Re: JNI types and usage newbie

• Subject: Re: JNI types and usage newbie
• From: Ken Hawkins <email@hidden>
• Date: Thu, 29 Apr 2004 20:04:41 -0700

Ken Hawkins <email@hidden> wrote:

can anyone point me toward a good sight that describes JNi types and
how to use them, in particular jobject. for example...

You can't cast or otherwise convert a jobject to or from anything. A
jobject is an opaque reference. You can't copy anything into or out of a
jobject except by making calls on it via the JNIEnv. That's the nature of
opaque references. The only things that aren't opaque in JNI are the
primitive types. Everything else, include arrays of primitive types, is
opaque.

I recommend the "JNI Programmer's Guide & Specification" (full book
available on-line in HTML and PDF):
<http://java.sun.com/docs/books/jni/>

It is the definitive reference. It could use some improvement in a few
examples, but is otherwise quite clear and concise.

i have an NSArray coming back from a native cocoa call and i am not
sure how i would cast that( er copy that) into a jobject. or is it a
jobjectArray? as well how to get items out of it once it is in a
relating jobject / jobjectArray...

As Dmitry showed, you have to manually munge all the references and values
to and fro. You can't cast a jobject to or from anything in ObjC (or in C,
for that matter), because it's opaque.

-- GG
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