一.介绍

JNI(Java Native Interface)是Java提供的编程接口，
通过JNIEnv指针,C/C++和java代码可以相互调用传递数据！

NDK是Android开发C/C++工具集(代码库和编译工具),能将C/C++编程成不同平台(CUP/ABI)的so库

jni.h文件位置:
1.Android-ndk/platforms/android-版本/arch-arm/usr/include/jni.h
2.JDK/include/jni.h

二.C/C++和Java数据类型映射

在jni.h中 
typedef为C/C++类型定义别名,和java类型建立映射

1.基本数据类型映射
typedef unsigned char   jboolean;       /* unsigned 8 bits */
typedef signed char     jbyte;          /* signed 8 bits */
typedef unsigned short  jchar;          /* unsigned 16 bits */
typedef short           jshort;         /* signed 16 bits */
typedef int             jint;           /* signed 32 bits */
typedef long long       jlong;          /* signed 64 bits */
typedef float           jfloat;         /* 32-bit IEEE 754 */
typedef double          jdouble;        /* 64-bit IEEE 754 */

2.类对象映射
2.1在C++中(全都继承于_jobject类)
class _jobject {};
class _jclass : public _jobject {};
class _jstring : public _jobject {};
class _jarray : public _jobject {};
class _jobjectArray : public _jarray {};
class _jbooleanArray : public _jarray {};
class _jbyteArray : public _jarray {};
class _jcharArray : public _jarray {};
class _jshortArray : public _jarray {};
class _jintArray : public _jarray {};
class _jlongArray : public _jarray {};
class _jfloatArray : public _jarray {};
class _jdoubleArray : public _jarray {};
class _jthrowable : public _jobject {};
typedef _jobject*       jobject;
typedef _jclass*        jclass;
typedef _jstring*       jstring;
typedef _jarray*        jarray;
typedef _jobjectArray*  jobjectArray;
typedef _jbooleanArray* jbooleanArray;
typedef _jbyteArray*    jbyteArray;
typedef _jcharArray*    jcharArray;
typedef _jshortArray*   jshortArray;
typedef _jintArray*     jintArray;
typedef _jlongArray*    jlongArray;
typedef _jfloatArray*   jfloatArray;
typedef _jdoubleArray*  jdoubleArray;
typedef _jthrowable*    jthrowable;
typedef _jobject*       jweak;

2.2在C中(没有类概念,全都是void*别名)
typedef void*           jobject;
typedef jobject         jclass;
typedef jobject         jstring;
typedef jobject         jarray;
typedef jarray          jobjectArray;
typedef jarray          jbooleanArray;
typedef jarray          jbyteArray;
typedef jarray          jcharArray;
typedef jarray          jshortArray;
typedef jarray          jintArray;
typedef jarray          jlongArray;
typedef jarray          jfloatArray;
typedef jarray          jdoubleArray;
typedef jobject         jthrowable;
typedef jobject         jweak;

三.在java中调用C/C++函数

1.在java中定义native方法(C/C++函数接口)

package com.example;
public class JavaDemo { 
    public native String getHello();
}

2.实现C/C++函数接口

#include <stdio.h>
#include <jni.h>

// JNIEnv*封装很多java方法，jobject obj代表调用该函数的java对象
jstring Java_com_example_JavaDemo_getHello(JNIEnv* env,jobject obj){
    return (*env)->NewStringUTF(env,"hello...");
}

3.使用NDK编译C/C++，生成libhello.so

在Android Studio编译C/C++
https://developer.android.google.cn/studio/projects/add-native-code.html
Google官方文档中国版, 专为中国开发者建的网站！

4.加载so库,调用C/C++函数

public class Main{
    
    // 加载C/C++库libhello.so, 去除前缀lib
    static{
        System.loadLibrary("hello");
    }
    
    public void onCreate() {        
        new JavaDemo().getHello(); // 调用C/C++函数
    }
}

四.在C/C++中调用java方法

// 调用java方法, 把Java字符串解析为C语言字符串
char* Jstring2CStr(JNIEnv* env, jstring jstr){
    // 1.获取String类(java反射)
    jclass clsstring = (*env)->FindClass(env,"java/lang/String");
    
    // 2.获取String.getBytes方法(java反射)
    jmethodID mid = (*env)->GetMethodID(env,clsstring, "getBytes", "(Ljava/lang/String;)[B");
    
    // 3.调用getBytes方法(java反射),把java字符串变为字节数组jbyteArray
    jstring strencode = (*env)->NewStringUTF(env,"GB2312");
    jbyteArray barr= (jbyteArray)(*env)->CallObjectMethod(env,jstr,mid,strencode);
    
    // 4.让jbyte*(char*别名) ba引用Java数组jbyteArray barr
    jsize alen = (*env)->GetArrayLength(env,barr);
    jbyte* ba = (*env)->GetByteArrayElements(env,barr,JNI_FALSE);
    
    // 5.把Java端数组复制到本地内存(char*指向的)
    char* rtn = NULL;
    if(alen > 0){
        rtn = (char*)malloc(alen+1);
        memcpy(rtn,ba,alen);
        rtn[alen]=0;
    }
    
    // 6.释放jbyte* ba对数组jbyteArray barr的引用, 好让java回收jbyteArray barr占用的内存
    (*env)->ReleaseByteArrayElements(env,barr,ba,0);
    return rtn ;
}

简书: http://www.jianshu.com/p/52b3774fbe2b
CSDN博客: http://blog.csdn.net/qq_32115439/article/details/72567183
GitHub博客：http://lioil.win/2017/05/19/C_JNI_NDK_Java.html
Coding博客：http://c.lioil.win/2017/05/19/C_JNI_NDK_Java.html