原文链接:http://blog.youkuaiyun.com/dawanganban/article/details/42399953
一、什么是JNI
JNI是Java Native Interface的缩写(Java本地调用),Java程序中的函数可以调用Native语言写的函数(一般指的是C/C++编写的函数),Native语言写的函数可以调用Java层的函数。
二、为什么要有JNI
Java语言的跨平台是因为在不同平台上可以运行Java虚拟机,而虚拟机是跑在具体平台上的,而本质上Java是通过JNI技术实现的跨平台,很多基层的模块在Java语言诞生之前已经有了比较优秀的实现,为了避免重复造轮子所以我们要使用JNI技术来使用已有的模块。
三、Mac OS上的环境搭建
在这里说明一下Max OS上的所需环境搭建,Windows和Linux的请搜索相关资料。
1、安装JDK(此处省略)。
2、安装ADT(Android Develop Tools),包括SDK和ADT插件,下载地址:http://pan.baidu.com/s/1o6OBIHG
3、安装Xcode可以去苹果商店下载安装(免费)。
4、安装Apache ANT(下载地址:http://ant.apache.org/bindownload.cgi)详细安装过程可以参考:http://blog.youkuaiyun.com/song_hui_xiang/article/details/14315529
5、安装GNU Make(默认已经安装,所以不用安装)可以使用 make -version命令验证是否安装。
6、安装NDK(下载地址:http://pan.baidu.com/s/1i3l5L8T),解压后在用户根目录下新建文件.bash_profile然后添加如下两行(配置环境变量,可以暂时不配置)。
- export ANDROID_NDK_HOME=/Users/lixiaoqiang/Documents/install_tools/ndk/android-ndk-r10c
- export PATH=${PATH}:${ANDROID_NDK_HOME}
关于上面部分开发工具简要介绍:
1、Apache Ant,是一个将软件编译、测试、部署等步骤联系在一起加以自动化的一个工具,大多用于Java环境中的软件开发。
2、NDK是Android原生开发工具包,可以支持C/C++等原生编程语言开发Android应用,它提供头文件、库和交叉编译工具链。
四、第一个示例程序
转载请说明出处:http://blog.youkuaiyun.com/dawanganban
1、为eclipse指定NDK路径
2、导入Android NDK中的示例代码(导入hello-jni工程),做过Android开发的朋友应该很熟悉,这里就不啰嗦了。
3、向项目中添加原生支持
项目——>右击——>Android Tools——>Add Native Support
该项目其实已经包含了一个原生项目,所以这一步可以跳过,我们直接Finish继续。
4、插上手机(模拟器太慢了,建议使用真机)运行项目。在C/C++界面视图我们可以看到如下信息
- **** Build of configuration Default for project HelloJni ****
- /Users/lixiaoqiang/Documents/install_tools/ndk/android-ndk-r10c/ndk-build all
- Android NDK: WARNING: APP_PLATFORM android-19 is larger than android:minSdkVersion 3 in ./AndroidManifest.xml
- [arm64-v8a] Gdbserver : [aarch64-linux-android-4.9] libs/arm64-v8a/gdbserver
- [arm64-v8a] Gdbsetup : libs/arm64-v8a/gdb.setup
- [x86_64] Gdbserver : [x86_64-4.9] libs/x86_64/gdbserver
- [x86_64] Gdbsetup : libs/x86_64/gdb.setup
- [mips64] Gdbserver : [mips64el-linux-android-4.9] libs/mips64/gdbserver
- [mips64] Gdbsetup : libs/mips64/gdb.setup
- [armeabi-v7a] Gdbserver : [arm-linux-androideabi-4.6] libs/armeabi-v7a/gdbserver
- [armeabi-v7a] Gdbsetup : libs/armeabi-v7a/gdb.setup
- [armeabi] Gdbserver : [arm-linux-androideabi-4.6] libs/armeabi/gdbserver
- [armeabi] Gdbsetup : libs/armeabi/gdb.setup
- [x86] Gdbserver : [x86-4.6] libs/x86/gdbserver
- [x86] Gdbsetup : libs/x86/gdb.setup
- [mips] Gdbserver : [mipsel-linux-android-4.6] libs/mips/gdbserver
- [mips] Gdbsetup : libs/mips/gdb.setup
- [arm64-v8a] Compile : hello-jni <= hello-jni.c
- [arm64-v8a] SharedLibrary : libhello-jni.so
- [arm64-v8a] Install : libhello-jni.so => libs/arm64-v8a/libhello-jni.so
- [x86_64] Compile : hello-jni <= hello-jni.c
- [x86_64] SharedLibrary : libhello-jni.so
- [x86_64] Install : libhello-jni.so => libs/x86_64/libhello-jni.so
- [mips64] Compile : hello-jni <= hello-jni.c
- [mips64] SharedLibrary : libhello-jni.so
- [mips64] Install : libhello-jni.so => libs/mips64/libhello-jni.so
- [armeabi-v7a] Compile thumb : hello-jni <= hello-jni.c
- [armeabi-v7a] SharedLibrary : libhello-jni.so
- [armeabi-v7a] Install : libhello-jni.so => libs/armeabi-v7a/libhello-jni.so
- [armeabi] Compile thumb : hello-jni <= hello-jni.c
- [armeabi] SharedLibrary : libhello-jni.so
- [armeabi] Install : libhello-jni.so => libs/armeabi/libhello-jni.so
- [x86] Compile : hello-jni <= hello-jni.c
- [x86] SharedLibrary : libhello-jni.so
- [x86] Install : libhello-jni.so => libs/x86/libhello-jni.so
- [mips] Compile : hello-jni <= hello-jni.c
- [mips] SharedLibrary : libhello-jni.so
- [mips] Install : libhello-jni.so => libs/mips/libhello-jni.so
- **** Build Finished ****
五、项目结构及主要目录介绍
1、jni目录:包含原生组件的源代码及描述原生组件构建方法的Make文件(Android.mk),该目录作为NDK构建项目时的构建目录。
2、libs目录:包含指定目标平台的独立子目录,在打包时该目录被包含在apk文件中。
3、obj目录:这是一个中间目录,编译源码后产生的目标文件都保存在该目录下,我们最好不用访问该目录。
六、 实例工程解析
Android.mk的内容如下
- # 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)
第一行:Android.mk文档必须以LOCAL_PATH变量的定义开头,my-dir是一个系统的宏定义,来定义源文件的目录位置。
第二行:Android构建系统将CLEAR_VARS变量设置为clear-vars.mk片段的位置,更多片段的makefile文件请看ndk\build\core目录,如下:
作用是清除除了LOCAL_PATH以外的LOCAL_<name>变量,这样可以避免冲突。
第三行:每一个原生组件被称为一个模块,LOCAL_MODULE变量用来给这些模块设定一个唯一的名称。
第四行:LOCAL_SRC_FILES变量定义用来建立和组装这个模块的源文件列表,用空格隔开。
第五行:指明了build-shared-library.mk文件的保存位置,该片段包含了将源文件构建成共享库的必要过程。
下面我们来看看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");
- }
- }
- #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 )
- {
- #if defined(__arm__)
- #if defined(__ARM_ARCH_7A__)
- #if defined(__ARM_NEON__)
- #if defined(__ARM_PCS_VFP)
- #define ABI "armeabi-v7a/NEON (hard-float)"
- #else
- #define ABI "armeabi-v7a/NEON"
- #endif
- #else
- #if defined(__ARM_PCS_VFP)
- #define ABI "armeabi-v7a (hard-float)"
- #else
- #define ABI "armeabi-v7a"
- #endif
- #endif
- #else
- #define ABI "armeabi"
- #endif
- #elif defined(__i386__)
- #define ABI "x86"
- #elif defined(__x86_64__)
- #define ABI "x86_64"
- #elif defined(__mips64) /* mips64el-* toolchain defines __mips__ too */
- #define ABI "mips64"
- #elif defined(__mips__)
- #define ABI "mips"
- #elif defined(__aarch64__)
- #define ABI "arm64-v8a"
- #else
- #define ABI "unknown"
- #endif
- return (*env)->NewStringUTF(env, "Hello from JNI ! Compiled with ABI " ABI ".");
- }