背景

Demo代码

代码结构

JNI.java

package com.stone.javacallc;

/**
 * Created by stoneWang
 * Created on 2024/1/16
 * java调用C
 */
public class JNI {

    {
        System.loadLibrary("javacallc");
    }
    /**
     * 让C代码做加法运算，把结果返回
     *
     * @param x
     * @param y
     * @return
     */
    public native int add(int x, int y);

    /**
     * 从Java传入字符串,C代码进行拼接
     *
     * @param s I am from java
     * @return  I am from java add I am from C
     */
    public native String sayHello(String s);

    /**
     * 让C代码给每个元素加上10
     *
     * @param intArray
     * @return
     */
    public native int[] increaseArrayEles(int[] intArray);
}

MainActivity.java

package com.stone.javacallc;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;

public class MainActivity extends AppCompatActivity {

    private JNI jni;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        jni = new JNI();
    }

    public void add(View view) {
        int result = jni.add(99, 1);
        Log.e(MainActivity.class.getSimpleName(), "result:" + result);
    }

    public void string(View view) {
        String result = jni.sayHello("I am from java ");
        Log.e(MainActivity.class.getSimpleName(), "result:" + result);
    }

    public void array(View view) {

    }

    public void checkpw(View view) {

    }


}

Android.mk

LOCAL_PATH :=$(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE :=javacallc
LOCAL_SRC_FILES :=javacallc.c
include $(BUILD_SHARED_LIBRARY)

Application.mk

App_ABI:=all

com_stone_javacallc_JNI.h

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

#ifndef _Included_com_stone_javacallc_JNI
#define _Included_com_stone_javacallc_JNI
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_stone_javacallc_JNI
 * Method:    add
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_com_stone_javacallc_JNI_add
  (JNIEnv *, jobject, jint, jint);

/*
 * Class:     com_stone_javacallc_JNI
 * Method:    sayHello
 * Signature: (Ljava/lang/String;)I
 */
JNIEXPORT jstring JNICALL Java_com_stone_javacallc_JNI_sayHello
  (JNIEnv *, jobject, jstring);

/*
 * Class:     com_stone_javacallc_JNI
 * Method:    increaseArrayEles
 * Signature: ([I)[I
 */
JNIEXPORT jintArray JNICALL Java_com_stone_javacallc_JNI_increaseArrayEles
  (JNIEnv *, jobject, jintArray);

#ifdef __cplusplus
}
#endif
#endif

javacallc.c

//
// Created by wanglei on 2024/1/16.
//
# include "com_stone_javacallc_JNI.h"
# include "string.h"

/**
 * 把一个jstring转换成一个c语言的char* 类型.
 */
char* _JString2CStr(JNIEnv* env, jstring jstr) {
    char* rtn = NULL;
    jclass clsstring = (*env)->FindClass(env, "java/lang/String");
    jstring strencode = (*env)->NewStringUTF(env,"GB2312");
    jmethodID mid = (*env)->GetMethodID(env, clsstring, "getBytes", "(Ljava/lang/String;)[B");
    jbyteArray barr = (jbyteArray)(*env)->CallObjectMethod(env, jstr, mid, strencode); // String .getByte("GB2312");
    jsize alen = (*env)->GetArrayLength(env, barr);
    jbyte* ba = (*env)->GetByteArrayElements(env, barr, JNI_FALSE);
    if(alen > 0) {
        rtn = (char*)malloc(alen+1); //"\0"
        memcpy(rtn, ba, alen);
        rtn[alen]=0;
    }
    (*env)->ReleaseByteArrayElements(env, barr, ba,0);
    return rtn;
}

/**
 *
 * @param env
 * @param jobj
 * @param ji
 * @param jj
 * @return
 */
jint Java_com_stone_javacallc_JNI_add
        (JNIEnv *env, jobject jobj, jint ji, jint jj){
    int result = ji + jj;
    return result;
};


jstring Java_com_stone_javacallc_JNI_sayHello
        (JNIEnv *env, jobject jobj, jstring jstr){
    char* fromJava = _JString2CStr(env, jstr);
    char* fromc = "add I am from C";
    // 拼接函数strcat
    strcat(fromJava, fromc); // 把拼接的结果放在第一参数里面
    // 将char转为jstring
    return (*env)->NewStringUTF(env, fromJava);
};

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 30
    buildToolsVersion "30.0.2"

    defaultConfig {
        applicationId "com.stone.javacallc"
        minSdkVersion 26
        targetSdkVersion 30
        versionCode 1
        versionName "1.0"

        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        ndk{
            moduleName "javacallc"
            abiFilters "armeabi", "armeabi-v7a", "arm64-v8a" // cpu类型
        }
    }

    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    externalNativeBuild {
        ndkBuild {
            path "src/main/jni/Android.mk"
        }
    }
}

dependencies {
    implementation fileTree(dir: "libs", include: ["*.jar"])
    implementation 'androidx.appcompat:appcompat:1.2.0'
    implementation 'androidx.constraintlayout:constraintlayout:2.0.1'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'androidx.test.ext:junit:1.1.2'
    androidTestImplementation 'androidx.test.espresso:espresso-core:3.3.0'

}