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Sample Java JNI example calling C++ native code

This is an example code to pass information between Java and C++ program using Protobuf.

Java code calls C++ using JNI(Java Native Interface).

This demo gives you 3 options to read and read/write protos.

  1. Pass file name to C++ and read proto using C++.
  2. Read file in Java and then pass byte array to C++. Convert byte array into proto in C++ and print.
  3. Read file in Java and then pass byte array to C++. Add extra elements to proto in C++ and return byte array in Java. Then read byte array in Java.

Build instructions

This example only has been tested on Linux environment. As native C++ code depends on the platform it runs on, you need to modify your build process and java code(native library name in java) to make it work on the platform.

Pre-requisites

Install Protobuf

This example leverages Protobuf to transfer data from Java to native code. Here's a link that provides instructions to install protobuf: Click here

Set PROTOBUFJAR environment variable to Protobuf jar location

I am using Makefile to build files in order. I am using PROTOBUFJAR environment variable to keep the location of protobuf jar file. I have included protobuf-java-3.6.0.jar along with this code.

  $ export PROTOBUFJAR=/home/<user>/java-native-demo/protobuf-java-3.6.0.jar

Build

Makefile

I am using Makefile to perform build activities. Run following command to build your classes.

  $ make all

Build order if you are planning to build manually.

  1. Build protobuf file to generate java class and C++ class based on proto definition.

  2. Build Java file that generates a header file for C++ code.

  3. Build C++ file that will be run using JNI by Java code.

How to test

  1. Standalone C++ application

     $ ./list_people.out <outputfile>

  2. Pass filename from java to C++ and read a file using C++:

     $ java -classpath .:$PROTOBUFJAR -Djava.library.path=. ListPeople <outputfile> 1

  3. Read a file in Java, convert it to byte array and read this byte array in C++, convert it to Proto in C++ and print it.

     $ java -classpath .:$PROTOBUFJAR -Djava.library.path=. ListPeople <outputfile> 2

  4. Read a file in Java, convert it to byte array and read this byte array in C++ and convert it to Proto in C++. Now, add a record to this proto in C++, return an updated byte array to Java, convert it to Proto in Java and print there.

     $ java -classpath .:$PROTOBUFJAR -Djava.library.path=. ListPeople <outputfile> 3