Android 1.5 NDK Native C原生庫開發包

今日android平台將支持Native C原生代碼的開發。Google發布了Android NDK作為Native Developer Kit的開發工具包將會適用於部分底層開發人員。需要說明的是這種方式並不適合大多數開發人員，官方表示他將不會支持android Framework API，以及不允許一個純使用C/C++的程序出現，而是通過Java代碼嵌入Native C或者說通過JNI方式調用一個靜態庫方式執行本地代碼，最終將會打包在apk文件中，還是運行在Dalvik VM之上，所以我們可以理解為這是一個針對性能敏感應用的解決方法。

Windows版本的android NDK下載鏈接 http://dl.google.com/android/ndk/android-ndk-1.5_r1-Windows.zip 文件大小為21.46MB

What is the Android NDK? (android NDK是什麼)

The Android NDK provides tools that allow Android application developers to embed components that make use of native code in their Android applications. (提供了一個工具允許android程序開發人員嵌入組件利用本地代碼在一個android應用程序中)

Android applications run in the Dalvik virtual Machine. The NDK allows developers to implement parts of their applications using native-code languages such as C and C++. This can provide benefits to certain classes of applications, in the form of reuse of existing code and in some cases increased speed. (android應用程序運行在Dalvik虛擬機上面，NDK允許開發人員實現部分的代碼使用本地語言比如C和C++)

The NDK provides:

• A set of tools and build files used to generate native code librarIEs from C and C++ sources
• A way to embed the corresponding native librarIEs into application package files (.apks) that can be deployed on android devices
• A set of native system headers and librarIEs that will be supported in all future versions of the Android platform, starting from android 1.5 (NDK支持從1.5 SDK以後的所有新版開發包)
• Documentation, samples, and tutorials

This release of the NDK supports the ARMv5TE Machine instruction set and provides stable headers for libc (the C library), libm (the Math library), the JNI interface, and other librarIEs.

The NDK will not benefit most applications. As a developer, you will need to balance its benefits against its drawbacks; notably, using native code does not result in an automatic performance increase, but does always increase application complexity. Typical good candidates for the NDK are self-contained, CPU-intensive Operations that don't allocate much memory, such as signal processing, physics simulation, and so on. Simply re-coding a method to run in C usually does not result in a large performance increase. The NDK can, however, can be an effective way to reuse a large corpus of existing C/C++ code. (NDK不適合大多數程序，作為卡開發人員你將需要權衡使用本地代碼的性能提高，但是可能會使其增加程序的復雜性，典型的是針對CPU敏感操作可以不需要分配太多的內存，例如信號處理，屋裡模擬以及簡單的算法描述等等使用C，主要是幫助移植一些現成的C/C++代碼庫)

Please note that the NDK does not enable you to develop native-only applications. android's primary runtime remains the Dalvik virtual Machine.(NDK不能緊緊開發使用本地代碼的程序，最終主要運行在Dalvik這個Java虛擬機上，同時android123表示它將不會支持完整的框架，也就是說無法描述UI，我們可以通過本地代碼生成一個so文件類似Windows下的Dll文件進行執行，比如在處理PDF和Office文件上將會有質的改變從性能角度上講)

Contents of the NDK

Development tools

The NDK includes a set of cross-toolchains (compilers, linkers, etc..) that can generate native ARM binarIEs on Linux, OS X, and Windows (with Cygwin) platforms.

It provides a set of system headers for stable native APIs that are guaranteed to be supported in all later releases of the platform:

• libc (C library) headers
• libm (math library) headers
• JNI interface headers
• libz (Zlib compression) headers
• liblog (android logging) header
• A Minimal set of headers for C++ support

The NDK also provides a build system that lets you work efficIEntly with your sources, without having to handle the toolchain/platform/CPU/ABI details. You create very short build files to describe which sources to compile and which android application will use them — the build system compiles the sources and places the shared librarIEs directly in your application project.

Important: With the exception of the libraries listed above, native system librarIEs in the android 1.5 platform are not stable and may change in future platform versions. Your applications should only make use of the stable native system librarIEs provided in this NDK.

Documentation

The NDK package includes a set of documentation that describes the capabilities of the NDK and how to use it to create shared librarIEs for your android applications. In this release, the documentation is provided only in the downloadable NDK package. You can find the documentation in the <ndk>/docs/ directory. Included are these files:

• INSTALL.TXT — describes how to install the NDK and configure it for your host system
• OVERVIEW.TXT — provides an overview of the NDK capabilitIEs and usage
• ANDROID-MK.TXT — describes the use of the android.mk file, which defines the native sources you want to compile
• APPLICATION-MK.TXT — describes the use of the Application.mk file, which describes the native sources required by your android application
• HOWTO.TXT — information about common tasks associated with NDK development.
• SYSTEM-ISSUES.TXT — known issues in the android system images that you should be aware of, if you are developing using the NDK.
• STABLE-APIS.TXT — a complete list of the stable APIs exposed by headers in the NDK.

Additionally, the package includes detailed information about the "bionic" C library provided with the android platform that you should be aware of, if you are developing using the NDK. You can find the documentation in the <ndk>/docs/system/libc/ directory:

• OVERVIEW.TXT — provides an overvIEw of the "bionic" C library and the features it offers.

Sample applications

The NDK includes two sample Android applications that illustrate how to use native code in your android applications:

• hello-jni — A simple application that loads a string from a native method implemented in a shared library and then displays it in the application UI.
• two-libs — A simple application that loads a shared library dynamically and calls a native method provided by the library. In this case, the method is implemented in a static library that is imported by the shared library.

For more information, see Using the Sample Applications.

System and Software Requirements

The sections below describe the system and software requirements for using the android NDK, as well as platform compatibility considerations that affect appplications using librarIEs produced with the NDK.

The android SDK

• A complete android SDK installation (including all dependencIEs) is required.
• android 1.5 SDK or later version is required.

Supported Operating systems

• Windows XP (32-bit) or Vista (32- or 64-bit)
• Mac OS X 10.4.8 or later (x86 only)
• Linux (32- or 64-bit, tested on Linux Ubuntu Dapper Drake)

Required development tools

• For all development platforms, GNU Make 3.81 or later is required. EarlIEr versions of GNU Make might work but have not been tested.
• For Windows, a recent release of Cygwin, including both the gmake and gcc packages, is required.

android platform compatibility

• The native librarIEs created by the Android NDK can only be used on devices running the android 1.5 platform version or later. This is due to toolchain and ABI related changes that make the native librarIEs incompatible with 1.0 and 1.1 system images.
• For this reason, you should use native librarIEs produced with the NDK in applications that are deployable to devices running the android 1.5 platform version or later. To ensure compatibility, an application using a native library produced with the NDK must declare a <uses-library> element in its manifest file, with the attribute android:minSdkVersion="3".

Installing the NDK

Installing the NDK on your development computer is straightforward and involves extracting the NDK from its download package and running a host-setup script.

Before you get started make sure that you have downloaded the latest Android SDK and upgraded your applications and environment as needed. The NDK will not work with older versions of the android SDK. Also, take a moment to revIEw the System and Software Requirements for the NDK, if you haven't already.

To install the NDK, follow these steps:

1. From the table at the top of this page, select the NDK package that is appropriate for your development computer and download the package.
2. Uncompress the NDK download package using tools available on your computer. When uncompressed, the NDK files are contained in a directory called android-ndk-<version>. You can rename the NDK directory if necessary and you can move it to any location on your computer. This documentation refers to the NDK directory as <ndk>.
3. Open a terminal, change to the NDK directory, and run the host-setup script. The script sets up your environment and generates a host configuration file used later, when building your shared librarIEs. The path to the host-setup script is:

   <ndk>/build/host-setup.sh

   If the script completes successfully, it prints a "Host setup complete." message. If it fails, it prints instructions that you can follow to correct any problems.

Once you have run the host-setup script, you are ready start working with the NDK.

Getting Started with the NDK

Once you've installed the NDK successfully, take a few minutes to read the documentation included in the NDK. You can find the documentation in the <ndk>/docs/ directory. In particular, please read the OVERVIEW.TXT document completely, so that you understand the intent of the NDK and how to use it.

Here's the general outline of how you work with the NDK tools:

1. Place your native sources under <ndk>/sources/<my_src>/.... If you want, you can place a symlink to your sources, rather than the sources themselves. The sources you reference here are not strictly associated with a specific shared library or android application. Instead, they are Accessible to any build configuration and can be used to produce any number of shared librarIEs that can be used by any android application.
2. Create <ndk>/sources/<my_src>/android.mk to describe your native sources to the NDK build system
3. Create <ndk>/aPPS/<my_app>/Application.mk to describe your Android application and native sources it needs to the NDK build system. This file sets up the link between an android SDK application project and any number of shared librarIEs defined in the <ndk>/sources/ folder and it specifIEs the path to the application project that will receive the shared library built from the sources.
4. Build your native code by running this make command from the top-level NDK directory:

   $ make APP=<my_app>

   The build tools copy the stripped, shared librarIEs needed by your application to the proper location in the application's project directory.

5. Finally, compile your application using the SDK tools in the usual way. The SDK build tools will package the shared librarIEs in the application's deployable .apk file.

For complete information on all of the steps listed above, please see the documentation included with the NDK package.

Using the Sample Applications

The NDK includes two sample applications that illustrate how to use native code in your android applications:

• hello-jni — A simple application that loads a string from a native method implemented in a shared library and then displays it in the application UI.
• two-libs — A simple application that loads a shared library dynamically and calls a native method provided by the library. In this case, the method is implemented in a static library imported by the shared library.

For each sample, the NDK includes an Android application project, as well as the corresponding C source code and the necessary android.mk and Application.mk files. The application projects are provided in <ndk>/aPPS/<app_name>/project/ and the C source for each application is provided in <ndk>/sources/samples/<library>/.

Once you have installed the NDK, you can build the shared librarIEs from the NDK by using these commands from the root of the NDK directory:

• $ make APP=hello-jni — compiles <ndk>/sources/samples/hello-jni/hello-jni.c and outputs a shared library to <ndk>/aPPS/hello-jni/project/libs/armeabi/libhello-jni.so.
• $ make APP=two-libs — compiles <ndk>/sources/samples/two-libs/second.c and first.c and outputs a shared library to <ndk>/aPPS/two-libs/project/libs/armeabi/libtwolib-second.so.

Next, build the sample android applications that use the shared librarIEs:

• If you are developing in Eclipse with ADT, use the New Project Wizard to create a new android project for each sample, using the "Import from Existing Source" option and importing the source from <ndk>/aPPS/<app_name>/project/. Then, set up an AVD, if necessary, and build/run the application in the emulator. For more information about creating a new android project in Eclipse, see Developing in Eclipse.
• If you are developing with Ant, use the android tool to create the build file for each of the sample projects at <ndk>/aPPS/<app_name>/project/. Then set up an AVD, if necessary, build your project in the usual way, and run it in the emulator