android ndk 内容

Concepts

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Before Beginning
Introduction
How It Works
Native Activities and Applications

Before Beginning

This guide assumes假定 that you are:
Already familiar with concepts inherent in native programming and in Android development.
Working in Eclipse, and using the Android Development Tools (ADT), except where otherwise noted.

Introduction

This section provides a high-level explanation of how the NDK works. The Android NDK is a set of tools allowing you to embed潜入 C or C++ (“native code”) into your Android apps. The ability to use native code in Android
apps can be particularly特别是 useful to developers
who wish to do one or more of the following:
Port(港口 端口n 持v) their apps between platforms.
Reuse重用 existing libraries, or provide their own libraries for reuse.
Increase performance in certain cases, particularly computationally计算 intensive密集 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.

Main components

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 scripts:
Automatically probe探索  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 

.dex

 executable执行档 file within which the native component runs.
When developing Java components, use the 

native

 keyword to indicate表明  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 

.so

 files, from your native source code.

Note: If two libraries implement respective（各自的
分别的 单独的） methods with the same signature签名, 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 

.a

 files, 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假定  knowledge of the JNI; for information about it, consult the Java
Native Interface Specification.
Application Binary Interface (ABI): The ABI defines exactly正确地 how your app's machine code is expected预期 to interact相互作用 with the system
at runtime. The NDK builds 

.so

 files against these definitions定义. 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 

NativeActivity

 class
in the manifest. Native
Activities and Applications provides more detail on how to do this, under “Using the

native_activity.h

 interface.”
The following two items are only required必需的 for building using the 

ndk-build

 script,
and for debugging using the

ndk-gdb

 script.

Android.mk

: You must create an 

Android.mk

 configuration
file inside your 

jni

 folder. The 

ndk-build

script looks at this file, which defines the module组件  and its name, the source files to be compiled, build flags and libraries to link.

Application.mk

: This file enumerates列举 and describes the modules
that your app requires. This information includes:
ABIs used to compile for specific platforms.
Toolchains.
Standard libraries to include (static and dynamic动态  STLport or default system).

Flow流程

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 native code.

Note: While it is possible to completely avoid避免 Java, you are likely to find the Android Java framework useful for tasks including controlling the display and UI.

Create an Android app Project in Eclipse as you would for any other Android project.
If you are writing a native-only app, declare the 

NativeActivity

 class
in 

AndroidManifest.xml

. You can do so from the Eclipse/ADT Android Manifest Editor, or by hand-editing the file. For more information, see theNative
Activities and Applications.
Create an 

Android.mk

 file describing the native library, including name, flags, linked libraries, and source files to be compiled in the ‘JNI’ directory.
Optionally可选择地, you can create an 

Application.mk

 file configuring the target ABIs, toolchain, release/debug mode, and STL（标准模板库(Standard Template Library)）.
For any of these that you do not specify指定, 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 

jni

 directory.
Use ndk-build to compile the native (

.so

, 

.a

) libraries.
Build the Java component, producing生产 the executable可执行的 

.dex

 file.
Package everything into an APK file, containing 

.so

, 

.dex

, and other files needed for your app to run.
Note that Eclipse can perform steps 7. through 9. in a single operation.

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沙盒 from other applications. You can therefore因此 still access Android framework APIs through the JNI. In certain cases在某些特定的情况下, 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建议 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:
The 

native_activity.h

 header defines the native version
of the 

NativeActivity

 class. 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.
The 

android_native_app_glue.h

 file defines a static helper library built on top of the 

native_activity.h

interface.
It spawns生成 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.
The 

<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检查 the native-activity sample application and its documentation. Further reading is also available in the comments in the

<ndk_root>/sources/android/native_app_glue/android_native_app_glue.h

 file.

Using the native_activity.h interface

To implement a native activity with the 

native_activity.h

 interface:
Create a 

jni/

 directory in your project's root directory. This directory stores all of your native code.
Declare your native activity in the 

AndroidManifest.xml

 file.
Because your application has no Java code, set 

android:hasCode

 to 

false

.

<application android:label="@string/app_name" android:hasCode="false">

You must set the 

android:name

 attribute of the activity tag to 

NativeActivity

.

<activity android:name="android.app.NativeActivity"
            android:label="@string/app_name">

Note: You can subclass 

NativeActivity

. If you do, use the name of
the subclass instead of

NativeActivity

.
The 

android:value

 attribute of the 

meta-data

 tag specifies指定 the name of the shared library containing the entry point to the application
(such as C/C++ 

main

), omitting省略  the 

lib

 prefix前缀 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>

Create a file for your native activity, and implement the function named in the 

ANativeActivity_onCreate

variable变量.
The app calls this function when the native activity starts. This function, analogous（相似的
模拟的 ） to 

main

 in C/C++, receives a pointer to an 

ANativeActivity

 structure,
which contains function pointers to the various callback implementations that you need to write. Set the applicable callback function pointers in

ANativeActivity->callbacks

 to the implementations of your callbacks.
Set the 

ANativeActivity->instance

 field 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 file

 in 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.mk

 file, compile your native code using
the 

ndk-build

 command.

$ cd <path>/<to>/<project>
$ <ndk>/ndk-build

Build and install your Android project as usual, using Ant or Eclipse. If your native code is in the 

jni/

directory, the build script automatically packages the 

.so

 file(s) built from
it into the APK.