# Exercise repository for Java Native Interface (JNI)

At times, it is necessary to use native codes (C/C++) to overcome the memory management and performance constraints in Java. Java supports native codes via the Java Native Interface (JNI). This repository contains a "Hello World" example for using the JNI which is based on this [tutorial](https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html).


# Prerequisites
1. **Java Development Kit >= 8**
2. **C/C++ Compiler**	
	* **[windows-only]**: Cygwin or MinGW on Windows ([Read "How to Setup Cygwin and MinGW](https://www3.ntu.edu.sg/home/ehchua/programming/howto/Cygwin_HowTo.html)
    * GCC compiler and Makefile ([Read "GCC and Make"](https://www3.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html))
3. **Eclipse for Java Developers** with **C/C++ Development Tool (CDT) extension** installed ([Read "Eclipse CDT"](https://www3.ntu.edu.sg/home/ehchua/programming/howto/EclipseCpp_HowTo.html)).

# Clone project
```git clone https://es.technikum-wien.at/embedded_systems_public/Java-Exercises-7-JNI-HW-Interfacing.git```

# Create Eclipse project

1. *File/New/Project*
2. Select *General/Project* and click *Next*
3. Unselect *Use default location* and browse to cloned diretory, click *OK*
4. Enter *Project name* and click *Finish*

# Build project

Building the project involves two steps

1. Building the Java code and the native code using the make targets:
    * **Linux**:```make -f makefile.linux all```
    * **Windows**:```make -f makefile.windows all```
    

# Run project

Before the application can be run, you must provide the library path to the native library (Linux: ```libhello.so```, Windows: ```hello.dll```) file, if it is not located in the *current working directory*.
This can be done via the VM argument ```-Djava.library.path```. Right-click on the project and select *Run As/Run Configurations*, then select *Java Application*. In the *Main tab*, enter the main class "HelloJNI" and in *Arguments/VM Arguments*, enter ```-Djava.library.path=jni```. Finally click on *Run*.

You should see the output "Hello World!" displayed on the console.

## Command line

### Example 1
```java -Djava.library.path=jni -cp bin java_exercises7_1_1.HelloJNI```
### Example 2
```java -Djava.library.path=jni -cp bin java_exercises7_2_1.HelloJNI```