实现原理
原理其实很简单，就是利用了反射。
实现步骤第一步：新建一个 JNI 类（名字可以自己随便取）
第二步：native 方法的编写
package com.wust.ccallj;
public class cCallJava {
public native void callPrintString();
public void printString(String s){
System.out.println("我被C语言调用了，他传来了：" + s);
}
}
第三步：新建 cpp 文件夹
这个cpp文件夹可以自己从头开始建，也可以用集成开发工具创建 jni入门级别教程，这次我们自己从零创建
第四步：生成 cCallJava 的 .h 文件，并放于 cpp 文件夹下
生成头文件时 报的 错误可以忽略
出现下面这个警告 是因为 我们没有配置 cMakeLists.txt 所以不用紧张
第五步：编写对应的 .cpp 源文件
可以发现，我们建的 .cpp 文件也是报这个警告，下面我们就通过配置 cMakeLists.txt 让他彻底消失
cMakeLists.txt 文件内容
# 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 的文件名，也是你在 java 代码里调用的名字，填一个就好了，记住！！！
cCallJava
# Sets the library as a shared library.
#设置库的类型 一种静态文件 STATIC .a 一种动态文件 SHARED .so
SHARED
# Provides a relative path to your source file(s).
# 需要编译的c/c++ 文件，这里填相对路径
testCallJava.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.
# 指定链接的目标库
cCallJava
# Links the target library to the log library
# included in the NDK.
${log-lib} )同时，在 build.gradle 里关联到 cMakeLists.txt，右键 cMakeLists.txt，点击我圈出来的东西
选择该 cMakeLists.txt 对应的路径
你会发现 build.gradle 文件中多这么个配置
其实这个你也可以手写进去，如果你记得的话
externalNativeBuild {
cmake {
path file('src/main/cpp/CMakeLists.txt')
}
}这个时候你就会发现那个警告没了，头文件只是起代码提示作用，就让他红着吧（报错原因是 jni.h 文件没找到），我们开始编写 .cpp 文件
//
// Created by yiqi on 2021/4/3.
//
#include <jni.h>
extern "C" {
/*
* Class: com_wust_ccallj_cCallJava
* Method: callPrintString
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_com_wust_ccallj_CallJava_callPrintString
(JNIEnv * env, jobject jobj){
//1、获取字节码
jclass jclazz = (*env).FindClass("com/wust/ccallj/aCallJava");
//2、获取方法
jmethodID jmethodId = (*env).GetMethodID(jclazz,"printString","(Ljava/lang/String;)V");
//3、实例化对象
jobject jobject1 = (*env).AllocObject(jclazz);
//4、调用方法
jstring jstr = (*env).NewStringUTF("这句话来自C");
(*env).CallVoidMethod(jobject1,jmethodId,jstr);
};
}注意，如果大家和我的类名一样，首字母小写了，它就会报下面这个错误，把首字母改成大写即可
//
// Created by yiqi on 2021/4/3.
//
#include <jni.h>
extern "C" {
/*
* Class: com_wust_ccallj_cCallJava
* Method: callPrintString
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_com_wust_ccallj_CallJava_callPrintString
(JNIEnv * env, jobject jobj){
//1、获取字节码
jclass jclazz = (*env).FindClass("com/wust/ccallj/CallJava");
//2、获取方法
jmethodID jmethodId = (*env).GetMethodID(jclazz,"printString","(Ljava/lang/String;)V");
//3、实例化对象
jobject jobject1 = (*env).AllocObject(jclazz);
//4、调用方法
jstring jstr = (*env).NewStringUTF("这句话来自C");
(*env).CallVoidMethod(jobject1,jmethodId,jstr);
};
}第六步：静态引入库，并调用
package com.wust.ccallj;
public class CallJava {
static {
System.loadLibrary("cCallJava");
}
public native void callPrintString();
public void printString(String s){
System.out.println("我被C语言调用了，他传来了：" + s);
}
}
package com.wust.ccallj;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
CallJava callJava = new CallJava();
callJava.callPrintString();
}
}
完美结束
