android NDK学习篇1之hello-jni

最近因项目需要，要将window下的C库移植到android中来，于是决定将NDK好好研究下。之前接触过，但感觉都是皮毛，没有对其提供的demo好好研究过，于是对NDK提供的demo（在android-ndk-r8e/sample目录下）进行系统学习下。

NDK版本：android-ndk-r8e

系统：Ubuntu 12.04 LTS 

最简单的demo,hello-jni将整个流程展现出来，麻雀虽小，五脏俱全，下面以NDK提供的hello-jni为例子，对这两天对NDK的学习进行小结。

1、建立android工程，编写java对应JNI层的本地接口：

package com.example.hellojni;

import android.app.Activity;
import android.widget.TextView;
import android.os.Bundle;


public class HelloJni extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        /* Create a TextView and set its content.
         * the text is retrieved by calling a native
         * function.
         */
        TextView  tv = new TextView(this);
        tv.setText( stringFromJNI() );  //调用本地接口
        setContentView(tv);
    }

    /* A native method that is implemented by the
     * 'hello-jni' native library, which is packaged
     * with this application.
     */
    public native String  stringFromJNI();// Java本地接口

    /* This is another native method declaration that is *not*
     * implemented by 'hello-jni'. This is simply to show that
     * you can declare as many native methods in your Java code
     * as you want, their implementation is searched in the
     * currently loaded native libraries only the first time
     * you call them.
     *
     * Trying to call this function will result in a
     * java.lang.UnsatisfiedLinkError exception !
     */
    public native String  unimplementedStringFromJNI();

    /* this is used to load the 'hello-jni' library on application
     * startup. The library has already been unpacked into
     * /data/data/com.example.hellojni/lib/libhello-jni.so at
     * installation time by the package manager.
     */
    static {
        System.loadLibrary("hello-jni"); //加载动态库
    }
}

2、编写jni层代码，在其中可以调用C代码：

注意：jni层中的方法名一定要以Java_开头，用下划线将包名和类名方法名连接在一起组成jni里面对应Java本地接口名。

#include <string.h>
#include <jni.h>
#include <stdio.h>
#include <android/log.h>  
//修改日志tag中的值  
#define LOG_TAG "logfromc"  
//日志显示的等级  
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)  
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)  


/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java
 */
jstring
Java_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                  jobject thiz )
{
     LOGI("hello,world in jni ");//jni 里面添加调试信息方法
    return (*env)->NewStringUTF(env, "Hello from JNI !");// 返回字符串
}

3、编写Android.mk文件：

具体解释见android-ndk-r8e/documentation.html文档中的说明。

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello-jni
LOCAL_SRC_FILES := hello-jni.c
LOCAL_LDLIBS:=-L$(SYSROOT)/usr/lib -llog

include $(BUILD_SHARED_LIBRARY)

4、用NDK来编译生成动态库.so文件:

用android-ndk-r8e目录下的ndk-build指令进行编译生成动态.so文件，此时可查看工程目录下自动添加libs和obj这两个目录，其中libs/ armeabi存放此.so文件。

当然可以配置NDK的临时环境变量，直接在jni目录下键入ndk-build即可，配置变量命令行：

NDKROOT=/home/NDK/android-ndk-r8e/
export PATH=$NDKROOT:$PATH

运行程序，手机界面显示Hello from JNI!

two-libs这个demo中使用了静态库来编译生成动态库，折腾了好一会才搞定。

参考资料：

http://developer.android.com/tools/sdk/ndk/index.html

http://hi.baidu.com/joec3/item/0e4fca153eb0e3dcbf904211