前言

基于C++学习和JNI流程学习，相信大家对NDK开发充满了期待，本篇博客将从一个简单的例子入手，带着大家熟悉NDK开发。

文件的加密与解密

创建Java文件Encryptor：

public class Encryptor {
    static {
        System.loadLibrary("file_encryption");
    }

    //加密
    public native static void cryption(String normalPath, String cryptPath);
    //解密
    public native static void decryption(String cryptPath, String decryptPath);
}

创建cpp文件file_encryption.c，加密核心实现：

#include "jni.h"
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <android/log.h>

#define LOGD(FORMAT,...) __android_log_print(ANDROID_LOG_DEBUG,"zp",FORMAT,##__VA_ARGS__);

char password[] = "ChinaIsPowerful";
//加密
JNIEXPORT void JNICALL
Java_com_zhangpan_myjnicmake_Encryptor_cryption(JNIEnv *env, jclass jcla, jstring normal_path_jstr, jstring crypt_path_jstr) {
    //jstring --> char*
    char* normal_path = (*env)->GetStringUTFChars(env, normal_path_jstr, NULL);
    char* crypt_path = (*env)->GetStringUTFChars(env, crypt_path_jstr, NULL);

    LOGD("%s", normal_path);
    //打开文件
    FILE* nomal_fp = fopen(normal_path, "rb");
    FILE* crypt_fp = fopen(crypt_path, "wb");

    if (nomal_fp == NULL) {
        LOGD("%s", "文件打开失败");
        return;
    }

    //一次读取一个字符
    int ch = 0;
    int i = 0;
    int pwd_length = strlen(password);
    while ((ch = fgetc(nomal_fp)) != EOF) { //End of File
        //写入(异或运算)
        fputc(ch ^ password[i % pwd_length], crypt_fp);
        i++;
    }

    //关流
    fclose(nomal_fp);
    fclose(crypt_fp);
}

CMakeLists配置：

add_library( # Sets the name of the library.
             file_encryption

             # Sets the library as a shared library.
             SHARED

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

target_link_libraries( # Specifies the target library.
                       file_encryption

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

解密核心实现：

JNIEXPORT void JNICALL
Java_com_zhangpan_myjnicmake_Encryptor_decryption(JNIEnv *env, jclass jcla, jstring crypt_path_jstr, jstring decrypt_path_jstr) {
    //jstring --> char*
    char *crypt_path = (*env)->GetStringUTFChars(env, crypt_path_jstr, NULL);
    char *decrypt_path = (*env)->GetStringUTFChars(env, decrypt_path_jstr, NULL);
    FILE* crypt_fp = fopen(crypt_path, "rb");
    FILE* decrypt_fp = fopen(decrypt_path, "wb");

    int ch;
    int i = 0;
    int pwd_length = strlen(password);
    while ((ch = fgetc(crypt_fp)) != EOF)
    {
        fputc(ch ^ password[i % pwd_length], decrypt_fp);
        i++;
    }

    fclose(crypt_fp);
    fclose(decrypt_fp);
}

创建FileEncryptionActivity：

public class FileEncryptionActivity extends AppCompatActivity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_file_encryption);
    }

    public void mCryption(View view) {
        String normalPath = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator + "Shape@2x.png";
//        String normalPath = "/sdcard/Shape@2x.png";
        String cryptPath = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator + "Shape@2xcrypt.png";
        Encryptor.cryption(normalPath, cryptPath);
        Log.d("zhangpan", "加密完成了...");
    }

    public void mDecryption(View view) {
        String cryptPath = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator + "Shape@2xcrypt.png";
        String decryptPath = Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator + "Shape@2xdecrypt.png";
        Encryptor.decryption(cryptPath, decryptPath);
        Log.d("zhangpan", "解密完成了...");
    }
}

布局文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent" android:layout_height="match_parent"
    android:orientation="vertical"
    android:gravity="center">

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="加密"
        android:onClick="mCryption"/>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="解密"
        android:onClick="mDecryption"/>

</LinearLayout>

编译运行，点击加密，再点击解密，Logcat输出：

11-30 12:13:50.548 6981-6981/? D/zhangpan: 加密完成了...
11-30 12:14:01.358 6981-6981/? D/zhangpan: 解密完成了...

注意：
1.添加文件的读写权限

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

2.文件存储路径/mnt/sdcard/，此例中是将Shape@2x.png放到sd卡中，加密是需要确定sd卡中存在此图片。
3.Android 6.0动态权限申请，如果你的手机是Android 6.0+，没有动态申请权限，需要手动去设置里面打开权限。

项目地址

https://github.com/fsrmeng/MyJniCmake-Master

展望

本篇博客简单介绍了JNI中文件的加密与解密，接下来我将介绍JNI中文件的拆分与合并，敬请期待！
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