Android NDK编程入门笔记
为何要用到NDK?
前段时间，工作中需要用android的应用程序访问android的根文件系统中的文件，但是由于应用程序的权限限制，无法访问这些文件，怎么办？ 所以这就要用到了NDK编程了，既用C/C++代码实现访问系统文件，并将其生成本地库，供android中的java代码调用，这样就可以在java代码中通过调用C/C++编写的库的接口来实现对系统文件的访问。
下面就介绍下Android NDK的入门学习过程：
入门的最好办法就是学习Android自带的例子， 这里就通过学习Android的NDK自带的demo程序：hello-jni来达到这个目的。
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"); }}a: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。b: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。Hello.java中 stringFromJNI() 方法对应于 C/C++中的 Java_com_example_hellojni_HelloJni_stringFromJNI() 方法HelloJni.java中的 unimplementedStringFromJNI() 方法对应于 C/C++中的 Java_com_example_hellojni_HelloJni_unimplementedStringFromJNI() 方法
# 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)#包含下边这两句就可以调用__android_log_print往Log中输出日志LOCAL_C_INCLUDES := $(LOCAL_PATH)/include LOCAL_LDLIBS += -L$(SYSROOT)/usr/lib -llog LOCAL_MODULE := hello-jniLOCAL_SRC_FILES := hello-jni.cinclude $(BUILD_SHARED_LIBRARY)
iaiai@ubuntu:~/workspace/android/NDK/hello-jni$ cd libs/iaiai@ubuntu:~/workspace/android/NDK/hello-jni/libs$ lsarmeabiiaiai@ubuntu:~/workspace/android/NDK/hello-jni/libs$ cd armeabi/iaiai@ubuntu:~/workspace/android/NDK/hello-jni/libs/armeabi$ lsgdbserver gdb.setup libhello-jni.so
