JDBC common interview questions

Database connection pool

Allow the program to reuse... In the connection pool Database connection , Instead of destroying the original connection , Create a new connection

working principle

When the server starts , Create a certain number of database connections Connection, Unified management （ Abstract out a concept of connection pool ）.

When the client needs to connect , Return an idle connection from the pool , Mark it as busy

If there is no idle connection , Just create a certain number of connections ( By configuration parameters )

After the connection is no longer used , Mark it as free , Other threads can use this connection

When the number of connections requested by the client exceeds the maximum number of connections , The request will be added to the waiting queue

Implementation scheme

Another blog : Simple implementation of database connection pool

JDBC Driving process of

1. Registration drive
   Class.forName("com.mysql.jdbc.Driver")
   Reflection to create Driver object , load Driver Class time , Execute static methods DriverManager.registerDriver(new Driver()); Registration drive
2. Establishing a connection
   adopt Connection build ⽴ Connect
   Connection coon = DriverManager.getConnection(URL,USER,PWD);
3. Create run SQL The object of Statement
   Statement statement = coon.prepareStatement(sql);
4. perform SQL sentence
   statement.executeQuery(); Inquire about ; return ResultSet
   statement.executeUpdate(); Additions and deletions ; Returns the number of rows affected
5. Processing result set
   ResultSet Save the results of execution
   The method of traversing query results is resultSet.next()
6. Release resources
   Release database connection object Connection
   Release execution SQL The object of Statement
   Release the objects of the result set ResultSet

Statement and PreparedStatement difference

PreparedStatement Interface inherited Statement Interface
It's all through Connection Connect object to get

Readability 、 Maintainability

Statement Only strings can be used to splice parameters

stmt.executeUpdate("insert into tb_name (col1,col2,col2,col4) " +
                "values ('"+var1+"','"+var2+"',"+var3+",'"+var4+"')");

PreparedStatement Use placeholders , Dynamic setting parameters

ps = con.prepareStatement("insert into tb_name (col1,col2,col2,col4) " +
                                       "values (?,?,?,?)");
        ps.setString(1, var1);
        ps.setString(2, var2);
        ps.setString(3, var3);
        ps.setString(4, var4);

performance

• Statement perform SQL Statement is not precompiled , Every execution needs to be recompiled
• PrepareStatement Will the precompiled SQL sentence , When executed many times , Just run , Don't compile it every time

Security

because Statement Concatenate parameters with strings , It's easy to get caught. SQL Inject

PrepareStatement Precompiling , Only one value can be inserted per placeholder , Will filter other statements ( Improved security )

JDBC How to page medium and large amounts of data