第一篇：AndroidStudio NDK的配置和使用，jni的第一个demo

1、NDK 是一组允许您将 C 或 C++（“原生代码”）嵌入到 Android 应用中的工具。

2、好处：

①在平台之间移植其应用。

②重复使用现有库，或者提供其自己的库供重复使用。

③在某些情况下提高性能，特别是像游戏这种计算密集型应用。

3、怎么使用？

现在要做的是从jni中把数据返回，在java中把数据显示。

开发环境是AndroidStudio,先把NDK的环境配置好。

在项目目录下的gradle.properties文件中，添加NDK支持，

android.useDeprecatedNdk=true

然后在app目录下的build.gradle文件中写入生成so库的名字和输入指定的平台。

ndk{
moduleName "hello-jni"       //生成的so文件名字，调用C程序的代码中会用到该名字
abiFilters "armeabi", "armeabi-v7a", "x86" //输出指定三种平台下的so库
}

具体放在：

defaultConfig {
applicationId "com.example.hellojni"
minSdkVersion 15
targetSdkVersion 25
versionCode 1
versionName "1.0"
testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
ndk{
moduleName "hello-jni"       //生成的so文件名字，调用C程序的代码中会用到该名字
abiFilters "armeabi", "armeabi-v7a", "x86" //输出指定三种平台下的so库
}
}

到此为，NDK的环境已经配置好了。

使用：

在main目录下，建一个jni文件夹，把以下三个文件复制进去，Android.mk,Application.mk,hello-jni.c



Android.mk:

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0 #
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
LOCAL_PATH := $(call my-dir)#此变量表示源文件在开发树中的位置。在这里，构建系统提供的宏函数 my-dir 将返回当前目录（包含 Android.mk 文件本身的目录）的路径。

include $(CLEAR_VARS)#CLEAR_VARS 变量指向特殊 GNU Makefile，可为您清除许多 LOCAL_XXX 变量，例如 LOCAL_MODULE、LOCAL_SRC_FILES 和 LOCAL_STATIC_LIBRARIES。 请注意，它不会清除 LOCAL_PATH。此变量必须保留其值，因为系统在单一 GNU Make 执行环境（其中所有变量都是全局的）中解析所有构建控制文件。 在描述每个模块之前，必须声明（重新声明）此变量。

LOCAL_MODULE    := hello-jni#LOCAL_MODULE 变量将存储您要构建的模块的名称.生成一个名为 libhello-jni.so 的库。
LOCAL_SRC_FILES := hello-jni.c#LOCAL_SRC_FILES 变量必须包含要构建到模块中的 C 和/或 C++ 源文件列表。

include $(BUILD_SHARED_LIBRARY)#帮助系统将所有内容连接到一起.BUILD_SHARED_LIBRARY 变量指向 GNU Makefile 脚本，用于收集您自最近 include 后在 LOCAL_XXX 变量中定义的所有信息。 此脚本确定要构建的内容及其操作方法。

Application.mk:

APP_ABI := all
#Application.mk 文件实际上是定义要编译的多个变量的微小 GNU Makefile 片段。
#它不会实际进入生成的共享库或最终软件包。APP_ABI := all指的是所有支持的指令集。
#也可以指定多个值，将它们放在同一行上，中间用空格分隔。例如：
#APP_ABI := armeabi armeabi-v7a x86 mips

hello-jni.c:

/*
* Copyright (C) 2009 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the Lic
4000
ense at
*
*      http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*
*/
#include <string.h>
#include <jni.h>

/* 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 )
{
#if defined(__arm__)
#if defined(__ARM_ARCH_7A__)
#if defined(__ARM_NEON__)
#if defined(__ARM_PCS_VFP)
#define ABI "armeabi-v7a/NEON (hard-float)"
#else
#define ABI "armeabi-v7a/NEON"
#endif
#else
#if defined(__ARM_PCS_VFP)
#define ABI "armeabi-v7a (hard-float)"
#else
#define ABI "armeabi-v7a"
#endif
#endif
#else
#define ABI "armeabi"
#endif
#elif defined(__i386__)
#define ABI "x86"
#elif defined(__x86_64__)
#define ABI "x86_64"
#elif defined(__mips64)  /* mips64el-* toolchain defines __mips__ too */
#define ABI "mips64"
#elif defined(__mips__)
#define ABI "mips"
#elif defined(__aarch64__)
#define ABI "arm64-v8a"
#else
#define ABI "unknown"
#endif

return (*env)->NewStringUTF(env, "Hello from JNI !  Compiled with ABI " ABI ".");
}

新建一个HelloJni的类：

/*
* Copyright (C) 2009 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
*      http://www.apache.org/licenses/LICENSE-2.0 *
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
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() );
setContentView(tv);
}

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

/* 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");
}
}

ps:注意，包名是com.example.hellojni，如果你的包名不是这个的，可以改hello-jni.c文件中的Java_com_example_hellojni_HelloJni_stringFromJNI。要不然会找不到生成的so库。比如说你的包名是com.xxx.xxx。那改成Java_com_xxx.xxx_hellojni_HelloJni_stringFromJNI

System.loadLibrary("hello-jni");//"hello-jni"是so库的名字。

在这里可以看到生成的so库：



可能会有疑问，生成的不是libhello-jni么，怎么调用的时候，只是写hello-jni。有可能查找的时候，是去掉或者是默认加上lib，再查找文件名。我猜的，没有去验证。哈哈

Android.mk：必须在 jni 文件夹内创建 Android.mk 配置文件。 ndk-build 脚本将查看此文件，其中定义了模块及其名称、要编译的源文件、版本标志以及要链接的库。

Application.mk：此文件枚举并描述您的应用需要的模块。 这些信息包括：

用于针对特定平台进行编译的 ABI。

工具链。

要包含的标准库（静态和动态 STLport 或默认系统）。

源码下载：http://download.csdn.net/detail/u013147860/9802553