How to fix JNI project that does not run

Question

I have a simple JNI project, which compiles. But when I run it, I get the following error:

$ java HelloJNI
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.ClassFormatError: Extra bytes at the end of class file HelloJNI
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$100(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)

I had to change a lot of things. On Ubuntu using gcc; it works:

ghostrider@:~/programing/java/jni/HelloJNI$ java -Djava.library.path=. HelloWorld
Hello from C!
ghostrider@:~/programing/java/jni/HelloJNI$ ls
ctest.c  HelloWorld.class  HelloWorld.h  HelloWorld.java  libctest.so

I used this link to get it working.

So my question now is can I use gcc to compile for a Windows architecture or do I have to install MinGw to do that? Thanks for everyone's help.

I just validated that this works with MinGw-64

C:\Users\sansari\Downloads\x86_64-w64-mingw32-gcc-4.8.0-win64_rubenvb\mingw64\java>gcc -o ctest.dll -shared -I"%JAVA_HOME%\include" -I"%JAVA_HOME%\include\win32" ctest.c -m64

C:\Users\sansari\Downloads\x86_64-w64-mingw32-gcc-4.8.0-win64_rubenvb\mingw64\java>ls
HelloWorld.class  HelloWorld.h  HelloWorld.java  ctest.c  ctest.dll

C:\Users\sansari\Downloads\x86_64-w64-mingw32-gcc-4.8.0-win64_rubenvb\mingw64\java>java HelloWorld
Hello from C!

Answer 1

I believe you did something wrong as the classloader complains about a broken class.

Following steps are needed in general (you have not told us which one you did)

javac HelloJNI.java
javah -jni HelloJNI
# compile your HelloJNI.cpp

Fix the library name in your Java class

instead of  : System.loadLibrary("hello");
it should be: System.loadLibrary("HelloJNI");

Following steps produce a valid DLL on CentOS 7 (sorry no MinGW for Windows at hand)

javac HelloJNI.java
javah HelloJNI
x86_64-w64-mingw32-gcc -Wl,-kill-at -Wall -O0 -o HelloJNI.dll -shared \
   -I. -I/usr/lib/jvm/java-openjdk/include \
   -I/usr/lib/jvm/java-openjdk  /include/linux HelloJNI.cpp

Copy the HelloJNI.class and HelloJNI.dll into the same directory on a 64bit Windows machine.

java HelloJNI

output

Hello World!