Re: XCode JNI project

You should probably turn the .c file into a .m file, because you won't be able to include any of the Cocoa headers in a C file. Java will be able to find the C function symbols fine, as long as they have the proper export statements in front of them.

Since you are calling into Cocoa from Java, there are a number of things you should be aware of:

1) To call most of AppKit safely, you should -performSelectorOnMainThread, because the thread from Java you are coming in from is almost certainly not Thread 0, which is reserved for AppKit's run loop. See http://developer.apple.com/technotes/tn2005/tn2147.html for more detail on these issues.

2) If you are using thread safe Foundation classes, and making Obj-C message sends from a thread that originated in Java, you will need an autorelease pool setup to clean up objects expecting to be deallocated by a run loop's autorelease pool.

3) To be completely pedantic, you should also wrap your calls into Cocoa with @try/@catch blocks to prevent an Objective-C exception throw from completely blowing though a bunch of HotSpot stack frames and bringing your app to unceremonious ending.

To help with these issues (at least internally) we have created the JavaNativeFoundation.framework inside of /System/Library/Frameworks/JavaVM.framework/Frameworks. Now, this is not officially API (yet), but you can use this as a reference on how to handle the JNI entry and exit points the same way we do inside of Java runtime. Check out the sections around JNF_COCOA_ENTER() and JNF_COCOA_EXIT() to get a feel for how to structure these checks.

Best of luck (your going to need it writing JNI),

Mike Swingler

Java Runtime Engineer

Apple Inc.

On Sep 27, 2008, at 10:13 AM, Ken Orr wrote:

Thanks for the suggestions guys - all have helped with my understanding of the situation. Here's what I have now:

1) A C file with an auto generated header file (auto-generated from the corresponding Java class's methods marked "native - not exactly sure how this happens)

2) An Objective-C M file, which will have my Cocoa calls

3) I've added the Cocoa.Framework under Targets->JNILib->Link Binary With Library by right clicking it and adding the framework under "Linked Libraries".

Everything builds fine until I include the Objective-C's header file my C file. When i include the header, I get all sorts of errors (see attached screen shot).

Any thoughts?

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On Sep 27, 2008, at 11:00 AM, Alexei Svitkine wrote:

Have two files. A .m file that does what you want, and exposes that as a C function. And the C file that just calls that function.

-Alexei

On Sat, Sep 27, 2008 at 10:05 AM, Michael Hall <email@hidden> wrote:

On Sep 27, 2008, at 8:41 AM, Ken Orr wrote:

C file via "import <Cocoa/Cocoa.h>

First you wouldn't include that in a C '.c' file, it would have to be an Objective-C '.m' file. A '.c' file will imply a straight 'C' compile and the compiler won't know how to handle the Objective-C specific syntax it runs into in the Cocoa.h file. This should get real errors, not warnings, and should not in any way succeed - so maybe this isn't what you mean?

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