Native Method example using JNI

Trivial JNI example

1. Declare your native methods in an otherwise normal Java class.

   public class nativetest
   {
   	public native String sayHello(String s);
   	public static void main(String[] argv)
   	{
   		String retval = null;
   		nativetest nt = new nativetest();
   		retval = nt.sayHello("Beavis");
   		System.out.println("Invocation returned " + retval);
   	}
   }
   

2. javac nativetest.java
   Standard Java compile
3. javah -jni nativetest
   Generates .h files. Here is the code.

   /* DO NOT EDIT THIS FILE - it is machine generated */
   #include 
   /* Header for class nativetest */
   
   #ifndef _Included_nativetest
   #define _Included_nativetest
   #ifdef __cplusplus
   extern "C" {
   #endif
   /*
    * Class:     nativetest
    * Method:    sayHello
    * Signature: (Ljava/lang/String;)Ljava/lang/String;
    */
   JNIEXPORT jstring JNICALL Java_nativetest_sayHello
     (JNIEnv *, jobject, jstring);
   
   #ifdef __cplusplus
   }
   #endif
   #endif
   

4. Write your native code, using the .h file generated above Copy the function prototype from the .h file and paste it in.

   #include "nativetest.h"	/*double quotes tells it to search current directory*/
   
   JNIEXPORT jstring JNICALL Java_nativetest_sayHello
     (JNIEnv *env, jobject thisobject, jstring js)
   
   {
   	return js;
   }
   

5. Compile

   cl -c /Ic:\jdk1.1.6\include /Ic:\jdk1.1.6\include\win32 nativetest.c 
   link /libpath=c:\jdk1.1.6\lib nativetest.obj /dll
   

   The above uses Microsoft Visual C++ command-line tools.
6. Modify your Java class to load the library.

   public class nativetest
   {
   	static {
   		System.loadLibrary("nativetest");
   	}
   	public native String sayHello(String s);
   	public static void main(String[] argv)
   	{
   		String retval = null;
   		nativetest nt = new nativetest();
   		retval = nt.sayHello("Beavis");
   		System.out.println("Invocation returned " + retval);
   	}
   }
   

7. run the example

   C:\jni\hello>java nativetest
   Invocation returned Beavis
   

nativetest.java

nativetest.c