Android JNI和NDK学习(2)--编程入门
以下文章转自我的GitHub Blog CommonQ's Blog
NDK代码编写
1. 首先是Java代码的编写
Android NDK Sample里面的Hello-jni工程.
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,因为这是属于类的静态方法。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。
2.编写C/C++代码
利用javah这个工具生成相应的.h文件，然后根据这个.h文件编写相应的C/C++代码。
2.1 生成相应.h文件：
就拿我这的环境来说，首先在终端下进入刚刚建立的HelloJni工程的目录：
~$ cd workspace/android/NDK/hello-jni/ls查看工程文件
~/workspace/android/NDK/hello-jni$ ls
AndroidManifest.xml assets bin default.properties gen res src可以看到目前仅仅有几个标准的android应用程序的文件（夹）。
首先我们在工程目录下建立一个jni文件夹：
~/workspace/android/NDK/hello-jni$ mkdir jni
~/workspace/android/NDK/hello-jni$ ls
AndroidManifest.xml assets bin default.properties gen jni res src下面就可以生成相应的.h文件了：
~/workspace/android/NDK/hello-jni$ javah -classpath bin -d jni com.example.hellojni.HelloJni-classpath bin：表示类的路劲
-d jni: 表示生成的头文件存放的目录
com.example.hellojni.HelloJni 则是完整类名
执行这个命令的时候可能会出错,因为需要加载android.jar关联类库.所以需要加上这个参数
-bootclasspath .../android.jar
这一步的成功要建立在已经在 bin/com/example/hellojni/ 目录下生成了 HelloJni.class的基础之上。
现在可以看到jni目录下多了个.h文件：
~/workspace/android/NDK/hello-jni$ cd jni/
~/workspace/android/NDK/hello-jni/jni$ ls
com_example_hellojni_HelloJni.h我们来看看com_example_hellojni_HelloJni.h的内容：
/* DO NOT EDIT THIS FILE - it is machine generated */
#include 上面代码中的JNIEXPORT 和 JNICALL 是jni的宏，在android的jni中不需要
按照：java_pacakege_class_mathod 形式来命名。
也就是说：
Hello.java中 stringFromJNI() 方法对应于 C/C++中的Java_com_example_hellojni_HelloJni_stringFromJNI() 方法
2.2 编写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 在这里我们先不对Android.mk文件做改动.android.mk在jni目录下，Android.mk 文件是Android 的 makefile文件，内容如下：
# 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.
#
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := hello-jni
LOCAL_SRC_FILES := hello-jni.c
include $(BUILD_SHARED_LIBRARY)在eclipse下进行重新编译.将生成的so库打包在apk内运行,就可以看到结果了.
To Be Continued
我会继续对Android.mk文件做研究