Ubuntu下第一个Jni例子


终端切换到 <sdk>/tools/ 目录下执行 Android 命令就可以把AVD Manager 打开了。 可是,如果你没有添加 <sdk>/tools 到你的环境变量中时,输入 android  回车后终端只会提示你

android:找不到命令

 

而只要在android前加上 ./ 就可以解决问题:

./android

更简单的办法是将 <sdk>/tools 路径添加进 PATH 环境变量。可以添加进用户级环境变量,也可以添加到系统环境变量中。通过命令或编辑文件均可,在这里我只提供一个办法,打开终端,输入:

sudo gedit /etc/environment

回车,在PATH=”………………….”的双引号中追加上:

:<sdk>/tools:<sdk>/platform-tools

比如:

:/opt/android-sdk/tools:/opt/android-sdk/platform-tools

注意,:是分隔符。

重启一下或者 source  /etc/environment(立即生效) 在终端输入 android 回车就会有反应了。

 

安装Android NDK

下载Android NDK : http://developer.android.com/sdk/ndk/index.html

echo 解压缩得到android-ndk-r6b目录,即可。
tar -jxvf android-ndk-r6b-linux-x86.tar.bz2

 

也将其路径加入到source路径中

 

 

 

经过了上述步骤,在命令行下敲:

ndk-bulid

弹出如下的错误,而不是说ndk-build not found,就说明ndk环境已经安装成功了。

Android NDK: Could not find application project directory !   
Android NDK: Please define the NDK_PROJECT_PATH variable to point to it.   
/home/braincol/workspace/android/android-ndk-r5/build/core/build-local.mk:85: *** Android NDK: Aborting    .  Stop.

 

二、代码的编写

1.首先是写java代码

建立一个Android应用工程HelloJni,创建HelloJni.java文件:

HelloJni.java :

/*



 * 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"


);
    }
}

这段代码很简单,注释也很清晰,这里只提两点::

static{
System.loadLibrary("hello-jni" );
}

表明程序开始运行的时候会加载hello-jni, static区声明的代码会先于onCreate方法执行。如果你的程序中有多个类,而且如果HelloJni这个类不是你应用程序的入口,那么 hello-jni(完整的名字是libhello-jni.so)这个库会在第一次使用HelloJni这个类的时候加载。

public native String stringFromJNI();
public native String unimplementedStringFromJNI();

可以看到这两个方法的声明中有 native 关键字, 这个关键字表示这两个方法是本地方法,也就是说这两个方法是通过本地代码(C/C++)实现的,在java代码中仅仅是声明。

用eclipse编译该工程,生成相应的.class文件,这步必须在下一步之前完成,因为生成.h文件需要用到相应的.class文件。

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