Data Access with JDBC

Data Access with JDBC Svetlin Nakov Bulgarian Association of Software Developers www.devbg.org

Contents Introduction to JDBC Querying the database Different statements Handling exceptions Transactions Best practices

About JDBC JDBC is a standard interface for connecting to relational databases from Java The JDBC Core API package in java.sql JDBC 2.0 Optional Package API in javax.sql JDBC 3.0 API includes the Core API and Optional Package API

JDBC Architecture Application Connection Driver Manager Statement Result Set Driver Driver Driver Database Database Database

JDBC Components Driver Manager Loads database drivers, and manages the connection between application & driver Driver Translates API calls to operations for a specific data source Connection A session between an application and a database driver

JDBC Components (2) Statement A SQL statement to perform a query or an update operation Metadata Information about the returned data, driver and the database Result Set Logical set of columns and rows returned by executing a statement

Data Access with JDBC Querying the Database

Stage 1: Loading Drivers Loading the JDBC driver is done by a single line of code: Your driver documentation will give you the class name to use Loading Oracle JDBC driver Loading the JDBC-ODBC bridge driver Class.forName(" < jdbc d river class> "); Class.forName("oracle.jdbc.driver.OracleDriver"); Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

Stage 2: Establishing a Connection The following line of code illustrates the process: If you are using the JDBC-ODBC bridge driver, the JDBC URL will start with " jdbc:odbc: " If you are using a JDBC driver developed by a third party, the documentation will tell you what subprotocol to use Connection con = DriverManager. getConnection( " url " , "user", "pass");

Stage 2: Establishing a Connection to ODBC Connecting to ODBC driver – example ODBC data source is called " Library " DBMS login name is " admin " The password is " secret " Establishing a connection: Connection dbCon = DriverManager. getConnection("jdbc:odbc:Library", "admin", "secret");

Stage 2: Establishing a Connection to Oracle Connecting to Oracle – example The server is Oracle 10g Express Edition, locally installed Oracle database schema is called " HR " The password is " hr " Establishing a connection: Connection dbCon = DriverManager.getConnection( "jdbc:oracle:thin:@localhost:1521/xe", "HR", "hr");

Stage 3: Creating Statement A Statement object sends the SQL commands to the DBMS executeQuery() is used for SELECT statements executeUpdate() is used for statements that create/modify tables An instance of active Connection is used to create a Statement object Connection dbCon = DriverManager.getConnection( "jdbc:odbc:Library", "admin", "secret"); Statement stmt = dbCon.createStatement() ;

Stage 4: Executing Query executeQuery() executes SQL command through a previously created statement Returns the results in a ResultSet object Connection dbCon = DriverManager.getConnection( "jdbc:odbc:Library", "admin", "secret"); Statement stmt = dbCon.createStatement(); ResultSet rs = stmt.executeQuery ( "SELECT first_name FROM employees");

Stage 4: Executing Statement executeUpdate() is used to submit DML/DDL SQL statements DML is used to manipulate existing data in objects (using UPDATE , INSERT , DELETE statements) DDL is used to manipulate database objects ( CREATE , ALTER , DROP ) Statement stmt = dbCon.createStatement(); int rowsAffected = stmt.executeUpdate ( "UPDATE employees SET salary = salary*1.2");

Stage 5: Process The Returned Results The ResultSet object Maintains a cursor pointing to its current row of data Provides methods to retrieve column values ResultSet rs = stmt.executeQuery( "SELECT last_name, salary FROM employees"); while (rs .next() ) { String name = rs .getString ("last_name"); double salary = rs .getDouble ("salary"); System.out.println(name + " " + salary); }

Stage 6: Closing the Connection Explicitly close a Connection , Statement , and ResultSet to release resources that are no longer needed try { Connection conn = ...; Statement stmt = ...; ResultSet rset = stmt.executeQuery( ...); ... } finally // clean up rset.close(); stmt.close(); conn.close(); }

SQLException SQL statements can throw java.sql . SQLException during their execution try { rset = stmt.executeQuery( "SELECT first_name, last_name FROM employee"); } catch (SQLException sqlex) { ... // Handle SQL errors here } finally { // Clean up all used resources try { if (rset != null) rset.close(); } catch (SQLException sqlex) { ... // Ignore closing errors } ... }

Querying Oracle through JDBC Live Demo

JDBC Statements Statement , PreparedStatement and CallableStatement Interfaces

Submitting DML Statements That Change the Database Create an empty statement object: Use executeUpdate() to execute the statement: Example: Statement stmt = conn.createStatement(); int count = stmt.executeUpdate(sql_dml_statement); Statement stmt = conn.createStatement(); int rowsDeleted = stmt.executeUpdate("DELETE FROM order_items WHERE order_id = 2354");

Submitting DDL Statements Create an empty statement object: Use executeUpdate() to execute the statement: Example: Statement stmt = conn.createStatement(); int count = stmt.executeUpdate(sql_ ddl _statement); Statement stmt = conn.createStatement(); stmt.executeUpdat e (" CREATE TABLE temp(col1 NUMBER(5,2), col2 VARCHAR2(30)) ");

Unknown Statements Create an empty statement object Use execute() to execute the statement Process the statement accordingly Statement stmt = conn.createStatement(); boolean result = stmt.execute(SQLstatement); if (result) { // was a query - process results ResultSet r = stmt.getResultSet(); ... } else { // was an update or DDL - process result int count = stmt.getUpdateCount(); ... }

Prepared Statements PreparedStatement is used to Execute a statement that takes parameters Execute given statement many times String insert SQL = "INSERT INTO employees(" + "first_name, last_name, salary) VALUES(?,?,?)"; PreparedStatement stmt = c on.prepareStatement(insert SQL ); stmt.setString( 1 , "Svetlin" ); stmt.setString( 2 , "Nakov" ); stmt.set Double (3, 25000.0 ); stmt.executeUpdate();

Retrieving Auto Generated Primary Key Some databases support "auto increment" primary key columns E. g. MS SQL Server, MS Access, MySQL, ... JDBC can retrieve auto generated keys // Insert row and return PK int rowCount = stmt.executeUpdate( "INSERT INTO Messages(Msg) VALUES ('Test')", Statement.RETURN_GENERATED_KEYS); // Get the auto generated PK ResultSet rs = stmt.getGeneratedKeys(); rs.next(); long primaryKey = rs.getLong(1);

Retrieving Auto Generated Primary Key in Oracle Oracle does not support "auto increment" primary key Use sequences to generate unique values stmtSeq = dbCon.createStatement(); rsNextId = stmtSeq.executeQuery( "SELECT <some_sequence>.nextval FROM dual"); rsNextId.next(); long nextId = rsNextId.getLong(1); psIns = dbCon.prepareStatement( "INSERT INTO Table(id, ...) VALUES(?, ?)"); psIns.setLong(1, nextId); psIns.setString(2, ...); psIns.executeUpdate();

Callable Statements CallableStatement interface: Is used for executing stored procedures Can pass input parameters Can retrieve output parameters Example: CallableStatement callStmt = dbCon.prepareCall("call SP_Insert_Msg(?,?)"); callStmt.setString(1, msgText); callStmt.registerOutParameter(2, Types.BIGINT); callStmt.executeUpdate(); long id = callStmt.getLong(2);

Transactions Management in JDBC

Database Transactions A database transaction is a set of database operations that must be either entirely completed or aborted A simple transaction is usually in this form: Begin the transaction Execute several SQL DML statements Commit the transaction If one of the SQL statements fails, rollback the entire transaction

JDBC Transactions JDBC transaction mode: Auto-commit by default Can be turned off by calling setAutoCommit(false) In auto-commit mode each statement is treated as a separate transaction If the auto-commit mode is off, no changes will be committed until commit() is invoked Auto-commit mode can be turned back on by calling setAutoCommit(true )

JDBC Transactions –Example If we don’t want certain changes to be made permanent, we can issue rollback () dbCon.setAutoCommit(false); try { Statement stmt = con.createStatement(); stmt.executeUpdate("INSERT INTO Groups " + "VALUES (101, ' Administrators ')"); stmt.executeUpdate("INSERT INTO Users " + "VALUES (NULL, 'Mary', 101)"); dbCon.commit(); } catch (Exception ex) { dbCon.rollback(); throw ex; }

Transactions in JDBC Live Demo

JDBC Advanced Features and Best Practices

Database Specific Properties You can pass database specific information to the database by using Properties object Example for Oracle database: Properties props = new java.util.Properties(); props.put("user", "scott"); props.put("password", "tiger"); props.put("defaultRowPrefetch", "30"); props.put("defaultBatchValue", "5"); Connection dbCon = DriverManger.getConnection( "jdbc:oracle:thin:@hoststring", props);

Connection Pool Connection pool contains a number of open database connections There are a few choices when using connection pool: Depend on the application server Use JDBC 2.0 interfaces ( ConnectionPoolDataSource and PooledConnection ) Create your own connection pool

Optimal Isolation Level You can set the transaction isolation level by calling setTransactionIsolation( level ) Transaction Level Permitted Phenomena Impact Dirty Reads Non-Repeatable Reads Phantom Reads TRANSACTION_NONE - - - FASTEST TRANSACTION_READ_UNCOMMITED YES YES YES FASTEST TRANSACTION_READ_COMMITED NO YES YES FAST TRANSACTION_REPEATABLE_READ NO NO YES MEDIUM TRANSACTION_SERIALIZABLE NO NO NO SLOW

ResultSet Metadata ResultSetMetaData class Used to get information about the types and properties of the columns in a ResultSet object ResultSet rs = stmt.executeQuery( "SELECT * FROM employees"); ResultSetMetaData rsm = rs.getMetaData(); int number = rsm.getColumnCount(); f or (int i=0; i<number; i++) { System.out.println(rsm.getColumnName(i)); }

Close Unneeded Resources Closing connections, statements and result sets explicitly allows garbage collector to recollect memory and free resources as early as possible Close statement object as soon as you finish working with them Use try-finally statement to guarantee that resources will be freed even in case of exception

Choose the Right Statement Use PreparedStatement when you execute the same statement more than once Use CallableStatement when you want result from multiple and complex statements for a single request

Use Batch Updates/Retrieval Send multiple queries to reduce the number of JDBC calls and improve performance: You can improve performance by increasing number of rows to be fetched at a time statement.addBatch("sql_query1"); statement.addBatch("sql_query2"); statement.addBatch("sql_query3"); statement.executeBatch(); s tatement.setFetchSize(30);

Optimize the SQL Queries Bad: Good: Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery( " SELECT * FROM EMPLOYEE WHERE ID = 1 "); Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery( " SELECT SALARY FROM EMPLOYEE WHERE ID=1 ");

Problems Write a program that prints the names of all employees and their salaries from the standard HR schema in Oracle 10g. Don't forget to ensure that all exceptions are handled appropriately and used resources are cleaned. Write a program that reads a last name and a salary range from the console and prints all matched employees and their salaries from the standard HR schema in Oracle 10g. Use PreparedStatement with parameters. Handle the possible exceptions and close all used resources.

Problems (2) Write a program that creates a table Countries(country_id, country_name) and a sequence SEQ_Countries . Define a class Country with the same fields like the columns in the Countries table. Write a method to insert new country. Write a method to list all countries (it should return Country[] ). Write a method to find country by country_id . Write a method to find country by part of its name. Finally write a method to drop the Countries table and the sequence SEQ_Countries . Test all methods.

Problems (3) Create tables Persons(person_id, person_name) , Accounts(account_id, acc_holder_id ,amount ) and Log(log_id, msg_date, log_msg) . Define sequences for populating the primary keys in these tables (without triggers). Write stored procedures for inserting persons, accounts and log messages and Java methods that call them. Write method for transferring funds between accounts. It should keep track of all successful transfers in the Log table. Use transactions to maintain data consistency.

Homework Write a program that prints the names of all employees, their managers and departments from the standard HR schema in Oracle 10g. Handle the possible exceptions and close all used resources. Write a program that reads a department name from the console and prints all employees in this department and their average salary. Use the standard HR schema in Oracle 10g. Use PreparedStatement with parameters. Handle the possible exceptions and close all used resources.

Homework (2) Write a program that creates tables Users(user_id, user_name, group_id) and Groups(group_id, group_name) along with sequences fro populating their primary keys. Write classes User and Group that correspond to these tables. Write methods for adding new users and groups. Write methods for listing all groups, all users and all users by given group. Write methods for updating and deleting users and groups. Finally write a method to drop the tables Users and Groups and their sequences. Test all these methods. Handle the exceptions appropriately and close all used resources.

Homework (3) Modify the previous program to add also the table Log(log_id, msg_date, msg_text) that keeps track of all changes in the all other tables. Modify all methods to maintain the log. Don't forget to use transactions if a business operation modifies several tables. Add a method for adding user and group in the same time (by given user name and group name). If the group does not exists, it should be created on demand. Use transactions to guarantee the data consistency. Don't forget to register the operations in the logs.

Data Access with JDBC

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Data Access with JDBC

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