Android JNI学习笔记

搞了好久Android 一直没搞过NDK开发
以前被卡在环境搭建
后面Android Studio 支持NDK之后
又被没有C++代码提示卡住了
最近发现用vscode写C++代码有代码提示后
封装一个sqlite3的库试试水

总结

我发现如果不去熟悉源码,很多方法使用都是不科学的。

首先就是Android自带的Cusor.getColumnIndex 我发现源码是读取查询结果所有列名之后将列名放在hashmap里面再从haspmap里面查找index 如果sql语句是自己写的那个列在第几个位置完全知道不需要掉这个方法虽然浪费不了多少时间
其次就是Cusor.getCount 方法 sqlite3的源码里面我就没有找到获取查询结果行数的方法
我也没有找到源码是怎么实现的我猜估计是执行过一次sql select count(*) from 不然我也想不到其他高效的方法了这个方法只能说能不用尽量不用吧

开发环境

怎么搭建就不说了百度一大堆
写说明下最重要的2点

第一 VSCODE代码提示
c_cpp_properties.json 配置JNI的头文件地址配置好之后就有代码提示了

{
  "configurations": [
    {
      "name": "Win32",
      "includePath": [
        "${workspaceFolder}/**",
        "${workspaceFolder}/include",
        "E:\\android_sdk\\ndk\\21.4.7075529\\toolchains\\llvm\\prebuilt\\windows-x86_64\\sysroot\\usr\\include\\",
        "E:\\android_sdk\\ndk\\21.4.7075529\\toolchains\\llvm\\prebuilt\\windows-x86_64\\sysroot\\usr\\include\\c++\\v1",
        "E:\\android_sdk\\ndk\\21.4.7075529\\toolchains\\llvm\\prebuilt\\windows-x86_64\\sysroot\\usr\\include\\x86_64-linux-android"
      ],
      "defines": ["_DEBUG", "UNICODE", "_UNICODE"],
      "compilerPath": "E:\\soft\\TDM-GCC-64\\bin\\gcc.exe",
      "cStandard": "gnu11",
      "cppStandard": "gnu++98",
      "intelliSenseMode": "windows-gcc-x64"
    }
  ],
  "version": 4
}

第二 NDK编译
Android现在推荐用的CMAKE编译需要把编译的文件添加到这个列表
网上好像有自动添加所有文件的代码问题我加上之后就一直卡在编译中
只能放弃了
CMakeLists.txt

# 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(native-lib SHARED
 native-lib.cpp
 include/changeType.cpp
 sqlite/shell.c
 sqlite/sqlite3.c
 sqlite/sqliteDb.cpp
 )

 #include的文件夹
 include_directories(jniSTDualCamPreview
                     ${CMAKE_SOURCE_DIR}/include
                     ${CMAKE_SOURCE_DIR}/sqlite
                     )

# 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.
                       native-lib

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

sqlite3库的引入

可以去官网下载源码SQLite Home Page

sqlite3操作介绍

打开数据库

bool SqliteDb::open(const char* file)
{  
    //返回值: 成功返回SQLITE_OK,失败返回其他值。
        // 一个打开的数据库实例

    // 根据文件路径打开数据库连接。如果数据库不存在，则创建。
    // 数据库文件的路径必须以C字符串传入。
    int result = sqlite3_open_v2(file, &db, SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_NOMUTEX | SQLITE_OPEN_SHAREDCACHE, NULL);

    if (result == SQLITE_OK) {
        LOGE("打开数据库连接成功");
        return true;
    }
    else {
         LOGE("打开数据库连接失败");
        return false;
    }
}

关闭数据库

void SqliteDb::close(){
    if(db!=NULL)
    {
        if(inTransaction)
        {
            commitTransaction();
        }

        sqlite3_close(db);
        db=NULL;
    }
}

执行sql

bool SqliteDb::execSql(const char* sql)
{
    if(db==NULL)
    {
        return false;
    }

    // const char *sqlSentence = "INSERT INTO t_person(name, age)     VALUES('夏明', 22); ";        //SQL语句
    sqlite3_stmt *stmt = NULL;        //stmt语句句柄

    //进行插入前的准备工作——检查语句合法性
    //-1代表系统会自动计算SQL语句的长度
    int result = sqlite3_prepare_v2(db, sql, -1, &stmt, NULL);

    if (result == SQLITE_OK) {
       
        //执行该语句
        sqlite3_step(stmt);
        LOGE("执行SQL成功");
    }
    else {
        LOGE("SQL语句有误");
    }
    //清理语句句柄，准备执行下一个语句
    sqlite3_finalize(stmt);

    return result == SQLITE_OK;
}

执行查询语句

sqlite3_stmt * SqliteDb::querySql(const char* sql){
    // const char *sqlSentence = "SELECT name, age FROM t_person WHERE age < 30;";    //SQL语句
    sqlite3_stmt *stmt = NULL;    // stmt语句句柄

    //进行查询前的准备工作——检查语句合法性
    //-1代表系统会自动计算SQL语句的长度
    int result = sqlite3_prepare_v2(db, sql, -1, &stmt, NULL);

    if (result == SQLITE_OK) {
        //每调一次sqlite3_step()函数，stmt语句句柄就会指向下一条记录
        // while (sqlite3_step(stmt) == SQLITE_ROW) {
        //     // 取出第0列字段的值
        //     const unsigned char *name = sqlite3_column_text(stmt, 0);
        //     // 取出第1列字段的值
        //     int age = sqlite3_column_int(stmt, 1);
        //     //输出相关查询的数据
        //     // std::clog << "name = " << name <<", age = "<< age;
        // }

        // LOGE("执行SQL成功");
        return stmt;
    }
    else {
        LOGE("SQL语句有误");
        //清理语句句柄，准备执行下一个语句
        sqlite3_finalize(stmt);
        return NULL;
    }
    
}

查询结果的读取
要封装的方法太多了只封装了几个重要的方法

//获取查询结果的列数
int getColumnCount(sqlite3_stmt* stmt){
    return sqlite3_column_count(stmt);
}

//销毁以防止内存泄露
void closeCusor(sqlite3_stmt* stmt){
    if(stmt!=NULL)
    {
        sqlite3_finalize(stmt);
        stmt=NULL;
    }   
}

//查询结果读取下一行
extern "C" JNIEXPORT jboolean JNICALL Java_com_test_sqlite_SqliteDb_moveNext(JNIEnv* env,
                                                                                 jobject /* this */
                                                                                 ,jlong ptr)
{
    if(ptr==0)
    {
        return 0;
    }
    sqlite3_stmt* stmt=(sqlite3_stmt*)ptr;
    if(sqlite3_step(stmt) == SQLITE_ROW)
    {
        return 1;
    }
    else{
        return 0;
    }
}

//获取列名
extern "C" JNIEXPORT jstring JNICALL Java_com_test_sqlite_SqliteDb_getColumnName(JNIEnv* env,
                                                                                 jobject /* this */
                                                                                 ,jlong ptr,int index)
{
    if(ptr==0)
    {
        return NULL;
    }
    sqlite3_stmt* stmt=(sqlite3_stmt*)ptr;
    int count=getColumnCount(stmt);
    if(index>=count)
    {
        return NULL;
    }
    const char* name=sqlite3_column_name(stmt,index);
    jstring str=env->NewStringUTF(name);

    return str;
}

//获取结果中的int值
extern "C" JNIEXPORT jint JNICALL Java_com_test_sqlite_SqliteDb_getColumnInt(JNIEnv* env,
                                                                                 jobject /* this */
                                                                                 ,jlong ptr,int index)
{
    if(ptr==0)
    {
        return 0;
    }
    sqlite3_stmt* stmt=(sqlite3_stmt*)ptr;
    int count=getColumnCount(stmt);
    if(index>=count)
    {
        return 0;
    }
    int value=sqlite3_column_int(stmt,index);

    return value;
}

//获取结果中的double值
extern "C" JNIEXPORT jdouble JNICALL Java_com_test_sqlite_SqliteDb_getColumnDouble(JNIEnv* env,
                                                                                 jobject /* this */
                                                                                 ,jlong ptr,int index)
{
    if(ptr==0)
    {
        return 0;
    }
    sqlite3_stmt* stmt=(sqlite3_stmt*)ptr;
    int count=getColumnCount(stmt);
    if(index>=count)
    {
        return 0;
    }
    double value=sqlite3_column_double(stmt,index);

    return value;
}

//获取结果中的string值
extern "C" JNIEXPORT jstring JNICALL Java_com_test_sqlite_SqliteDb_getColumnString(JNIEnv* env,
                                                                                 jobject /* this */
                                                                                 ,jlong ptr,int index)
{
    if(ptr==0)
    {
        return NULL;
    }
    sqlite3_stmt* stmt=(sqlite3_stmt*)ptr;
    int count=getColumnCount(stmt);
    if(index>=count)
    {
        return NULL;
    }
    const unsigned char* value=sqlite3_column_text(stmt,index);
    jstring str=unsigchar2jstring(env,value);
    return str;
}

分享代码

链接：https://pan.baidu.com/s/1PrSV8Ry9tvcSjl3yB5YqEA?pwd=4fow
提取码：4fow

Android JNI学习笔记

Android JNI学习笔记

总结

开发环境

sqlite3库的引入

sqlite3操作介绍

分享代码

相关阅读更多精彩内容

友情链接更多精彩内容