JNI/NDK编程（三）——C 调用 Java 成员变量

本文源码：https://github.com/jt1024/JNIBasicForAndroid

环境

1.Android Studio 3.0
2.gradle-4.1-all
3.android sdk 26

一、在Android Studio 新建安卓工程

二、基本配置

参照:
JNI/NDK编程（一）——无参函数之Hello world !或JNI/NDK编程（二）——带参函数之模拟登录

三、编写代码

1.编写C++文件 login_lib.cpp
路径：NDKThirdDemo/app/src/main/cpp/login_lib.cpp

#include 
#include "string.h"

extern "C" JNIEXPORT jstring JNICALL
Java_com_tao_ndkthirddemo_Java2CLogin_login(JNIEnv *env, jobject instance, jstring username, jstring password, jint authcode) {
    jclass jclazz;

    //1.加载java类得到class对象
    jclazz = env->FindClass("com/tao/ndkthirddemo/Java2CLogin");

    //2.获取java类成员变量
    //2.1 获取java类成员变量ID
    jfieldID codeErrorID = env->GetFieldID(jclazz, "codeError", "Ljava/lang/String;");
    jfieldID usernameOrPasswordErrorID = env->GetFieldID(jclazz, "usernameOrPasswordError", "Ljava/lang/String;");
    jfieldID loginSuccessErrorID = env->GetStaticFieldID(jclazz, "loginSuccess", "Ljava/lang/String;");
    //2.2 通过成员变量ID获取java类成员变量
    jstring jcodeError = (jstring) env->GetObjectField(instance, codeErrorID);
    jstring jusernameOrPasswordError = (jstring) env->GetObjectField(instance, usernameOrPasswordErrorID);
    jstring jloginSuccess = (jstring) env->GetStaticObjectField(jclazz, loginSuccessErrorID);
    //return jloginSuccess;

    //3.验证
    if (authcode == 888) {
        const char *cStr;
        const char *pStr;
        jboolean isCopy;
        cStr = env->GetStringUTFChars(username, &isCopy);
        pStr = env->GetStringUTFChars(password, &isCopy);
        int reUsername = strcmp(cStr, "admin");
        int rePassword = strcmp(pStr, "1234");
        if (reUsername == 0 && rePassword == 0) {
            return jloginSuccess;
        } else {
            return jusernameOrPasswordError;
        }
    } else {
        return jcodeError;
    }
}

2.编写Jave文件 Java2CLogin
路径：NDKThirdDemo/app/src/main/java/com.tao.ndkthirddemo.Java2CLogin

package com.tao.ndkthirddemo;

/**
 * 作者： 麦典威
 * 修改时间：2018/3/15 8:07
 * 版权声明：www.ekwing.com
 * 功能： ${TODO}
 */


public class Java2CLogin {

    private String codeError = "验证码错误！";
    private String usernameOrPasswordError = "用户名或密码错误！";
    private static String loginSuccess = "登录成功！";

    static {
        System.loadLibrary("login_lib");
    }

    /**
     * 带参数的 native 函数
     *
     * @param username 用户名
     * @param password 密码
     * @param authcode 验证码
     * @return
     */
    public native String login(String username, String password, int authcode);

}

3.编写Activity文件 MainActivity
路径：NDKThirdDemo/app/src/main/java/com.tao.ndkthirddemo.MainActivity

package com.tao.ndkthirddemo;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";
    private EditText edtUsername, edtPassword, edtAuthcode;
    private Button btnLogin;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
    }

    private void initView() {
        edtUsername = findViewById(R.id.edt_username);
        edtPassword = findViewById(R.id.edt_password);
        edtAuthcode = findViewById(R.id.edt_authcode);
        btnLogin = findViewById(R.id.btn_login);

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String username = edtUsername.getText().toString().trim();
                String password = edtPassword.getText().toString().trim();
                String authcodeStr = edtAuthcode.getText().toString().trim();
                int authcode = Integer.parseInt(authcodeStr.equals("") ? "0" : authcodeStr);
                String result = new Java2CLogin().login(username, password, authcode);
                Log.e(TAG, "C 调用 Java 成员变量获取的结果是：" + result);
                Toast.makeText(MainActivity.this, result, Toast.LENGTH_LONG).show();
            }
        });
    }
}

4.编写 activity_main.xml

5.编写CMakeLists.txt文件，以后如果编写其他jni类，只需替换里面的“login_lib”

# 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.
             login_lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/login_lib.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.
                       login_lib

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

四、编译

点击build——>Rebuild Project 或 Make Progject
参照:
JNI/NDK编程（一）——无参函数之Hello world !或JNI/NDK编程（二）——带参函数之模拟登录

五、运行程序

附：JNI/NDK编程基础系列导航

JNI/NDK编程（一）——无参函数之Hello world !

JNI/NDK编程（二）——带参函数之模拟登录

JNI/NDK编程（三）——C 调用 Java 成员变量

JNI/NDK编程（四）——C 调用 Java 类中的函数/方法

JNI/NDK编程（五）——通过打印日志debug

JNI/NDK编程（六）——综合实战：通过编译lame实现wav转mp3