Extra 12. Java Native Interface

Main steps

1. To add support for C development in Eclipse, install Eclipse CDT plugin, which you can search in Eclipse Marketplace by entering cdt.
2. Review content of jtm.extra12 package and src/main/c/jnifunctions.c file.

3. Install GNU C compiler:

   sudo apt-get update
   sudo apt-get install gcc
4. Implement TODO's for JNIClass.java class and jnifunctions.c file.

5. Compile JNIClass.java file considering classpath and target location:

   javac -cp target/classes -d target/classes src/main/java/jtm/extra12/JNIClass.java

6. Generate jtm_extra12_JNIClass.h file in lib folder:

   javac -cp target/classes -h lib/ src/main/java/jtm/extra12/JNIClass.java

   This file will be located in lib folder of the project.

7. Check that list of methods is generated in jtm_extra12_JNIClass.h file.

8. Implement generated interfaces from lib/jtm_extra12_JNIClass.h file in /src/main/c/jnifunctions.c file:

   JNIEXPORT void JNICALL Java_jtm_extra12_JNIClass_printHello(JNIEnv * env, jclass class) {
   // TODO call printHello();
   }
   
   JNIEXPORT void JNICALL Java_jtm_extra12_JNIClass_printArray
   (JNIEnv * env, jobject obj, jstring string) {
           // TODO #1: convert jstring to array of C chars
           // TODO #2: call printArray(yourarray);
           // TODO #3: release resourches for passed jstring
   }
   
   JNIEXPORT jstring JNICALL Java_jtm_extra12_JNIClass_createArray(JNIEnv * env,
   jobject obj, jint size) {
           // TODO #1: call createArray(size);
           // TODO #2: convert created array of C chars to jstring
           // TODO #3: return jstring
   }

9. Compile jnifunctions.c file including JNI libraries for 64-bit Linux:

   gcc -fPIC -I/usr/lib/jvm/java-11-openjdk-amd64/include \
   -I/usr/lib/jvm/java-11-openjdk-amd64/include/linux \
   ./src/main/c/jnifunctions.c -c -o ./lib/jnifunctions.o

10. Make shared library for Linux:

    gcc -shared ./lib/jnifunctions.o -o ./lib/jnifunctions.so

11. Test application by calling its main() method:

    java -cp target/classes/ jtm.extra12.JNIClass
12. Ensure JNIClassTest1.java unit tests are passing.

Optional tasks

1. You can compile jnifunctions.c as independent executable with following parameters:

   gcc -I/usr/lib/jvm/java-11-openjdk-amd64/include \
   -I/usr/lib/jvm/java-11-openjdk-amd64/include/linux \
   ./src/main/c/jnifunctions.c -o ./lib/hello.o

2. Then execute created binary by:

   ./lib/hello.o

Links

• Java Native Interface Specification
• JNI Functions

Alternatives

• Java Native Access (JNA)