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 <jni.h>
/* 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 <jni.h>
#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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