Step By Step Instructions below:
Step #1: Compile the project using ant. Some snippets are shown below.
NativeAdd.java class (with Native method)
package org.apache;
import android.util.Log;
public class NativeAdd {
static {
try {
Log.i("JNI", "Trying to load libNativeAdd.so");
System.loadLibrary("NativeAdd");
}
catch (UnsatisfiedLinkError ule) {
Log.e("JNI", "WARNING: Could not load libNativeAdd.so");
}
}
public static native long add(long a, long b);
}
Snippet that shows how this class/method is invoked from the main activity
public void onClick(View view) {
Log.i(LOG_TAG, "onClick");
EditText a = (EditText) findViewById(R.id.a);
EditText b = (EditText) findViewById(R.id.b);
EditText c = (EditText) findViewById(R.id.c);
Log.i(LOG_TAG, "calling native method");
long sum = NativeAdd.add(Long.parseLong(a.getText().toString()),
Long.parseLong(b.getText().toString()));
Log.i(LOG_TAG, "back from native method");
String text = Long.toString(sum);
c.setText("Native add returns = " + text.subSequence(0, text.length()));
}
Step #2: Generate header files using java by running the following command.
javah -classpath ../../android.jar;../bin/classes; org.apache.NativeAdd
Here’s how the header file (org_apache_CallNative.h) looks like
/* DO NOT EDIT THIS FILE - it is machine generated */
#include
/* Header for class org_apache_NativeAdd */
#ifndef _Included_org_apache_NativeAdd
#define _Included_org_apache_NativeAdd
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: org_apache_NativeAdd
* Method: add
* Signature: (JJ)J
*/
JNIEXPORT jlong JNICALL Java_org_apache_NativeAdd_add
(JNIEnv *, jclass, jlong, jlong);
#ifdef __cplusplus
}
Step #3: Code a tiny C file (org_apache_NativeAdd.c) as shown below:
#include "org_apache_NativeAdd.h"
JNIEXPORT jlong JNICALL Java_org_apache_NativeAdd_add
(JNIEnv *env, jclass c, jlong a, jlong b)
{
return a + b;
}
Step #4: Please read the following article before you try the next step.
Shared library “Hello World!” for Android
Step #5: Compile and link the org_apache_NativeAdd.c/org_apache_NativeAdd.h into a shared library.
arm-none-linux-gnueabi-gcc -I/usr/lib/jvm/java-1.5.0-sun/include -I/usr/lib/jvm/java-1.5.0-sun/include/linux -fpic -c org_apache_NativeAdd.c
arm-none-linux-gnueabi-ld -T armelf_linux_eabi.xsc -shared -o libNativeAdd.so org_apache_NativeAdd.o
NOTE: details on armelf_linux_eabi.xsc are in the Step #4.
Step #6: copy the library to the emulator. Copy the Application as well.
adb push native/libNativeAdd.so /system/libadb install bin/CallNative.apk
Step #7: Run the app to try some values. That’s it!
