Examples

Some usage examples.

Created Nov 30, 2012 by rk3dov

Introduction

This page contains some examples of javacpp usage.

Quick start: Wrapping simple C++ class

For example, we have c++ library named foo, i.e. binary named libfoo.so in Linux or libfoo.dylib in Mac OS X or foo.dll in Windows, that contains some classes including Abc. Header file Abc.hpp:

class Abc {
public:
    void testMethod(int a);
};

To use it from Java we need Java wrapper class for each class that we want to use. javacpp also uses this Java class to generate JNI wrapper library. For example for class Abc.java:

import org.bytedeco.javacpp.*;
import org.bytedeco.javacpp.annotation.*;

@Platform(include = "Abc.hpp", link = "foo")
// "link" tells javacpp which original library should be linked (if not specified, "Abc" will be used)
public class Abc extends Pointer {
    static {
        Loader.loadLibrary("jnifoo");  // Specify name of our wrapper library
        //Loader.load();  // This method can be used if JNI wrapper named as class, i.e. "jniAbc"
    }
    
    public Abc() {
        allocate();  // To be able to create Abc instances from Java we should wrap allocator
    }
    
    public native void allocate();  // Define allocator

    public native void testMethod(int a);  // Method we want to use

    
    public static void main(String[] args) {
        Abc abc = new Abc();
        abc.testMethod(123);
    }
}

Now we should compile Abc.java:

$ javac -cp javacpp Abc.java

And generate JNI wrapper (assume that all headers and libraries located in current directory):

$ java -jar javacpp.jar -d . -o jnifoo -Xcompiler -L. Abc

-d option can be used to specify output directory, -o - resulting JNI library name. Using -Xcompiler we can pass options to compiler, e.g. -L. - to search libraries in current path.

Run it:

$ java -cp javacpp.jar:. Abc
testMethod: 123

String parameters and result

String parameters are also supported gracefully. Methods:

    void testStringMethod(std::string s);
    void testCharMethod(const char* s);

Can be wrapped with:

    public native void testStringMethod(String s);
    public native void testCharMethod(String s);