NDK開發入門（一）

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開發NDK我選擇最新版的Android Studio2.2.2,主要原因是最新版已經很好的支持了c/c++的代碼提示、代碼調試和源代碼跳轉等功能，個人感覺非常好用。

開始實踐

1.首先下載最新版Android studio並且下載ndk開發中需要的工具，如下圖所示：

2.安裝好三個工具後我們就可以進行開發了，首先我們新建一個項目，其中選中“Include C++ Support”，如下圖所示：

3.創建好之後，點擊“run”就可以安裝運行了。

使用c/c++進行開發

創建一個Java類

public class NdkTest {


    public static native String callFromNativ(String str);

    public static native void callJavaCode(String str);

    public static native void getBitmapInfo(Bitmap bitmap);

    public static native void callJavaInstaceCode(String str);

}

public class CallFromNative {
    public static final String TAG = "CallFromNative";

    public static void showLog(String str) {
        Log.e(TAG,"callByNative = "+str);
    }

    public void showInstanceLog(String str) {
         Log.e(TAG,"callByNative = "+str);
    }

}

生成頭文件

通過命令行生成頭文件，首先進入到java目錄下然後輸入命令

然後你就可以看到生成的頭文件了,打開頭文件如下所示：

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class test_com_example_liaojd_myapplication_NdkTest */

#ifndef _Included_test_com_example_liaojd_myapplication_NdkTest
#define _Included_test_com_example_liaojd_myapplication_NdkTest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     test_com_example_liaojd_myapplication_NdkTest
 * Method:    callFromNativ
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callFromNativ
  (JNIEnv *, jclass, jstring);

/*
 * Class:     test_com_example_liaojd_myapplication_NdkTest
 * Method:    callJavaCode
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callJavaCode
  (JNIEnv *, jclass, jstring);
/*
 * Class:     test_com_example_liaojd_myapplication_NdkTest
 * Method:    callJavaInstaceCode
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callJavaInstaceCode
  (JNIEnv *, jclass, jstring);

/*
 * Class:     test_com_example_liaojd_myapplication_NdkTest
 * Method:    getBitmapInfo
 * Signature: (Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_getBitmapInfo
  (JNIEnv *, jclass, jobject);

#ifdef __cplusplus
}
#endif
#endif

實現函數聲明

在cpp文件夾下面創建一個main.c的文件，如下所示：

//
// Created by liaojd on 2017/1/13.
//
#include <android/bitmap.h>
#include "jni.h"
#include "android/log.h"
#include "android/bitmap.h"

#define  TAG    "NDK_TEST"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, TAG,  __VA_ARGS__)

/**
 * 實現打印輸出並返回一個字符串
 */
JNIEXPORT jstring JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callFromNativ
        (JNIEnv *env, jclass obj, jstring jstr) {
    LOGD("test ndk");
    return (*env)->NewStringUTF(env,"Test NDK");
}


/**
 *回調Java靜態方法
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callJavaCode
        (JNIEnv *env, jclass thiz, jstring jstr) {
    //得到類的實例對象
    jclass clazz = (*env)->FindClass(env,"test/com/example/liaojd/myapplication/CallFromNative");

    jmethodID showLog= (*env)->GetStaticMethodID(env, clazz, "showLog","(Ljava/lang/String;)V");
    //調用java函數
    jstring str =(*env)->NewStringUTF(env, "Hi,I'm From C");
    (*env)->CallStaticVoidMethod(env,clazz,showLog,str);
}

/**
 * 回調Java實例方法
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_callJavaInstaceCode
        (JNIEnv *env, jclass thiz, jstring jstr) {
    //得到類的實例對象
    jclass clazz = (*env)->FindClass(env,"test/com/example/liaojd/myapplication/CallFromNative");

    jmethodID showInstanceLog= (*env)->GetMethodID(env, clazz, "showInstanceLog","(Ljava/lang/String;)V");
    //得到一個實例對象
    jobject instance = (*env)->AllocObject(env,clazz);
    //調用java函數
    jstring  str =(*env)->NewStringUTF(env, "Hi,I'm From C");
    (*env)->CallVoidMethod(env,instance,showInstanceLog,str);
}

/**
 * 獲取圖片信息
 */
JNIEXPORT void JNICALL Java_test_com_example_liaojd_myapplication_NdkTest_getBitmapInfo
        (JNIEnv *env, jclass obj, jobject jbitmap) {
    AndroidBitmapInfo info;
    AndroidBitmap_getInfo(env,jbitmap,&info);
    LOGD("bitmap width = %d height = %d", info.width, info.height);
}

上面的例子中獲取方法的ID的時候需要傳入方法描述符，在Java中它表示方法的簽名，Java類型簽名映射表如下：

Java類型	簽名
Boolean	Z
Byte	B
Char	C
Short	S
Int	I
Long	J
Float	F
Double	D
fully-qualified-class	Lfully-qualified-clas;
type[]	[type
method type	(arg-type)ret-type

用類型簽名映射手工生成的方法描述符是一件非常繁瑣的任務，不一定每次都記住，所以JDK爲我們提供了javap反彙編命令行程序，如下所示：

加載動態庫

    static {
        System.loadLibrary("native-lib");
    }

添加運行的代碼

  NdkTest.callFromNativ("callFromNativ");
  NdkTest.callJavaCode("callJavaCode");
  NdkTest.callJavaInstaceCode("callInstanceCode");
  NdkTest.getBitmapInfo(bitmapDrawable.getBitmap());

因爲bitmap.h在libjnigraphics.so中，所以修改CMakeLists.txt文件，如下所示：

# Sets the minimum version of CMake required to build the native
# library. You should either keep the default value or only pass a
# value of 3.4.0 or lower.

cmake_minimum_required(VERSION 3.4.1)

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds it for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             # Associated headers in the same location as their source
             # file are automatically included.
             src/main/cpp/main.c)

# Searches for a specified prebuilt library and stores the path as a
# variable. Because system libraries are included in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

find_library( # Sets the name of the path variable.
              jnigraphics-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              jnigraphics )

# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in the
# build script, prebuilt third-party libraries, or system libraries.

target_link_libraries( # Specifies the target library.
                       native-lib

                       # Links the target library to the log library
                       # included in the NDK.
                       ${jnigraphics-lib}
                       ${log-lib} )

涉及到相關的知識點

1.Cmake參考資料
cmake document
2.最新版Android studio關於如何使用c/c++請參考官方文檔
官方使用教程