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：

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。

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。

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