Lecture 21

Database Programming and JDBC

Database Programming Objective: to access a database from an application

program As opposed to interactive interfaces like MySQL’s

Motivation: Interactive interfaces are convenient but Not all queries can be expressed in SQL (recursive)

Impedance mismatch problem (difference between database and programming language models) Different data types Sets/Multi-sets VS tuple-at-a-time (declarative VS non-

declarative/procedural actions) E.g. display selected tuples in a GUI

Database Programming Approaches

(1) Embedded commands: DB commands are embedded in a general-purpose host programming language such as COBOL, C, Java, etc… Embedded SQL commands are identified by a

special prefix E.g. EXEC SQL

A pre-compiler or a preprocessor first scans the code to identify database statements and extracts them for processing by the DBMS

Replaced by function calls to the DBMS-generated code Static (queries must be known at compile time)

Embedded Commands Example

Database Programming Approaches

(2) Library of database functions available to the host language for database calls known as an API (Application Programming Interface) Host language uses library functions to connect to a

database, execute a query, execute an update, etc …

Database queries/update commands (along with other information) are sent as parameters to function calls

Dynamic E.g. ODBC (Object DataBase Connectivity) and JDBC

(used to stand for Java DataBase Connectivity) ODBC: SQL Access Group, chiefly Microsoft, in 1992 JDBC: Like ODBC but enables SQL function calls for Java

programming

Database Programming Approaches

Embedded SQL provides static database programming

API: dynamic database programming Advantage: no preprocessor needed (thus

more flexible) No compile-time restrictions

Drawback: SQL syntax checks to be done at run-time

E.g. When executing an SQL statement via ODBC/JDBC, it is not checked for errors until it is run

Java Database Connectivity

A Java program with JDBC functions can access any relational DBMS that has a JDBC

driver (JDBC compliant) A JDBC driver is an implementation of the function calls

specified in the JDBC API (java.sql.*) for a particular RDBMS A JDBC driver translates JDBC function calls into DBMS-

specific calls that are understood by DBMS and vice-versa can connect to several databases known as data

sources All JDBC drivers loaded (dynamically) by an application are

registered with a driver manager Driver manager is responsible to manage different drivers

for different DBMSs used within the same program

JDBC Architecture

JavaApplication

JDBC Driver

manager

Driver 3

Driver 2

Driver 1

DBMS

DBMS

DBMS

Application-Initiates and terminates connection with a data source-Submits SQL statements and retrieves results through the JDBC API

Driver Manager-Loads JDBC driver and passes JDBC function calls from application to correct driver

Driver(s)-Establishes the connection with the data source (thru Driver Manager)-Submits requests from and return results to application-Translates data and errors from data source format to JDBC standard

Data source(s)-Processes commands from the driver and returns results

The JDBC Steps Importing Packages Loading & Registering the JDBC Drivers

Registers driver with driver manager Opening a Connection to a Database

Also ties driver and DBMS to driver manager Creating a Statement Object Executing a Query and Returning a Result Set Object Processing the Result Set Garbage collection

Closing the Result Set, Statement and Connection Objects

Import JDBC libraryLoad & register

JDBC driver

Create a connection

object

Create a statement

objectExecute SQL statement

Process results

Steps in JDBC Database Access

(1) Import JDBC library (import java.sql.*) javax.sql.* (for advanced features, such as

scrollable result sets)

(2) Load & register JDBC driver with manager: Class.forName("com.mysql.jdbc.Driver"); This will load the driver and register it with the driver

manager in order to make it available to the program Can also be done in two steps

Initializing/loading a driver: (for oracle) driver=new com.mysql.jdbc.Driver()

Registering it with the DriverManager: DriverManager.registerDriver(driver)

Steps in JDBC Database Access

(3) Create a connection object (via getConnection)

= DriverManager.getConnection(“jdbc:mysql: //<host>: <port>/<db-account>”, <username>, <password>)

Connection myConnection = DriverManager. getConnection("jdbc:mysql:// devsrv.cs.csbsju.edu :3306/irahal_compnay","irahal",“****");

(4) Create a statement object : Statement

Statement stmt = myConnection.createStatement(); String queryString = "Select lname As LastName,

fname As FirstName, salary from employee where fname like '%"+name+"%'"; name is a variable of type string

PreparedStatment (notice the question mark ?) String queryString = "Select count(*) as COUNTER

from employee where salary>?"; preparedStmt = myConnection.prepareStatement

(queryString);

Prepared Statement motivation

Why use PreparedStatements: “prepared” = already compiled (querystring is a parameter) Used if the same query is to be executed multiple times since it

would be compiled only once for queries that are executed many times with possibly different

contents

Suppose we would like to run the query SELECT *

FROM EMPLOYEE

WHERE LNAME=‘Moon’;

But we would like to run this for a number of employees (separately), not only ‘Moon’…

Additional Steps for PreparedStatments in JDBC Database Access

(4-1) Identify statement parameters (designated by question marks (?) in the statement string)

String queryString = "Select count(*) as COUNTER from employee where salary>?";

preparedStmt = myConnection.prepareStatement (queryString);

(4-2) bind parameters to program variables pst.setString(1, var1) where pst is a

PreparedStatement variable preparedStmt.setFloat(1,salary);

Set 1st ? in my query to the value of salary which is a float

Connection

ResultSet

Statement

intExecuteUpdate(String Q)

ExecuteQuery(String Q)

createStatement()

Connection

ResultSet

PreparedStatement

intExecuteUpdate()

ExecuteQuery()

prepareStatement (String Q)

setString(int num,String val)

Statement

PreparedStatement

Querying with Prepared Statement

String queryStr =

"SELECT * FROM EMPLOYEE WHERE FName = ? AND SALARY < ?”;

PreparedStatement pstmt = con.prepareStatement(queryStr);

pstmt.setString(1, “Joe”);

pstmt.setFloat2, 100000);

ResultSet rs = pstmt.executeQuery();

1st question

mark

Value to insert

Affecting the DB with PreparedStatement

String deleteStr =

“DELETE FROM EMPLOYEE WHERE LNAME = ? and SALARY = ?”;

PreparedStatement pstmt = con.prepareStatement(deleteStr);

pstmt.setString(1, “RAHAL”);

pstmt.setFloat(2, 10000);

int delnum = pstmt.executeUpdate();

Create a statement

objectIdentify statement parameter

sExecute

SQL statement

Steps in JDBC Database Access

(5) Execute SQL statement (referenced by an object) via JDBC’s executeQuery (SELECT statements)

Returns a ResultSet resultset1 = stmt.executeQuery(queryString); (regular

statement) resultset2 = preparedStmt.executeQuery(); (prepared

statement) executeUpdate (DDL, INSERT, UPDATE, and DELETE

statements) Returns how many tuples have been affected (an integer) result1 = stmt.executeUpdate(updateString); (regular

statement) result2 = preparedStmt.executeUpdate(); (prepared

statement)

Steps in JDBC Database Access

(6) Process query results ResultSet is a 2-dimensional table

Only one ResultSet per Statement can be opened at once

ResultSet object maintains a cursor pointing to its current row of data

The 'next' method moves the cursor to the next row As of JDBC 2.0, scrollable ResultSets are available,

which also include ‘previous’, ’first’, ‘last’, etc.. Any methods on the ResultSet will occur on the current

row

ResultSet

The cursor is positioned before the 1st row upon creation

Statement stmt=con.createStatement();

ResultSet rs = stmt.executeQuery ("SELECT * FROM

Table1");

while (rs.next())

{ //something… }

ResultSet methods

Getting the value in some column (for the current row) - depends on attribute data type:

getString(int columnNum); getString(String columnName); getInt(int columnNum); getInt(String columnName); Etc…

To check if null was returned, you have to use wasNull() on the ResultSet after getting the value

String s = rs.getString(“column1");

Mapping Java Types to SQL Types

Java method

SQL type

The driver maps SQL types

(varchar, number,…) to the

appropriate Java method

(getString, getInt…)

Cleaning Up After Yourself

(7) Remember to close the Connections, Statements, PreparedStatements and ResultSets in the following order

rs.close()

stmt.close()

pstmt.close()

con.close()

Highly recommended

Recommended

Optional (will otherwise be closed by its calling statement)

Timeouts

You can use Stmt.setQueryTimeOut(int secs) to set a timeout for the driver to wait for a statement to be completed

If the operation is not completed in the given time, an SQLException is thrown

Dealing With Exceptionscatch (SQLException e) {

while (e != null) {//human readable message about the exceptionSystem.out.println(e.getMessage());

//String describing the reason of the exception

System.out.println(e.getSQLState());

//driver-dependant error code for the exception

System.out.println(e.getErrorCode());

e = e.getNextException();}

What will this do?String table1=“create table table1(col1 integer, col2 integer)”;

Statement st=con.createStatement();

int resCreate=st.executeUpdate(table1);

PreparedStatement ps1 = con.prepareStatement(“insert into Table1 values(?,?)”);

for(int i=0;i<10;i++){

ps1.setInt(1,i);

ps1.setInt(2,i*i);

ps1.executeUpdate();

}

Statement st2=con.createStatement();

ResultSet rs=executeQuery(“select col2 from Table1”);

while(rs.next()){

System.out.println(rs.getInt(1)); }

Useful JDBC links http://dev.mysql.com/doc/refman/5.0/en/

java-connector.html http://java.sun.com/j2se/1.3/docs/guide/jd

bc/getstart/GettingStartedTOC.fm.html http://notes.corewebprogramming.com/

student/JDBC.pdf