An Introduction to JNI using a Bottom Up Approach: Step 4

Continued from An Introduction to JNI using a Bottom Up Approach: Step 3.

Step 4: Java Main Function

Run Java without the CToCPPHelper Entry Point

1. Create the following Java Class:
   package com.mycompany.myapp;
   
   public class JavaToCHelper {
       public native void cToCPPHelperEntryPoint(); // Comment to ignore CToCPPHelper
   
       static // static initializer code
       {
   //        System.load(JavaToCHelper.class.getResource("/dlls/CToCPPHelper.dll").getPath()); // Comment to ignore CToCPPHelper
       }
   
       public void jToCHelperExitPoint() {
           System.out.println("JAVA_TO_C_HELPER EXIT START");
   //        cToCPPHelperEntryPoint(); // Comment to ignore CToCPPHelper
           System.out.println("JAVA_TO_C_HELPER EXIT FINISH");
       }
   
       public static void main(String[] args) {
           System.out.println("JAVA_TO_C_HELPER MAIN START");
           JavaToCHelper jToCHelper = new JavaToCHelper();
           jToCHelper.jToCHelperExitPoint();
           System.out.println("JAVA_TO_C_HELPER MAIN FINISH");
       }
   
       public void jToCHelperEntryPoint() {
           System.out.println("JAVA_TO_C_HELPER ENTRY START");
           jToCHelperExitPoint();
           System.out.println("JAVA_TO_C_HELPER ENTRY FINISH");
       }
   }
2. Compile and run JavaToCHelper, using the following commands:
   cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\java"
   
   "C:\Program Files\Java\jdk1.6.0_26\bin\javac.exe" -classpath "." com\mycompany\myapp\JavaToCHelper.java
   
   "C:\Program Files\Java\jdk1.6.0_26\bin\java.exe" -classpath "." com.mycompany.myapp.JavaToCHelper
   

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Run CToCPPHelper.exe with a Generated Header File

1. Generate CToCPPHelper.h from JavaToCHelper.class, using the following commands:
   cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\java"
   "C:\Program Files\Java\jdk1.6.0_26\bin\javah.exe" -classpath "." com.mycompany.myapp.JavaToCHelper
   mv com_mycompany_myapp_JavaToCHelper.h "C:\Users\Scott\workspace\myrepo\my-app\src\main\resources\dlls\CToCPPHelper.h"
   

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2. Verify the CToCPPHelper.h was generated in the same folder as CToCPPHelper.c:
   /* DO NOT EDIT THIS FILE - it is machine generated */
   #include <jni.h>
   /* Header for class com_mycompany_myapp_JavaToCHelper */
   
   #ifndef _Included_com_mycompany_myapp_JavaToCHelper
   #define _Included_com_mycompany_myapp_JavaToCHelper
   #ifdef __cplusplus
   extern "C" {
   #endif
   /*
    * Class:     com_mycompany_myapp_JavaToCHelper
    * Method:    cToCPPHelperEntryPoint
    * Signature: ()V
    */
   JNIEXPORT void JNICALL Java_com_mycompany_myapp_JavaToCHelper_cToCPPHelperEntryPoint
     (JNIEnv *, jobject);
   
   #ifdef __cplusplus
   }
   #endif
   #endif
3. Compile the code and run "CToCPPHelper.exe" (see An Introduction to JNI using a Bottom Up Approach: Step 3).

Run JavaToCHelper with the New DLL

1. Uncomment two lines of code:
    System.load(JavaToCHelper.class.getResource("/dlls/CToCPPHelper.dll").getPath()); // Comment to ignore CToCPPHelper
       cToCPPHelperEntryPoint(); // Comment to ignore CToCPPHelper
2. Generate a 64 bit dll file (for 32 bit Java, remove "-m64") using the following commands:
   cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\resources\dlls"
   
   "C:\MinGW64\bin\g++.exe" -m64 -Wl,--add-stdcall-alias -shared -I"C:\Program Files\Java\jdk1.6.0_26\include" -I"C:\Program Files\Java\jdk1.6.0_26\include\win32" -o CToCPPHelper.dll CToCPPHelper.c CPPHelper.cpp PrintOpenWindows.cpp
   

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3. Compile and run JavaToCHelper (adding the new dll to the class path), using the following commands:
   cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\java"
   
   "C:\Program Files\Java\jdk1.6.0_26\bin\javac.exe" -classpath "." com\mycompany\myapp\JavaToCHelper.java
   
   "C:\Program Files\Java\jdk1.6.0_26\bin\java.exe" -classpath ".;../resources" com.mycompany.myapp.JavaToCHelper
   

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Summary of commands

Anytime you change the JavaToCHelper.java, CToCPPHelper.c, CPPHelper.cpp or PrintOpenWindows.cpp, you can regenerate the appropriate files via:

REM Generate the New C Header File

cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\java"

"C:\Program Files\Java\jdk1.6.0_26\bin\javac.exe" -classpath "." com\mycompany\myapp\JavaToCHelper.java

"C:\Program Files\Java\jdk1.6.0_26\bin\javah.exe" -classpath "." com.mycompany.myapp.JavaToCHelper

mv com_mycompany_myapp_JavaToCHelper.h "C:\Users\Scott\workspace\myrepo\my-app\src\main\resources\dlls\CToCPPHelper.h"


REM Generate the New DLL

cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\resources\dlls"

"C:\MinGW64\bin\g++.exe" -m64 -Wl,--add-stdcall-alias -shared -I"C:\Program Files\Java\jdk1.6.0_26\include" -I"C:\Program Files\Java\jdk1.6.0_26\include\win32" -o CToCPPHelper.dll CToCPPHelper.c CPPHelper.cpp PrintOpenWindows.cpp


REM Run Java

cd /D "C:\Users\Scott\workspace\myrepo\my-app\src\main\java"

"C:\Program Files\Java\jdk1.6.0_26\bin\java.exe" -classpath ".;../resources" com.mycompany.myapp.JavaToCHelper

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A warning! If you followed these instructions and you received the error "Can't load this .dll (machine code=0xbd) on a AMD 64-bit platform", it may be because your project is a "Maven Project". You can "Disable Maven Nature" and try recompiling from the command line with the following command:

mvn eclipse:clean eclipse:eclipse clean compile test-compile

Also, you can run the JavaToCHelper by right clicking and "Run As Java Application" or executing this command.

mvn exec:java -Dexec.mainClass="com.mycompany.myapp.JavaToCHelper"

This assumes your src/main/resources folder will be copied to the target folder during the build phase.

    <build>
        <resources>
            <resource>
                <filtering>true</filtering>
                <directory>src/main/resources</directory>
            </resource>
        </resources>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.0</version>
            </plugin>
        </plugins>
    </build>

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This post was reposted from http://scottizu.wordpress.com/2013/08/21/an-introduction-to-jni-using-a-bottom-up-approach-step-4/, originally written on August 21st, 2013.