jni demo 求和

TestActivity.

package edu.usc.nsl.carma;

import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import edu.usc.nsl.carma.nativecaller.NativeCaller;

public class TestActivity extends ActionBarActivity {

	private NativeCaller nc;
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_test);
		
		//final TextView textview = (TextView) findViewById(R.id.textview1);
		
		nc = new NativeCaller();
		
		final Button button1 = (Button) findViewById(R.id.button1);
        button1.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
            	nc.call1();
            }
        });
        
        final Button button2 = (Button) findViewById(R.id.button2);
        button2.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
            	nc.call2();
            }
        });
        
        final Button button3 = (Button) findViewById(R.id.button3);
        button3.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
            	nc.call3();
            }
        });
	}

}

NativeCaller

package edu.usc.nsl.carma.nativecaller;

import android.util.Log;

/**
 * The NDK seems fairly restrictive in terms of what java classes may make a call
 * to the native functions since these functions must contain in their name, 
 * the name fo the java classes that are allowed to call them
 * 
 * This Native Caller is a central interface to all of the native functions written
 * for this application, as in all native function declarations contain "NativeCaller"
 * in their name.
 */
public class NativeCaller {

	private static final String DEBUG_TAG = "JAVA";
	
	// Don't make these static
	private native void helloLog(String logThis);
	private native String getString(int value1, int value2);
	private native String stringFromJNI();
	
	// The name (ndk1) of the native library is defined in the makefile
	static {
		System.loadLibrary("ndk1");
	}
	
	public NativeCaller() {
		super();
	}
	
	//c example accepting a string and making a call to the Android log util
	public void call1() {
		helloLog("This will log to LogCat via the native call.");
	}
	
	//c example doing some math and returning a string
	public void call2() {
		String result = getString(5,2);
		Log.v(DEBUG_TAG, "Result: "+result); 
		result = getString(105, 1232);
		Log.v(DEBUG_TAG, "Result2: "+result);
	}
	
	//c++ example returning a string
	public void call3() {
		Log.v(DEBUG_TAG, stringFromJNI());
	}

}

Android.mk

LOCAL_PATH := $(call my-dir)


include $(CLEAR_VARS)


LOCAL_LDLIBS := -llog


# Discussion on compiling multiple source files:
# https://groups.google.com/forum/#!topic/android-ndk/R8q-5bFc3SA
LOCAL_MODULE    := ndk1
LOCAL_SRC_FILES := native.c hello-jni.cpp


include $(BUILD_SHARED_LIBRARY)

hello-jni.cpp

#include <string.h>
#include <jni.h>

/**
 * Taken from here:
 * http://taylorpeer.com/hello-world-cpp-android-ndk/
 */

extern "C" {

	/**
	 * c++ example returning a string
	 *
	 * JNIEXPORT jstring JNICALL would be a more explicit way of defining the return data type,
	 * though this appears to not be neccessary
	 */
	jstring Java_edu_usc_nsl_carma_nativecaller_NativeCaller_stringFromJNI (JNIEnv *env, jobject obj) {
		return env->NewStringUTF("Hello from C++ over JNI!");
	}

}

native.c

#include <jni.h>
#include <stdio.h>
#include <string.h>
#include <android/log.h>

#define DEBUG_TAG "NDK"

/**
 * Taken from here:
 * http://code.tutsplus.com/tutorials/advanced-android-getting-started-with-the-ndk--mobile-2152
 */

/**
 * Naming convention for functions writen for the NDK:
 * T Java__package-name_class-name_method-name(){}
 */

//c example accepting a string and making a call to the Android log util
void Java_edu_usc_nsl_carma_nativecaller_NativeCaller_helloLog(JNIEnv * env,
		jobject this, jstring logThis) {
	jboolean isCopy;
	const char * szLogThis = (*env)->GetStringUTFChars(env, logThis, &isCopy);

	__android_log_print(ANDROID_LOG_DEBUG, DEBUG_TAG, "NDK:LC: [%s]",
			szLogThis);

	(*env)->ReleaseStringUTFChars(env, logThis, szLogThis);
}

//c example doing some math and returning a string
jstring Java_edu_usc_nsl_carma_nativecaller_NativeCaller_getString(
		JNIEnv * env, jobject this, jint value1, jint value2) {

	char *szFormat = "The sum of the two numbers is: %i";
	char *szResult;

	// add the two values
	jlong sum = value1 + value2;

	// malloc room for the resulting string
	szResult = malloc(sizeof(szFormat) + 20);

	// standard sprintf
	sprintf(szResult, szFormat, sum);

	// get an object string
	jstring result = (*env)->NewStringUTF(env, szResult);

	// cleanup
	free(szResult);

	return result;
}