C/C++在ndk开发中的区别

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)