Java Database Connectivity

Java and the database Database is used to store data. It is also known as persistent storage as the data is stored and can be retrieved anytime. Java and database are used almost everywhere to store persistent data and retrieve it when required.

SQL Information stored in the database is in tables and the language used to query information from the database is SQL. Using SQL we can query a table based on the requirement.

CRUD OPERATIONS CRUD stands for create, read, update, delete. Create statement in SQL looks like – Create table mytab ( mynum number, name varchar2(25)); READ statement looks like – Select * from mytab where mynum=25; UPDATE statement looks as – Update mytab set mynum=88 where mynum=25; DELETE statement like – Delete from mytab where mynum=88;

DDL – DML – DCL - TCL DDL – Data Definition Language – These queries are used to create database objects such as tables, indexes, procedures, constraints – Create, drop, alter,truncate,comment,rename DML – Data Manipulation Language – These queries are used to manipulate the data in the database tables. – Insert, update, delete, select (more available) DCL – Data Control Language – These are data control queries like grant and revoke permissions TCL– Transaction Control Language – These are transaction control queries like commit, revoke, savepoint, set transaction

CRUD and Java Java can invoke CRUD operations using JDBC JDBC is Java Database Connectivity and there are 4 types of drivers which form a bridge between java and a database The Operations communicated by java will be translated in a form understood by the database by the drivers.

JDBC Drivers Type 1 Driver - JDBC-ODBC bridge – This is an ODBC driver, which is open source Type 2 Driver – Native API driver – This is more like the OCI (Oracle Call Interface) call interface is converted to native calls of the database. Type 3 Driver – Network protocol driver – This is achieved by using a Java Driver in a middleware Type 4 Driver – Native Protocol Driver – This is a driver written purely in Java Language We Usually prefer to depend on the Type 4 driver – Eg: Oracle thin driver

Java-Oracle Example public void ConnectToAndQueryDatabase (String username, String password) { Connection con = DriverManager.getConnection( "jdbc:myDriver:myDatabase", username, password); Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery("SELECT a, b, c FROM Table1"); while (rs.next()) { int x = rs.getInt("a"); String s = rs.getString("b"); float f = rs.getFloat("c"); } In the example We have a method call ConnectToAndQueryDatabase which takes in a username, password The Connection Interface creates a connection. A connection is created by using the DriverManager Class with the connection string, username, password. The concrete implementation is OracleConnection. The statement Interface creates a statement. The concrete implementation is Oracle Statement. ResultSet stores the result after invoking execute Query on the statement object. We can iterate over the resultset to get the results from table.

Statement and Prepared Statement A Statement Interface is used to create a statement i.e. a query to be executed against a database. In this process, two calls are made to the database system, one to get the table metadata and other to get the data itself. The statement object is also compiled every time it executes. A Prepared statement comes in handy as it is compiled and cached and only makes on database call when invoked. The compilation would be skipped even if the values in the where clause change

Statement and Prepared Statement Statement Statement stmt= con.statment(“select * from mytab where num=22”); Prepared Statement PreparedStatement pstmt = con.preparestatement(select * from mytab where num=?”); pstmt.setInt(1,”11”); This sets the value of ? as 11 – If we run the statement in for loop every time changing the where clause, the statement is compiled every time before execution. – In the case of prepared statement, the prepared statement is cached and compiled only one time in the event of a for loop with changing where clause.

Stored Procedures Stored procedures are compiled sql programs which are stored in the database system and can be executed in the database. We use the call keyword in SQL to execute stored procedures Java can access the stored procedures in the database by using the Callable statements

Callable Statements Callable statements are used to call stored procedures in the database. CallableStatement cstmt = null; try { String SQL = "{call myprocedure(?, ?)}"; //? Can be set dynamically as in prepared statement cstmt = conn.prepareCall (SQL);... } catch (SQLException e) {... } finally {... }

Transactions Transactions imply a situation when we have one or more SQL statements to be committed only when everything goes right – Assume we have 10 databases which are always to be in sync. So we would have one sql query to be executed across all databases. So, the sql query should committed in all the databases only after successful execution in all 10 databases. This is called a transaction. The transaction is committed only after successful execution across all databases.

Closing Connections Always remember to close in the following sequence. – Resultset – Statement/Prepared Statement – Connections Do a null check always before closing the connections using close() method.