Detail the execution process of JDBC query method

Code

package jdbc;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

interface IRowMapper {
    
	void row(ResultSet resultSet);
}

public class Select {
    
	public static void select(String sql, IRowMapper rowMapper) {
    
		Connection connection = null;
		Statement statement = null;
		ResultSet resultSet = null;
		try {
    
			Class.forName("com.mysql.jdbc.Driver");//  load 
			connection = DriverManager.getConnection("jdbc:mysql://127.0.0.1/test", "root", "root");
			statement = connection.createStatement();
			resultSet = statement.executeQuery(sql);
			rowMapper.row(resultSet);
		} catch (Exception e) {
    
			e.printStackTrace();
		} finally {
    
			if (statement != null) {
    
				try {
    
					statement.close();
				} catch (SQLException e) {
    
					e.printStackTrace();
				}
				if (connection != null) {
    
					try {
    
						connection.close();
					} catch (SQLException e) {
    
						e.printStackTrace();
					}
				}
			}
		}
	}

	public static void main(String[] args) {
    
		class RowMapper implements IRowMapper {
    
			@Override
			public void row(ResultSet resultSet) {
    
				try {
    
					while (resultSet.next()) {
    
						String name = resultSet.getString("name");
						String address = resultSet.getString("address");
						System.out.print(name);
						System.out.print(address);
					}
				} catch (Exception e) {
    
					e.printStackTrace();
				}
			}
		}
		select("select name,address from student", new RowMapper());
	}
}

result

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Here we are MySQL Graphical client management tools are built on MySQL, Execute statement .
The code we need to use is ：

use test;
create table student(
	id char(50),
	name char(40),
	user_name varchar(50),
	address varchar(85),
	password varchar(150)
 );

Execution process

Because the judgment statement should be inserted from the outside , therefore , We use interfaces and inner classes to realize this function .
1. Let's take another look main How to execute the code in the method .
perform select sentence , Pass in select Method is a statement for querying information , And an object .
2. Jump to the select Method .
（1） Load driver class . Use Class.forName(“com.mysql.jdbc.Driver”); Statement to realize this function . Among them com.mysql.jdbc.Driver Can find Driver（） copied .
（2） Connect . Use connection = DriverManager.getConnection(“jdbc:mysql://127.0.0.1/test”, “root”, “root”); Realize this function ,jdbc, It means a kind of java API.MySQL Point out that we use MySQL Executive .127.0.0.1 It's local ,root Is the user name ,root It's a password . It doesn't have to be exactly the same as mine .
（3） Create statement . Use statement = connection.createStatement(); Statements for .createStatement Translation is to create sentences .
（4） Execute statement .executeQuery（） Method realization . This method is used to execute select sentence , But you can only execute query statements , After execution, it returns the query result ResultSet object . Then we have to judge , So we called row Method . Pass in resultSet.
3. Enter the internal class implementation row Method .resultSet.next() The original content is a continuous strip with its own space .（ Look at the picture below ）, There is a pointer above the first content , At this time, the pointer points to nothing , However , When we use it , It will see if there is anything below it , If there is content, keep going down , If not, it will stop . We use this feature to judge whether the statement is completed . stay while Inside , We define name and address, Then the output . Due to runtime error , We use try-catch To throw . return select Method , Release resources . namely finally Contents of Li . Because if you release it first connection Words , because statement There is still , Therefore, it cannot be released successfully . So let's release statement, Re release connection.