学步园

一、C/C++在ndk开发时的区别主要体现在2个方面

1、Android.mk中增加以下代码：

 LOCAL_CPP_EXTENSION := cpp

2、这时候jni目录下的文件名应该命名为Hello.cpp(以Hello为例)

而且,一般的情况下,加入涉及到字符串的转换,还需要

#include <malloc.h>
#include <stdlib.h>

代码的下载链接为：http://download.csdn.net/detail/caihongshijie6/6651737

二、代码实现

1、main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <Button
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="调用c++代码"
        android:onClick="click"
         />

</LinearLayout>

2、MainActivity

package com.njupt.cpp;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Toast;

public class MainActivity extends Activity {

    static{
    	System.loadLibrary("Hello");
    }
    
    public native String helloInC();
    public native String helloInC(String str);
    
    public void click(View v){
    	Toast.makeText(this, helloInC("haha in cpp..."), 1).show();
    }
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

}

3、Hello.cpp

#include <stdio.h>
#include <jni.h>
#include <malloc.h>
#include <stdlib.h>

#include "com_njupt_cpp_MainActivity.h"

#include <android/log.h>//include  D:\android-ndk-r7b\platforms\android-8\arch-arm\usr\include\android下的log.h这个目录
#define LOG_TAG "System.out"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)

/**
 * c++版的将java风格的字符串转化成c风格的字符串...
 */
char*   Jstring2CStr(JNIEnv*   env,   jstring   jstr)
{
	 char*   rtn   =   NULL;
	 jclass   clsstring   =   (env)->FindClass("java/lang/String");
	 jstring   strencode   =   (env)->NewStringUTF("UTF8");
	 jmethodID   mid   =   (env)->GetMethodID(clsstring,   "getBytes",   "(Ljava/lang/String;)[B");
	 jbyteArray   barr=   (jbyteArray)(env)->CallObjectMethod(jstr,mid,strencode); // String .getByte("GB2312");
	 jsize   alen   =   (env)->GetArrayLength(barr);
	 jbyte*   ba   =   (env)->GetByteArrayElements(barr,JNI_FALSE);
	 if(alen   >   0)
	 {
	  rtn   =   (char*)malloc(alen+1);         //"\0"
	  memcpy(rtn,ba,alen);
	  rtn[alen]=0;
	 }
	 (env)->ReleaseByteArrayElements(barr,ba,0);  //
	 return rtn;
}


JNIEXPORT jstring JNICALL Java_com_njupt_cpp_MainActivity_helloInC__
  (JNIEnv * env, jobject){

	return env->NewStringUTF("hello jni in cpp..");
}


JNIEXPORT jstring JNICALL Java_com_njupt_cpp_MainActivity_helloInC__Ljava_lang_String_2
  (JNIEnv * env, jobject, jstring jstr){

	char* str = Jstring2CStr(env,jstr);

	LOGD("%s",str);

	return env->NewStringUTF("我的女神是章泽天...");
}

4、Android.mk

LOCAL_PATH := $(call my-dir)

   include $(CLEAR_VARS)

   LOCAL_CPP_EXTENSION := cpp
   LOCAL_MODULE    := Hello   
   LOCAL_SRC_FILES := Hello.cpp
   LOCAL_LDLIBS += -llog
   
   include $(BUILD_SHARED_LIBRARY)