android studio 3.0 JNI使用

前提下是要装了NDK

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步骤1：

创建一个生成头文件的java文件，作为模板
public class Java2CJNI {

    static {
        //System的S是大写
        System.loadLibrary("Java2C");
    }

    //这个就是我们将来需要调用的方法
    public native String java2C();
}

通过Terminal输入命令生成头文件：
生成对应头文件所在的包名+类名
javah -classpath F:\project7\JNINDKStuddy\app\build\intermediates\classes\debug -jni com.jnindkstuddy.Java2CJNI

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其中JNINDKStuddy是项目的包名。按下enter键后，如果成功，就会生成.h文件。

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下面就是com_jnindkstuddy_Java2CJNI.h的代码

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class com_jnindkstuddy_Java2CJNI */

#ifndef _Included_com_jnindkstuddy_Java2CJNI
#define _Included_com_jnindkstuddy_Java2CJNI
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_jnindkstuddy_Java2CJNI
 * Method:    java2C
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_jnindkstuddy_Java2CJNI_java2C
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

步骤2：
新建一个Java2C.cpp文件（.cpp是c++，.c是C）
把上面com_jnindkstuddy_Java2CJNI.h的函数代码复制到新建的这个Java2C.cpp文件中。代码如下

#include 
#include "com_jnindkstuddy_Java2CJNI.h"

JNIEXPORT jstring JNICALL Java_com_jnindkstuddy_Java2CJNI_java2C(JNIEnv* env, jobject instance)
{
    return (env)->NewStringUTF("I am From Native C");
}

步骤3：
配置studio文件：
1.配置 CMakeLists.txt，as 3.0后台打so库需要在CMake下。

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CMakeLists.txt如下：

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

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 them for you.
# Gradle automatically packages shared libraries with your APK.

add_library( # Sets the name of the library.  设置生成so文件名
            Java2C  

            # Sets the library as a shared library.
            SHARED

            # Provides a relative path to your source file(s).  要编译的C/C++文件
            src/main/jni/Java2C.cpp)

# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries 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 )

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

target_link_libraries( # Specifies the target library. 指定连接的目标库
                      Java2C

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

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构建。

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如果没有报错的，就会生成so库。

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使用，JNI是JAVA和C++/C进行通信，和JAVA 和JS通信的用法差不多，毕竟是跨平台语言。

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