官方实例：android ubuntu ndk jni hello-jni

这里我贴一下其它网友有关为什么用jni的原因：

1. 你的Java代码，需要得到一个文件的属性。但是你找遍了JDK帮助文档也找不到相关的API。
2. 在本地还有一个别的系统，不过他不是Java语言实现的，这个时候你的老板要求你把两套系统整合到一起。
3. 你的Java代码中需要用到某种算法，不过算法是用C实现并封装在动态链接库文件（DLL）当中的。

参考资料：http://developer.android.com/sdk/ndk/index.html  、http://developer.android.com/sdk/ndk/overview.html

下 载 android-ndk-r6-linux-x86.tar.bz2 然后解压，那个解压后会有个README.txt文件大概对doc包下的一些说明文档进行了粗略的介绍，那个文档中有下面的指导，所以需要认真的看一下。

本地操作：

pateo@pateo-B86N53X:~$ pwd
/home/pateo
pateo@pateo-B86N53X:~$ gedit .bashrc

在文件中输入的内容：

export NDK_HOME=/home/pateo/work/android-ndk-r6
export PATH=$PATH:$NDK_HOME

然后保存

pateo@pateo-B86N53X:~$ source .bashrc

pateo@pateo-B86N53X:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/home/pateo/work/android-ndk-r6

pateo@pateo-B86N53X:~$ cd work/android-ndk-r6/samples/hello-jni

pateo@pateo-B86N53X:~/work/android-ndk-r6/samples/hello-jni$  ndk-build
Gdbserver      : [arm-linux-androideabi-4.4.3] libs/armeabi/gdbserver
Gdbsetup       : libs/armeabi/gdb.setup
Compile thumb  : hello-jni <= hello-jni.c
SharedLibrary  : libhello-jni.so
Install        : libhello-jni.so => libs/armeabi/libhello-jni.so
pateo@pateo-B86N53X:~/work/android-ndk-r6/samples/hello-jni$ $NDK_HOME/ndk-buildGdbserver      : [arm-linux-androideabi-4.4.3] libs/armeabi/gdbserver
Gdbsetup       : libs/armeabi/gdb.setup
Install        : libhello-jni.so => libs/armeabi/libhello-jni.so

把官方自带的hello-jni的工程导入到Eclipse中运行

把上面的代码贴出来让大家看一下：

java文件

/* * 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");    }}

c文件

/* * 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. * */#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 !");}