This guide assumes that you are already familiar with concepts inherent in native programming and in Android development.
This section provides a high-level explanation of how the NDK works. The Android NDK is a set of tools allowing you to embed C or C++ (“native code”) into your Android apps. The ability to use native code in Android apps can be particularly useful to developers who wish to do one or more of the following:
- Port their apps between platforms.
- Reuse existing libraries, or provide their own libraries for reuse.
- Increase performance in certain cases, particularly computationally intensive ones like games.
How it Works
This section introduces the main components used in building a native application for Android, and goes on to describe the process of building and packaging.
You should have an understanding of the following components as you build your app:
- ndk-build: The ndk-build script launches the build scripts at the heart of the NDK. These
- Automatically probe your development system and app project file to determine what to build.
- Generate binaries.
- Copy the binaries to your app's project path.
For more information, see ndk-build.
- Java: From your Java source, the Android build process generates
.dex(Dalvik EXecutable) files, which are what the Android OS runs in the Dalvik Virtual Machine (“DVM”). Even if your app contains no Java source code at all, the build process still generates a
.dexexecutable file within which the native component runs.
When developing Java components, use the
nativekeyword to indicate methods implemented as native code. For example, the following function declaration tells the compiler that the implementation is in a native library:
public native int add(int x, int y);
- Native shared libraries: The NDK builds these libraries, or
.sofiles, from your native source code.
Note: If two libraries implement respective methods with the same signature, a link error occurs. In C, "signature" means method name only. In C++, "signature" means not only method name, but also its argument names and types.
- Native static libraries: The NDK can also build static libraries, or
.afiles, which you can link against other libraries.
- Java Native Interface (JNI): The JNI is the interface via which the Java and C++ components talk to one another. This guide assumes knowledge of the JNI; for information about it, consult the Java Native Interface Specification.
- Application Binary Interface (ABI): The ABI defines exactly how your app's machine code is
expected to interact with the system at runtime. The NDK builds
.sofiles against these definitions. Different ABIs correspond to different architectures: The NDK includes ABI support for ARMEABI (default), MIPS, and x86. For more information, see ABI Management.
- Manifest: If you are writing an app with no Java component to it, you must declare the
NativeActivityclass in the manifest. Native Activities and Applications provides more detail on how to do this, under “Using the
Android.mk: You must create an
Android.mkconfiguration file inside your
ndk-buildscript looks at this file, which defines the module and its name, the source files to be compiled, build flags and libraries to link.
Application.mk: This file enumerates and describes the modules that your app requires. This information includes:
- ABIs used to compile for specific platforms.
- Standard libraries to include (static and dynamic STLport or default system).
The general flow for developing a native app for Android is as follows:
- Design your app, deciding which parts to implement in Java, and which parts to implement as
Note: While it is possible to completely avoid Java, you are likely to find the Android Java framework useful for tasks including controlling the display and UI.
- Create an Android app Project as you would for any other Android project.
- If you are writing a native-only app, declare the
AndroidManifest.xml. For more information, see the Native Activities and Applications.
- Create an
Android.mkfile describing the native library, including name, flags, linked libraries, and source files to be compiled in the "JNI" directory.
- Optionally, you can create an
Application.mkfile configuring the target ABIs, toolchain, release/debug mode, and STL. For any of these that you do not specify, the following default values are used, respectively:
- ABI: armeabi
- Toolchain: GCC 4.8
- Mode: Release
- STL: system
- Place your native source under the project's
- Use ndk-build to compile the native (
- Build the Java component, producing the executable
- Package everything into an APK file, containing
.dex, and other files needed for your app to run.
Native Activities and Applications
The Android SDK provides a helper class,
NativeActivity, that allows you to
write a completely native activity.
NativeActivity handles the communication
between the Android framework and your native code, so you do not have to subclass it or call its
methods. All you need to do is declare your application to be native in your
AndroidManifest.xml file, and begin creating your native application.
An Android application using
NativeActivity still runs in its own virtual
machine, sandboxed from other applications. You can therefore still access Android framework APIs
through the JNI. In certain cases, however–such as for sensors, input events, and
assets–the NDK provides native interfaces that you can use instead of having to call
across the JNI. For more information about such support, see
Android NDK Native APIs.
Regardless of whether or not you are developing a native activity, we recommend that you create your projects with the traditional Android build tools. Doing so helps ensure building and packaging of Android applications with the correct structure.
The Android NDK provides you with two choices to implement your native activity:
native_activity.hheader defines the native version of the
NativeActivityclass. It contains the callback interface and data structures that you need to create your native activity. Because the main thread of your application handles the callbacks, your callback implementations must not be blocking. If they block, you might receive ANR (Application Not Responding) errors because your main thread is unresponsive until the callback returns.
android_native_app_glue.hfile defines a static helper library built on top of the
native_activity.hinterface. It spawns another thread, which handles things such as callbacks or input events in an event loop. Moving these events to a separate thread prevents any callbacks from blocking your main thread.
<ndk_root>/sources/android/native_app_glue/android_native_app_glue.c source is
also available, allowing you to modify the implementation.
For more information on how to use this static library, examine the native-activity sample
application and its documentation. Further reading is also available in the comments in the
Using the native_activity.h interface
To implement a native activity with the
- Create a
jni/directory in your project's root directory. This directory stores all of your native code.
- Declare your native activity in the
- Create a file for your native activity, and implement the function named in the
ANativeActivity_onCreatevariable. The app calls this function when the native activity starts. This function, analogous to
mainin C/C++, receives a pointer to an
ANativeActivitystructure, which contains function pointers to the various callback implementations that you need to write. Set the applicable callback function pointers in
ANativeActivity->callbacksto the implementations of your callbacks.
- Set the
ANativeActivity->instancefield to the address of any instance of specific data that you want to use.
- Implement anything else that you want your activity to do upon starting.
- Implement the rest of the callbacks that you set in
ANativeActivity->callbacks. For more information on when the callbacks are called, see Managing the Activity Lifecycle.
- Develop the rest of your application.
- Create an
Android.mk filein the
jni/directory of your project to describe your native module to the build system. For more information, see Android.mk.
- Once you have an
Android.mkfile, compile your native code using the
- Build and install your Android project as usual. If your native code is in
jni/directory, the build script automatically packages the
.sofile(s) built from it into the APK.
Because your application has no Java code, set
<application android:label="@string/app_name" android:hasCode="false">
You must set the
android:name attribute of the activity tag to
<activity android:name="android.app.NativeActivity" android:label="@string/app_name">
android:value attribute of the
meta-data tag specifies the name of the shared
library containing the entry point to the application (such as C/C++
main), omitting the
lib prefix and
.so suffix from the library name.
<meta-data android:name="android.app.lib_name" android:value="native-activity" /> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> </manifest>
$ cd <path>/<to>/<project> $ <ndk>/ndk-build