- which you correctly coded in the GetStaticMethodID call as (Ljava/lang/String;[Ljava/lang/String;) .

So to call it you need two arguments: one String and one String[] (array) -- and (for your case) the array must contain zero elements, but such an empty array is not the same as NULL. Have a gander at NewObjectArray .