From: mickish AT cmu DOT edu (Andrew Mickish)
Subject: Re: Makefile for JNI on eNp-Ty
10 Aug 1998 17:09:42 -0700
Message-ID: <35CEEB02.5D1046CA.cygnus.gnu-win32@cmu.edu>
References: <35CE365C DOT 3365 AT telegenisys DOT com>
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To: "Henry J. Cobb" <hcobb AT telegenisys DOT com>
Cc: gnu-win32 AT cygnus DOT com

> Henry J. Cobb wrote:
>
> In my continuing mission to use the CygWin32 tools to build Java
> products,
> I now have a makefile for Java Naive Interface DLLs, but unfortunately
> it only works with the M$ C compiler so far...

Henry,

Attached are some hints I've accumulated for doing JNI with GNU-WIN32.

--Andrew Mickish
  http://www.andrew.cmu.edu/~mickish/

--------------------------------------------------------------------

Subject:  More Java JNI with GNU-WIN32 gcc
   Date:  Mon, 10 Aug 1998 08:35:16 -0400
   From: Andrew Mickish <mickish AT cmu DOT edu>
     To:  gnu-win32 AT cygnus DOT com

Here are a few more issues that came up when trying to compile
a JNI DLL with gcc.

*  There are two different problems that could cause Unsatisfied
   link errors.  One is if the DLL is not found, and the other is
   if the DLL is found but is unreadable or the specific function
   cannot be found in it.  These problems are distinguishable by
   the stack trace that you get below the "Unsatisfied link error,"
   and also by testing for changes in behavior when you run Java
   after deliberately deleting the DLL or deliberately renaming
   some arbitrary text file to be your DLL.

*  The directory containing the DLL ought to be in your PATH.
   The recommended way to set your path is to change it with the
   System control panel and open a new window.  The cygnus.bat file
   munges the path a little itself, so my own path ends up looking
   like this:

bash-2.01$ echo $PATH
/Cygnus/B19/H-i386-cygwin32/bin:/WINNT/system32:/WINNT:/utils:/enscript://E/jdk1.1.5/bin://E/java/HelloWorld

*  I had to hand-edit the DEF file and put in an alias for the
   exported function.  Without the alias, it seems that Cygnus is
   exporting the mangled name of the function with its offset
   ("...@8") and Java is looking for an unmangled name
   (or vice versa).  My DEF file looks like this:

bash-2.01$ cat hello.def
EXPORTS
Java_HelloWorld_displayHelloWorld=Java_HelloWorld_displayHelloWorld AT 8
dll_entry AT 12


Please let me know if any of this has been helpful.

--Andrew Mickish
  mickish AT cmu DOT edu


-----------------------------------------------------------------


 Subject:  Java JNI with GNU-WIN32 gcc
   Date:  Thu, 11 Jun 1998 19:48:11 -0400
   From:  Andrew Mickish <mickish AT CMU DOT EDU>
     To:  gnu-win32 AT cygnus DOT com

With significant help from Bill Pringlemeir, I figured out how to compile
Sun's JNI tutorial HelloWorld example under GNU-WIN32 with gcc.  I will keep
a
ZIP file of the gcc-compatible code at
http://www.andrew.cmu.edu/~am2q/HelloWorld.zip for future reference.

Please add this information to the GNU-WIN32 documentation and FAQ.  As of
version 19.1, it is significantly more complicated to compile a
Java-compatible DLL under gcc than under Microsoft Visual C++.  Even printf()

does not work in a gcc-compiled DLL, even thoough it works fine with VC++.
Will this be changing with future releases of GNU-WIN32?

Here are a few hints for how to compile a trivial JNI-compaible DLL with gcc:

1)  You need init.cc, an element of building relocatable DLLs

2)  There is a complicated linking procedure involving ld and intermediate
files:

     ld --base-file hello.base --dll -o hello.dll HelloWorldImp.o init.o
$(LIB) -e _dll_entry AT 12
     dlltool --as=as --dllname hello.dll --def hello.def --base-file
hello.base --output-exp hello.exp
     ld --base-file hello.base hello.exp --dll -o hello.dll HelloWorldImp.o
init.o $(LIB) -e _dll_entry AT 12
     dlltool --as=as --dllname hello.dll --def hello.def --base-file
hello.base --output-exp hello.exp
     ld hello.exp --dll -o hello.dll HelloWorldImp.o init.o $(LIB) -e
_dll_entry AT 12

3)  Printf will not work here.  Use a Windows dialog box or call back into
Java to invoke
     System.out.println():

    void jprintf (JNIEnv *env, char* str) {

      jstring jstr = (*env)->NewStringUTF(env, str);
      jclass System_class = (*env)->FindClass(env, "java/lang/System");
      jfieldID fid_out = (*env)->GetStaticFieldID(env, System_class, "out",
"Ljava/io/PrintStream;");
      jobject out = (*env)->GetStaticObjectField(env, System_class, fid_out);

      jclass PrintStream_class = (*env)->FindClass(env,
"java/io/PrintStream");
      jmethodID mid_println = (*env)->GetMethodID(env, PrintStream_class,
"println", "(Ljava/lang/String;)V");
      if (mid_println == 0) {
        exit(1);
      }
      (*env)->CallVoidMethod(env, out, mid_println, jstr);
    }


    JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld(JNIEnv *env,
jobject obj)
    {
      (void)env; (void)obj;
      MessageBoxA(NULL,"Hello Jupiter!", "Hello world!", 0);
      jprintf(env, "Hello Uranus!\n");
      return;
    }


A collection of files that implement Sun's JNI HelloWorld example may be
found
at http://www.andrew.cmu.edu/~am2q/HelloWorld.zip

--Andrew Mickish
  mickish AT cmu DOT edu


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