README.md

Call C from Java

This repository demonstrates how to compile a simple "Hello World" example using the Java Native Interface (JNI) to call a shared library written in C.

Building and running the example

With Docker

Build the image with docker build -t call-c-from-java:latest . and run it with docker run -it --rm call-c-from-java:latest.

With gcc and javac

First compile the java source using javac App.java Then compile the c source into a shared library, passing in the appropriate jvm includes:

gcc -I/usr/lib/jvm/java-1.8-opendjdk/include -I/usr/lib/jvm/java-1.8-opendjdk/include/linux -shared -fpic -o libSharedLibrary.so c/App.c

Finally, run the application, adding the shared library to the java library path:

java -Djava.library.path=./libSharedLibrary.so App

N.B. If running on Windows, substitute libSharedLibrary.so with SharedLibrary.dll

Explanation

Java:

The Java file App.java declares the native HelloWorld() function using the native keyword and invokes it in its main method. The static loader loads the appropriate shared library containing HelloWorld(), namely libSharedLibrary.so, at class loadtime.

C

The C header file App.h is created using javac -h App.java. This file contains a method stub derived from the method signature specified by the native declaration in App.java. The stub is implemented in App.c, and defined to print "Hello from C!" to stdout. In order to create a dynamic shared library, the -shared and -fpic flags are passed to gcc. These flags instruct gcc to create a shared library using position independent code. This means that the library can be executed from any memory location, therefore making it a dynamic library.