The Android NDK is a toolset that lets you embed components that make use of native code in your Android applications.
Android applications run in the Dalvik virtual machine. The NDK allows you to implement parts of your 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.
The NDK provides:
.apk) that can be deployed on Android devicesThe latest release of the NDK supports these ARM instruction sets:
Future releases of the NDK will also support:
ARMv5TE machine code will run on all ARM-based Android devices. ARMv7-A will run only on
devices such as the Verizon Droid or Google Nexus One that have a compatible CPU. The main
difference between the two instruction sets is that ARMv7-A supports hardware FPU, Thumb-2, and
NEON instructions. You can target either or both of the instruction sets — ARMv5TE is the
default, but switching to ARMv7-A is as easy as adding a single line to the application's
Application.mk file, without needing to change anything else in the file. You can also build for
both architectures at the same time and have everything stored in the final .apk.
Complete information is provided in the CPU-ARCH-ABIS.HTML in the NDK package.
The NDK provides stable headers for libc (the C library), libm (the Math library), OpenGL ES (3D graphics library), the JNI interface, and other libraries, as listed in the Development Tools section.
The NDK will not benefit most applications. As a developer, you need to balance its benefits against its drawbacks; notably, using native code does not result in an automatic performance increase, but always increases application complexity. In general, you should only use native code if it is essential to your application, not just because you prefer to program in C/C++.
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. When examining whether or not you should develop in native code, think about your requirements and see if the Android framework APIs provide the functionality that you need. The NDK can, however, can be an effective way to reuse a large corpus of existing C/C++ code.
The Android framework provides two ways to use native code:
Write a native activity, which allows you to implement the lifecycle callbacks in native
code. The Android SDK provides the NativeActivity class, which is a convenience class that notifies your
native code of any activity lifecycle callbacks (onCreate(), onPause(),
onResume(), etc). You can implement the callbacks in your native code to handle
these events when they occur. Applications that use native activities must be run on Android
2.3 (API Level 9) or later.
You cannot access features such as Services and Content Providers natively, so if you want to use them or any other framework API, you can still write JNI code to do so.
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:
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 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.
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:
cpufeatures static library that
lets your application code detect the target device's CPU family and the optional features at
runtime.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:
The NDK includes 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.san-angeles — a simple application that renders 3D graphics through the
native OpenGL ES APIs, while managing activity lifecycle with a GLSurfaceView object.hello-gl2 — a simple application that renders a triangle using OpenGL ES
2.0 vertex and fragment shaders.hello-neon — a simple application that shows how to use the
cpufeatures library to check CPU capabilities at runtime, then use NEON intrinsics
if supported by the CPU. Specifically, the application implements two versions of a tiny
benchmark for a FIR filter loop, a C version and a NEON-optimized version for devices that
support it.bitmap-plasma — a simple application that demonstrates how to access the
pixel buffers of Android Bitmap objects from native code, and uses
this to generate an old-school "plasma" effect.native-activity — a simple application that demonstrates how to use the
native-app-glue static library to create a native activitynative-plasma — a version of bitmap-plasma implemented with a native
activity.For each sample, the NDK includes the corresponding C source code and the necessary Android.mk
and Application.mk files. There are located under <ndk>/samples/<name>/
and their source code can be found under <ndk>/samples/<name>/jni/.
You can build the shared libraries for the sample apps by going into
<ndk>/samples/<name>/ then calling the ndk-build command.
The generated shared libraries will be located under
<ndk>/samples/<name>/libs/armeabi/ for (ARMv5TE machine code) and/or
<ndk>/samples/<name>/libs/armeabi-v7a/ for (ARMv7 machine code).
Next, build the sample Android applications that use the shared libraries:
<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.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. For more information, see Developing in Other IDEs.The hello-jni sample is a simple demonstration on how to use JNI from an Android application. The HelloJni activity receives a string from a simple C function and displays it in a TextView.
The main components of the sample include:
AndroidManifest.xml
file, a src/ and res directories, and a main activity)jni/ directory that includes the implemented source file for the native code
as well as the Android.mk filetests/ directory that contains unit test code.android tool to update the project so it generates a build.xml file that you can
use to build the sample.
<ndk-root>/samples/hello-jni directory.<ndk-root>/samples/hello-jni directory.android update project -p . -s
ndk-build command.
cd <ndk-root>/samples/hello-jni <ndk_root>/ndk-build
ant debug adb install bin/HelloJni-debug.apk
When you run the application on the device, the string Hello JNI should appear on
your device. You can explore the rest of the samples that are located in the
<ndk-root>/samples directory for more examples on how to use the JNI.
The native-activity sample provided with the Android NDK demonstrates how to use the android_native_app_glue static library. This static library makes creating a native activity easier by providing you with an implementation that handles your callbacks in another thread, so you do not have to worry about them blocking your main UI thread. The main parts of the sample are described below:
AndroidManifest.xml
file, a src/ and res directories). The AndroidManifest.xml declares
that the application is native and specifies the .so file of the native activity. See NativeActivity for the source or see the
<ndk_root>/platforms/samples/native-activity/AndroidManifest.xml file.jni/ directory contains the native activity, main.c, which uses the
android_native_app_glue.h interface to implement the activity. The Android.mk that
describes the native module to the build system also exists here.To build this sample application:
android tool to update the project so it generates a build.xml file that you can
use to build the sample.
<ndk-root>/samples/native-activity directory.<ndk-root>/samples/native-activity directory.android update project -p . -s
ndk-build command.
cd <ndk-root>/platforms/samples/android-9/samples/native-activity <ndk_root>/ndk-build
ant debug adb install bin/NativeActivity-debug.apk
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.
<uses-sdk> element in its manifest file, with an
android:minSdkVersion attribute value of "3" or higher. For example:
<manifest> ... <uses-sdk android:minSdkVersion="3" /> ... </manifest>
android:minSdkVersion attribute value, as given in the
table.| OpenGL ES Version Used | Compatible Android Platform(s) | Required uses-sdk Attribute |
|---|---|---|
| OpenGL ES 1.1 | Android 1.6 and higher | android:minSdkVersion="4" |
| OpenGL ES 2.0 | Android 2.0 and higher | android:minSdkVersion="5" |
For more information about API Level and its relationship to Android platform versions, see Android API Levels.
<uses-feature> element in its manifest, with an
android:glEsVersion attribute that specifies the minimum OpenGl ES version
required by the application. This ensures that Android Market will show your application only
to users whose devices are capable of supporting your application. For example:
<manifest> ... <uses-feature android:glEsVersion="0x00020000" /> ... </manifest>
For more information, see the <uses-feature>
documentation.
Bitmap pixel buffers or utilizes native activities, the application
containing the library can be deployed only to devices running Android 2.2 (API level 8) or
higher. To ensure compatibility, make sure that your application declares <uses-sdk
android:minSdkVersion="8" /> attribute value in its manifest.