JDBC Technology

One 、java Database programming foundation

        Java Database programming in is through JDBC（Java Database Connectivity） Realized . Use JDBC Technology involves three different roles ：Java official 、 Developers and database vendors .  As shown in the figure below ：

The three different roles involved in the above figure are responsible for different functions , As shown below ：

• Java Official supply JDBC Interface , Such as Connection、Statement and ResultSet etc.
• In order to support Java Language uses its own database , They provide specific implementation classes based on these interfaces , These concrete implementation classes are called JDBC Driver（JDBC The driver ）.
• For developers ,JDBC Provides a consistent API, Developers don't need to care about the details of implementing interfaces .

        JDBC API by Java Developers use the database to provide a unified programming interface , It consists of a group of Java Class and interface . Such classes and interfaces come from java.sql and javax.sql Two bags .

Two 、 Database programming process

         The general process of database programming is shown in the figure below , Among them, query （Read） The process requires at most 7 A step , modify （C Insert 、U to update 、D Delete ） The process requires at most 6 A step . This process uses precompiled statement objects for data operations , Therefore, it is possible to bind parameters , See the first 4 step .

         The above steps are basic general steps , The actual situation will change , For example, no parameters need to be bound , Is the first 4 The steps are omitted . also , If Connection object 、Statement Objects and ResultSet Objects use automatic resource management technology to release resources , So the first 7 Steps can also be omitted . 

1） When programming database connection ,JVM You must first load the database driver provided by a specific manufacturer . Use Class.forName() Method realization Driver loading The process , as follows ：

Class.forName("com.mysql.jdbc.Driver");

 2） Establish a database connection

         After the driver is loaded successfully, you can connect to the database . The database connection can be established by calling DriverManager Class getConnection() Method realization

3） establish SQL Statement object (Statement)（ Transmitter object ）

conn.createStatement();
conn.prepareStatement(sql);

4） Set parameters （ if necessary ）

5） Perform the operation , function SQL sentence

6）Resultset Object to get the result

        ResultSet Objects are used to encapsulate sql Statement query results , It's also a very important object . This object provides the method of traversing data and getting data .

7） Release resources

3、 ... and 、 The interface is introduced

        JDBC API There are three important interfaces in , Respectively Connection、Statement as well as ResultSet.

 3.1 Connection Interface

        Connection The implementation object of the interface represents the connection with the database , That is to say Java Establish a connection between the program and the database .Connection Methods commonly used in interfaces ：

• Statement createStatement()： Create a statement object , The statement object is used to put SQL Statements are sent to the database .
• PreparedStatement prepareStatement(String sql)： Create a precompiled statement object , Used to parameterize SQL Statements are sent to the database , The parameter contains one or more question marks “?” Place holder .
• CallableStatement prepareCall(String sql)： Create a statement object that calls the stored procedure , Parameters are called stored procedures , The parameter contains one or more question marks “?” As a placeholder .
• close()： Close the connection to the database , You must close the connection after using , Otherwise, the connection will remain for a long time , Until timeout .
• isClosed()： Determine whether the connection has been closed .

3.2 Statement Interface

        Statement It is called a statement object , It provides for sending SQL sentence , And visit the results .Connection Interface provides generation Statement Methods , Generally speaking, it can be through connection.createStatement() You can get Statement object .

         There are three kinds of Statement Interface ：Statement、PreparedStatement and CallableStatement, among PreparedStatement Inherit Statement Interface ,CallableStatement Inherit PreparedStatement Interface .Statement Implementation objects are used to perform basic SQL sentence ,PreparedStatement Implementation objects are used to perform precompiled SQL sentence ,CallableStatement Implementation objects are used to call stored procedures in the database .

Statement There are many ways , The most common methods are as follows ：

• executeQuery()： Run the query statement , return ResultSet object .
• executeUpdate()： Run update operation , Returns the number of updated rows .
• close()： Close the statement object .
• isClosed()： Judge whether the statement object has been closed .  

3.3 ResultSet Interface  

          stay Statement perform SQL When the sentence is , If it is SELET Statement will return the result set , The result set passes through the interface ResultSet Description of the , It provides a way to access the result set row by row , Through this method, you can access the contents of different fields in the result set .

ResultSet Provides methods to retrieve different types of fields , The most commonly used methods are introduced as follows ：

• close()： Close result set object .
• isClosed()： Judge whether the result set object has been closed .
• next()： Move the cursor of the result set back one line from its current position .
• getString()： Get in the database yes CHAR or VARCHAR Data of string type , The return value type is String.
• getFloat()： Get data of floating-point type in the database , The return value type is float.
• getDouble()： Get data of floating-point type in the database , The return value type is double.
• getDate()： Get data of date type in the database , The return value type is java.sql.Date.
• getBoolean()： Get the type of Boolean data in the database , The return value type is boolean.
• getBlob()： Get in the database yes Blob( Binary large objects ) Data of type , The return value type is Blob type .

Four 、 Example ：

    public static void main( String[] args )
    {
        Connection con = null; //  Define a MYSQL Link to 
        PreparedStatement ps = null; //  Statement object 
        ResultSet rs = null;
        String sql = "SELECT * FROM country where id = ?;"; //sql sentence ,？ Is the parameter 
        try{
            // load Mysql drive 
            Class.forName("com.mysql.cj.jdbc.Driver");
            // Get database connection object 
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/mybatis?useSSL=false", "root", ""); //  Link to local MYSQL
            // Instantiation PreparedStatement
            ps = con.prepareCall(sql);
            // Set parameters 
            ps.setInt(1,1);
            // perform sql
            rs = ps.executeQuery();
            // To get the results 
            if(rs.next()){
                System.out.println(rs.getString(2));
            }

        }catch (ClassNotFoundException | SQLException e){
            e.printStackTrace();
        }finally {
            // Release resources 
            try {
                if (ps != null) ps.close();
                if (con != null) con.close();
                if (rs != null) rs.close();
            } catch (SQLException ignored) {

            }
        }
    }