Framework step by step easy-to-use process

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1. dao layer

At first , I didn't learn orm( object relation mapping ) Frame time , Write the project using ： JdbcTemplate The template operates the database .

1. First use jdbc , establish connection object To get data , But trouble , Waste resources . So we used Database connection pool technology . Such as ：Druid Druid
2. establish Database connection pool object , Database password , The account number is written into the object , To get connenction To use .
3. Due to the coupling problem , Will configure ： Database password , Account number and other configurations , It's written in properties In file , Use classLoader Load injection into Database connection pool objects .
4. Encapsulate a tool class , obtain connection object , Execute corresponding sql that will do .
5. In the use of JdbcTemplate Gadget simplification jdbc The process of obtaining data , Just focus on writing sql, Just get the data .daoImpl Write the corresponding sql.

mybatis The process of ：

The above problems ： Use JdbcTemplate Gadget after , sql Statement is the same as Java Code Coupled （ It's written in java In the code ）, And then the project will go online , Need to be right sql Optimize and modify , You need to be right about java Modify the source code of , Is not in line with the situation .

that , take sql Written in In profile . Solve the problem of high coupling .

It's on it ： thought

1. Import mybaits Of jar package , Need to use mybatis frame , Need to write a mybatis Core profile , One is mapper The mapping file （ Write about sql Statement file ）
2. mybatis The bottom layer also needs to be used Database connection pool technology .
3. mybatis Core profile Write database Configuration of ： Password etc. , Global profile : There are data sources, some running environment information , Location of mapping file, etc

Import dependence ：

  <!--mybatis-->
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.4</version>
    </dependency>
    <!--druid-->
    <dependency>
      <groupId>com.alibaba</groupId>
      <artifactId>druid</artifactId>
      <version>1.1.21</version>
    </dependency>
    <!--mysql-->
    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.19</version>
      <scope>runtime</scope>
    </dependency>;https://mybatis.org/mybatis-3/zh/index.html

establish MyBatis Configuration file for , And create test Class to create sqlSessionFactory

Use ：

1. load mybatis Global profile , To create a SqlSessionFactory object
2. sqlSession The object is based on Specified in the global configuration file mapping Mapping file to query the corresponding sql To perform . according to id The query , Yes namespace Words , Write together .namespace.id

 @Test
    public  void test() throws IOException {
    
        String resource = "MyBatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory =new SqlSessionFactoryBuilder().build(inputStream);

        //2. obtain sqlSession example , It can directly execute the mapped sql sentence 
        /*sqlSession There are two parameters , The first is sql Unique identifier of , That is to say id  The second is execution sql Parameters of  */
        SqlSession sqlSession = sqlSessionFactory.openSession();
        Employee o = sqlSession.selectOne("com.Mybatis.EmployeeMapper.selectEMP", 1);

        System.out.println(o);
        sqlSession.close();

    }

modify

 Employee o = sqlSession.selectOne("com.Mybatis.EmployeeMapper.selectEMP", 1);
  Employee o = sqlSession.selectOne("com.Mybatis.EmployeeMapper.selectEMP", "abc"); Parameter type cannot be determined 

The version together obtains data in the above way , because Unable to determine the type of data , So interface programming is used

Mapping file corresponding to an interface . Mapping files namespace Write the full class name of the interface ,id Write the corresponding method name . that will do .

 EmployeeMapperDap mapper = sqlSession.getMapper(EmployeeMapperDap.class);   You can get the corresponding proxy object and directly execute sql.

How can this method find the corresponding sql Of ？？？

1. It specifies sql Mapping location of , stay mybatis In the global configuration file .
2. According to the interface class object , You can get its full class name , Compare namespace Value , The executed method can correspond to id, You can find the execution sql Statement .