在windows 上构建android NDK 开发环境

What

Android Native Development Kit

why

我们使用NDK很多情况是因为我们对以前c/c++代码的移植，或者对一些第三方开源native code的利用。

How

1.构建开发环境。

 1.0  构建Android 开发环境，这个我们稍后会讲到.

 1.1 android的NDK开发一般是基于linux 开发。这里我们基于window做，需要一个 叫做CYGWIN的环境。它是可以在windows平台上运行的unix模拟环境。cygwin 镜像站： http://cygwin.com. 建议从日本server 安装，速度好像快一点，可能是个人网络原因。配置cygwin的 "C:\cygwin\home\/%youName%/\.bash_profile"下面android NDK 路径。

 1.2.下载 android ndk 的开发包。我当前用到的是 android-ndk-r7

2. 用一个例子说明如何使用。

  2.0. 构建java 工程，放在NDK 下面APP文件下。这里我们借用ndk smaple下面一个例子。code如下。

/* * 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 HelloTest 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");    }}

 这里我们要注意几个point，public native String  stringFromJNI()， 另一个就是static {}，这个会先于Oncreat构建。调用hello-jni.so.即ndk-build 编译出的动态库。

2.1. 生成相应的.h文件。

在android工程下在工程目录下建立一个jni文件夹。然后启动cygwin console。

$ cd $NDK 指向到我们的ndk目录，

$ cd apps/hellotest  指向工程目录。

在bin目录中已经生成了HelloTest.class的前提下。我们使用 $ javah -classpath bin -d jni dogfood.test.app.HelloTest，可以在jni目录下生成一个.H 文件。打开该文件。找到 JNIEXPORT jstring JNICALL Java_dogfood_test_app_HelloTest_stringFromJNI
  (JNIEnv *, jobject); 方法。具体这个方法解释，参阅GOOGLE.

2.2. 编写c++ 文件。

#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 */jstringJava_com_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,                                                  jobject thiz ){    return (*env)->NewStringUTF(env, "Hello from JNI !");}

2.3 编写Android.mk 文件。

LOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := hello-jniLOCAL_SRC_FILES := hello-jni.cinclude $(BUILD_SHARED_LIBRARY)

2.4 我们来生成.so 文件吧。

$ ../../ndk-build，在obj目录下我们找到一个libhello-jni.so的文件，这就是我们要生成的动态库。

接下，在eclips下编译可以应用这个SO 文件。