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Pass and return numeric value to JNI method
  • If you didn't know how to write sample JNI please follow previous blog.
  • When pass data beetween C and Java it need to give corresponding types.
  • here are the java and c corresponding types
    JAVA C Bytes
    boolean jboolean 1
    byte jbyte 1
    char jchar 2
    short jshort 2
    int jint 4
    long long 8
    float jfloat 4
    double jdouble 8
  • Let's implement simple Calculater Class which is get two integer values and return a addition
  • Define the Calculator.java class as below.
                class Calculator {
                    public static native int addition(int x,int y);
  • Compile and generate corresponding header definition
                javac HelloNative.java
                javah HelloNative
  • Then implement the Calculator.c as below
                #include "Calculator.h"
                #include <stdio.h>
                JNIEXPORT jint JNICALL Java_Calculator_addition (JNIEnv *env, jclass cl, jint x, jint y)
                    return x+y;
  • Generate the dynamic library according to the environment
                //on Cygwin programming environment in windows
                gcc -mno-cygwin -D __int64="long long" -I JAVA_HOME/include/ -I JAVA_HOME/include/win32 -shared -Wl,--add-stdcall-alias -o Calculator.dll Calculator.c
                // Gnu C compiler on Linux
                gcc -fPIC -I JAVA_HOME/include -I JAVA_HOME/include/linux -shared -o libCalculator.so Calculator.c
                //With tSun compiler under the Solaris Operating System
                cc -G -I JAVA_HOME/include -I JAVA_HOME/include/solaris -o libCalculator.so Calculator.c
                //in Microsoft C++ compiler on Windows
                cl -I JAVA_HOME\include -I JAVA_HOME\include\win32 -LD Calculator.c -FeCalculator.dll
  • Create CalculatorTest class
                class CalculatorTest
                    public static void main(String[] args)
                       int re=Calculator.addition(10,15);
                       System.out.println("re: "+re);
  • Run the CalculatorTest class
                javac CalculatorTest.java
                java CalculatorTest
  • download the sample code here.

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